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Evidence of Mn-Ion Structural Displacements Correlated with Oxygen Vacancies in La0.7Sr0.3MnO3 Interfacial Dead Layers

Cite this: ACS Appl. Mater. Interfaces 2021, 13, 46, 55666–55675
Publication Date (Web):November 10, 2021
https://doi.org/10.1021/acsami.1c15599

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Abstract

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The properties of half-metallic manganite thin films depend on the composition and structure in the atomic scale, and consequently, their potential functional behavior can only be based on fine structure characterization. By combining advanced transmission electron microscopy, electron energy loss spectroscopy, density functional theory calculations, and multislice image simulations, we obtained evidence of a 7 nm-thick interface layer in La0.7Sr0.3MnO3 (LSMO) thin films, compatible with the formation of well-known dead layers in manganites, with an elongated out-of-plane lattice parameter and structural and electronic properties well distinguished from the bulk of the film. We observed, for the first time, a structural shift of Mn ions coupled with oxygen vacancies and a reduced Mn valence state within such layer. Understanding the correlation between oxygen vacancies, the Mn oxidation state, and Mn-ion displacements is a prerequisite to engineer the magnetotransport properties of LSMO thin films.

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1. Introduction

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Transition metal oxides with perovskite structures (ABO3) are fascinating systems that host a vast array of unique phenomena, such as high-temperature (and unconventional) superconductivity, colossal magnetoresistance, (1,2) all forms of magnetic and ferroelectric order parameters, and the interplay between these phases. The recent years have witnessed an increasing ability to grow thin-film heterostructures of these materials with atomic precision. With this level of control, the electrostatic boundary conditions at oxide surfaces and interfaces can be used to form new electronic configurations as a consequence of epitaxial strain, orbital hybridization, and reduced dimensionality. Notably, colossal magnetoresistive manganite films with the composition La0.7Sr0.3MnO3 (LSMO) are of great interest because of their half-metallicity, room-temperature ferromagnetism, and the competition between lattice, spin, and charge degrees of freedom (3−9) that create an ideal platform for exploring oxide spintronic applications. Although bulk LSMO is a metallic ferromagnet, it is well known that LSMO ultrathin films become insulators with a greatly reduced magnetization when their thickness is less than a critical value of 4 unit cells. (10,11) The layer characterized by suppressed magnetization and metallicity is commonly called “dead layer”. (12,13) Many mechanisms have been proposed to explain the formation of magnetic dead layers at interfaces, including cation intermixing, (14) strain-induced spin canting, (15) interfacial orbital reconstruction, (16) oxygen octahedral rotation, (17,18) and polar discontinuity across the interfaces. (19−23) The latter yields an uncompensated extra charge that can drive a potential divergence, that is, polar catastrophe, which affects the stoichiometry of manganites at the interface. In spite of abundant investigation, the origin of a magnetic dead layer at the LSMO/substrate interface is still not well understood. From the structural point of view, bulk LSMO (A site: La and Sr, B site: Mn) has rhombohedral symmetry with the space group R3̅C and the a–a–a– Glazer notation, (24) revealing equal and out-of-plane BO6 octahedral tilting in all three directions. Several studies have evidenced that not only the dead layer thickness but also the magnetic properties of epitaxially grown LSMO oxides can be tuned by controlling the oxygen coordination, (25) interfacial oxygen octahedral rotation, (18) interfacial strain, (26,27) and concentration of oxygen defects. (27)
In previous studies, scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) have proven effective in examining structures and cation charge states in thin films. For EELS, fine spectral features from the cation and oxygen edges, energy onset, and intensity ratio of the core-loss edges can be used to quantify B-site charge states as well as to determine the film stoichiometry. (14,28−35)
Here, we provide the first evidence of Mn-ion (B-site) displacement correlated with the reduction of the Mn valence state in the neighborhood of the film/substrate interface in LSMO thin films as grown on (001)-oriented (LaAlO3)0.3–(Sr2AlTaO6)0.7 (henceforth LSAT) substrates by means of pulsed laser deposition (PLD). First, we assess the quality and strain properties of the LSMO/LSAT heterostructure by high-resolution TEM (HRTEM) and geometrical phase analysis (GPA). (36) By using aberration-corrected STEM imaging with a high-angle annular dark-field (HAADF) detector and a low-angle annular dark-field (ADF) detector, EELS, atomistic calculations, and STEM image simulations, we correlate the observed Mn displacements with the changes in the valence state of Mn ions from its optimal value of 3.3, as well as with oxygen vacancies, throughout the film. In particular, STEM–ADF imaging shows enhanced defect contrast in the presence of oxygen vacancies, providing direct evidence of the formation of a 7 nm-thick interfacial region in the LSMO film that is structurally different from the bulk of the film and can be associated with the magnetic dead layer of manganites. EELS analysis indicates a preferential formation of oxygen vacancies in LSMO at the interfacial region with the substrate with respect to the rest of the film, which is accompanied by an out-of-plane lattice expansion. By comparing the STEM results and multislice simulations based on test structures as obtained from density functional theory (DFT) calculations, we probe the formation of an oxygen-deficient structure with displaced Mn ions and discuss the origin of the dead layer as consequent to the lattice strain and Mn-ion displacements.

2. Experimental and Calculation Details

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2.1. Thin-Film Fabrication

Epitaxial LSMO thin films were grown on cubic LSAT (001) substrates by PLD using a KrF excimer laser (248 nm, repetition rate 3 Hz, fluence of 1.4–2 J/cm2). The choice of LSAT substrate is driven by its great thermal stability and low lattice mismatch with the film (0.2%). The substrate temperature and oxygen partial pressure (PO2) during deposition were maintained at 720 °C and 0.35 mbar, respectively. After deposition, the films were cooled down to room temperature at a rate of 15 °C/min in 700 mbar O2 pressure. (37) Detailed structural characterizations (θ–2θ, reciprocal space maps, ω, and ϕ-scans) were performed by PANalytical X’Pert four-circle X-ray diffraction (XRD) and morphological characterizations by atomic force microscopy (AFM).

2.2. TEM Imaging and Spectroscopy

Cross-sectional TEM samples were prepared using the conventional sandwich technique. At first, the samples were mechanically ground, then dimpled and, eventually, thinned down to electron transperancy by Ar+ ion beam milling.
The HRTEM investigations were performed by using the JEOL 2010 UHR field emission gun microscope operated at 200 kV with a measured spherical aberration coefficient Cs of 0.47 ± 0.01. In all cases, the sample was tilted to [010] zone axes, aligning the film growth direction of the heterostructure perpendicular to the electron beam direction.
High-resolution STEM (HRSTEM) were performed on a probe-corrected FEI Titan3 G2 60-300 operating at 300 keV with a convergence angle of 19.6 mrad. A Gatan Quantum energy filter (GIF) and a four-quadrant Super-X detector (FEI) were used for EELS and EDX spectrum acquisition, respectively. Simultaneously, HAADF and ADF image acquisition was performed. The inner and outer detection angles of the HAADF detector were 116.7 and 177 and 24.1 and 55 mrad for the ADF detector, respectively. The collection semiangle and dispersion channel width of EELS acquisition were 24.59 mrad and 0.25 eV, respectively. To correct the intrinsic shift of energy of the electron beam, the zero-loss peak was used, which was recorded simultaneously using the Dual EELS mode of the spectrometer. The sample was oriented to the [010] axis of the LSMO thin films and the fast scanning direction was oriented perpendicular to the interface.

2.3. Data Evaluation

Gatan Digital Micrograph 3.13 (GMS 3.13) was used to process EELS and EDX data. For EELS quantification, at first, the raw EELS data sets were background-corrected using a pre-edge power law fit, and plural scattering was removed via zero-loss deconvolution. (38) The absolute energy onset was calibrated carefully with the simultaneously acquired zero-loss peak. Then, the valence state of Mn was determined by the energy difference between the pre-peak and the main peak of the O–K edges as described by Varela et al. (30) To compare all experimental techniques reliably and quantitatively, the atomic column positions have been extracted, and the displacements of the cations were quantified by using the StatSTEM software. (39)
GPA developed by Hÿtch et al. (36) was used to measure the strain within the film region using an HRTEM image. Strain map calculation was carried out with scripts in the FRWR tools menu (FRWR, 2012) (40) for Gatan Digital Micrograph software. Two [200] and [002] Bragg spots were used to calculate two-dimensional symmetric strain components. Depending on the aperture size used for the Bragg spots, the spatial sampling of strain measurement was 3 nm.

2.4. DFT Calculations

In order to confirm experimental observation theoretically, the LSMO supercell was relaxed with first-principles methods. DFT calculations have been performed using the Quantum Espresso (41) code using the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional (42) and ultrasoft pseudopotentials. (43) Semicore 3s and 3p states were included in the valence electron configuration for the Mn atom (for La, Mn, and O atoms, the valence electron configuration of 5s25p66s25d1, 3s23p63d54s2, and 2s22p4, respectively). The cutoff for the plane-wave wave functions and the kinetic energy were set as 50 and 450 Ry, respectively. The atomic positions and the free lattice parameters were relaxed using the BFGS (Broyden–Fletcher–Goldfarb–Shanno) algorithm until the forces on the atoms were below 1 meV/Å for the bulk and strain calculations.

2.5. STEM–HAADF Image Simulation

The STEM–HAADF images were simulated using a multislice algorithm with Frozen Phonon approximation to allow quantitative comparison with experimental micrographs. The simulations were executed using QSTEM, (40) with parameters as of the experiment: 300 kV acceleration voltage, 19.6 mrad convergence angle, 5 μm Cs, 0.8 eV energy spread (dE), and −3.8 nm defocus with a total of 15 phonon configurations. For simulation, the LSMO structure was oriented along the viewing direction [010] as of experiment with a thickness of 50 nm along the electron beam direction. To perform a multislice algorithm, the slice thickness of the LSMO structure was chosen 1.99 Å. The scan areas were 110 Å × 110 Å, which sampled by 220 × 220 pixels. The probe array and resolution were kept as 400 × 400 pixel and 0.037 × 0.037 Å, respectively.

3. Results and Discussion

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Figure 1a displays a representative cross-sectional HRTEM image of the LSMO/LSAT heterostructure taken in the [010] zone axis, showing that the LSMO films have a good crystalline quality as well as smooth and atomically flat interfaces with the substrate. The corresponding selected area electron diffraction (SAED) pattern shown in Figure 1b enlightens the very good crystalline matching between the LSMO film and the LSAT substrate with the LSMO (001)∥LSAT (001) epitaxial relationship.

Figure 1

Figure 1. Cross-sectional HRTEM image and corresponding strain analysis: (a) HRTEM image of the [001]-oriented LSMO thin film grown on the LSAT substrate, (b) SAED pattern from the imaged region, (c) experimental HRTEM image used for strain analysis, and (d,e) symmetric strain components: in-plane (εxx) and out-of-plane (εzz) respectively. A color bar for the strain map is shown. (f) Line profiles of the strain components, averaged over the width of the strain maps in the [100] direction of the film. All strain values indicated in this figure are in percentage.

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The quality and the strain distribution of the film were further explored by GPA. (36) The in-plane strain map εxx [where εxx = (aLSMO – aLSAT)/aLSAT, with aLSMO and aLSAT being the in-plane lattice parameters of LSMO and LSAT, respectively] and the out-of-plane strain maps εzz [where εzz = (cLSMO – cLSAT)/cLSAT, where cLSMO and cLSAT being the out-of-plane lattice parameters of LSMO and LSAT, respectively), as calculated from Figure 1c by keeping the LSAT substrate as a reference, are presented in Figure 1d,e, respectively. The integrated line profile of the strain distribution shown in Figure 1f provides evidence of minor strain variations of εxx in the rest of the film (i.e., beyond 7 nm), as expected for epitaxial growth. Moreover, the film grows perfectly coherent in-plane, with almost the same lattice parameter of the LSAT substrate (aLSAT = 3.868 Å (44)). Remarkably, in Figure 1f, an increase of the out-of-plane strain with a value of +2.4 ± 0.5% is measured over the first 7 nm of the film relative to LSAT, which corresponds to an out-of-plane lattice parameter ≈ 3.960 Å, that is, +2.1% compared to bulk LSMO (abulk = 3.876 Å (45)). The εzz value for the rest of the LSMO film (beyond 7 nm) is approximately +0.8 ± 0.3%, which corresponds to the out-of-plane lattice parameter of 3.898 Å.
The presence of out-of-plane strain gradients in the absence of dislocations or crystal twinning indicates an interesting strain relief mechanism. It is worth mentioning that in perovskite systems, the epitaxial strain is accommodated mainly by changes in the lattice constants or by structural distortions such as changing bond lengths or octahedral tilting. (46,47)
To elucidate the origin of the strain distribution in the LSMO/LSAT heterostructure, we performed aberration-corrected STEM with HA/ADF detectors. A representative cross-sectional STEM–HAADF image of the LSMO/LSAT heterostructure shown in Figure 2a reveals that the film has a uniform structure and the interface is atomically sharp with no visible defects (a magnified image is shown in the blue-colored box) nor cation interdiffusion between the LSAT substrate and the LSMO film (t/λ profiles are given in Figure S1 in the Supporting Information). Considering that STEM–HAADF experiments produce images whose contrast is approximately proportional to the square of the average atomic number, (48,49) in Figure 2a, La/Sr atomic columns have brighter contrast (marked with a green circle in the inset), while darker contrast is associated with Mn positions (a magnified image is shown in the inset with a red-colored box superimposed to the image with La/Sr and Mn columns labeled in green and purple, respectively). As the presence of oxygen vacancies is better emphasized by ADF, (50,51) we compare the HAADF and ADF signals of the same region.

Figure 2

Figure 2. Experimental high-resolution STEM images. (a) STEM–HAADF image showing the cross-sectional geometry of the LSMO/LSAT heterostructure along the [010] zone axis. The inset in the blue box shows that the interface is atomically flat and defect free. The inset in the red box shows a close-up image of the LSMO thin film. The scale bar of the insets corresponds to 1 nm. The upper right inset shows the projection of the LSMO unit cell along the [010] direction where green, purple, and red-colored circles represent La/Sr, Mn, and O atomic column positions, respectively. (b) STEM–ADF image showing the contrast variation at around 7 nm distance from the interface.

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Figure 2b shows a STEM–ADF image of the heterostructure acquired simultaneously with Figure 2a. Contrast variations can be appreciated within the film and observed to recur at measurable distances from the interface. In particular, a well-defined darker contrast layer of about 7 nm thickness is observed in the LSMO film at the interface with the substrate, which recalls the typical values of dead layers (52) in LSMO thin films. Differently from HAADF, the ADF signal is known to be very sensitive to the dechanneling of the incident electron beam caused by defects such as vacancies and lattice distortion around vacancy sites, as well as strain surrounding the defects. (50,53−57) Based on the experimental evidence, the presence of contrast variations throughout the film can be attributed to a different concentration of oxygen vacancies, with a distinct darker contrast in the first 7 nm interfacial region of the LSMO film where on average, a higher strain value is also measured (Figure 1f). To validate this, the out-of-plane (εzz) strain was also calculated from HAADF–STEM (presented in Figure S2 in the Supporting Information), and it also confirms the presence of higher strain values at the 7 nm interfacial region.
Energy-dispersive X-ray spectroscopy (EDXS) elemental mapping performed at the film/substrate interface further supports our interpretation of the STEM–HAADF results, ruling out the possibility of cation interdiffusion from the LSAT substrate to the LSMO film (see Figure S2 in the Supporting Information).
To obtain quantitative information on the oxygen distribution, we probe the local Mn valence (30) changes by performing site-selective EELS across the film thickness. The EELS line scans shown in Figure 3a highlight the changes in the fine structure of the O K and Mn L2,3 edge spectra from the interface to the surface. Twelve spectra were collected from the interface to the LSMO film surface, each one averaging over 2.5 nm along the [001] direction and 4.85 nm along the [100] direction of the film. In Figure 3b, the regions from which spectra were collected are numbered from 1 to 12, while in Figure 3a, all the corresponding 12 spectra are stacked vertically for ease of visualization. To determine the valence state of the Mn atoms, electron energy-loss near-edge structures of O K-edges have been used. The fine structure of the O K edge provides information on the electronic excitation from O 1s to 2p bands. The three main peaks of the O K edges are labeled “A”, “B”, and “C” in Figure 3a. It can be noted that peak “A” intensities change when moving from the interface to the surface of the film. The weakening of peaks “A” and “C” in conjunction with the broadening of main peak “B” has been recognized as the signature of the presence of oxygen vacancies. (58) Besides, the energy difference between peak “A” and peak “B” also increases steadily with distance from the interface. The change in the Mn valence state (details are in the Supporting Information) over the film region is summarized in Figure 3c. The nominal Mn valence of bulk LSMO (marked as a dashed line in Figure 3c) is 3.3. The Mn valence state increases gradually from approximately +3.1 ± 0.05 to +3.3 ± 0.05 toward the surface of the film, which reveals that the concentration of oxygen vacancies increases (Mn valence decreases) when nearing the interface with the substrate. As a matter of fact, the term “dead layer” indicates the interfacial region with depressed electric and magnetic properties, corresponding to the Mn valence value smaller than 3.3. Such a region is not fully insulating and antiferromagnetic since, according to the well-accepted manganite phase diagrams, (59,60) LSMO turns to be metallic when the Mn valence value is larger than 3.1, which occurs above 2 nm (i.e., 5 unit cells) from the interface with the substrate. Such a thickness is compatible with those reported in the literature for the insulating dead layer and with our best samples. The reduction of the Mn oxidation state has also been observed in (La, Ca)MnO3/STO6, (61) SrMnO3/LSAT, (44) and TbMnO3/STO, (62) and it was proposed as a possible explanation of the observed magnetic dead layer in such systems. (12)

Figure 3

Figure 3. EELS analysis of the Mn oxidation state in the film region. (a) Fine structures of O–K and Mn-L edges. Three prominent peaks for O–K edges are labeled “A”, “B”, and “C”. Peaks “A” and “C” correspond to O 2p–Mn 3d and 4sp hybrid states, respectively, while peak “B” is characteristic of O 2p Sr 4d or La 5d hybrid states. The energy difference ΔE between peaks “A” and “B” was measured to determine the Mn valency. (b) STEM–HAADF image of the LSMO/LSAT structure and the line shows the area from where EELS line profiles have been collected. The numbers indicate the area across which the average spectra were collected. (c) Estimated Mn valence in the LSMO film as a function of distance from the interface with the substrate.

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In addition, although the averaged values for the Mn valence state calculated from EELS spectra appear to slightly decrease when the film thickness reaches 20 nm, they still correspond to the optimal value of 3.3 within our experimental error, with no evidence of the presence of Mn2+ ions at the LSMO surface. (63) The slight decrease of the Mn valence state toward the surface can also be explained due to some extrinsic effects during TEM sample preparation. (64) Differently, the reduction of the Mn valence state in the proximity of the LSMO film/LSAT substrate interface is well above our experimental sensitivity.
The strain, the changes in the Mn valence state, and the variations in oxygen signal provide combined evidence of a deviation from the typical perovskite structure of LSMO in the proximity of the interface. The vacancies in ultrathin oxide films can form due to multiple reasons such as the interlayer effect, (65) chemical potential difference, (66) or during the growth stage of the LSMO film. (31) In our case, oxygen vacancies in the LSMO films cannot be attributed to any chemical effect since EDXS measurements do not show any cation inter-diffusion across the film/substrate interfaces (see Figure S3). On the other hand, similar to other oxide systems, (67) the Mn-ion displacements at the interface region suggest a possible role of residual strain in promoting oxygen vacancies at specific atomic sites. When oxygen vacancies are induced in LSMO, the charge compensating electrons localize on nearby Mn ions lowering their oxidation state (e.g., from Mn4+ to Mn3+). We observe from the STEM–HAADF images, a collective displacement of Mn ions from their centrosymmetric positions. To obtain quantitative information about the distribution and arrangement of the lattice distortions, we took a closer look at the STEM–HAADF image in Figure 4 and identified the presence of regions with different Mn displacements.

Figure 4

Figure 4. Quantitative analysis of the Mn-ion (B-site) displacements over the LSMO film. (a) Boxes marked in a different color on the STEM–HAADF image represent the regions used to map the displacement of Mn ions (B-site). (b–d) Map of atomic displacement vectors showing the displacement of Mn atoms (arrows) from the center of the projected La atoms superposed on the STEM–HAADF image [010] zone axis. (e–g) Out-of-plane (Δz) component and (h–j) in-plane (Δx) component of Mn displacements calculated from the corresponding regions. The scale bar on the images represents 1 nm. (k) Plot shows the variation in the magnitude of Δz and Δx from three different regions. (l) Projection of the LSMO unit cell along the [010] direction, where Δx and Δz denote the shift of Mn-ion (B-site) (blue) atoms along the in-plane and out-of-plane directions, respectively, concerning the centrosymmetric position (open circle).

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The atomic positions are obtained precisely from the HAADF images by using a center-of-mass refinement approach with picometer resolution, as implemented with StatSTEM (39) software. The relative displacements of Mn ions (B-site) from centrosymmetric locations are measured using La ion (A-site) positions as a reference. A schematic of one projected unit cell of LSMO is provided in Figure 4l where the positions of Mn, La/Sr, and O atomic columns are labeled blue, green, and orange circles, respectively. The relative displacements of the Mn ions from the center of the pseudocubic perovskite cell along the a and c crystallographic directions are indicated by Δx and Δz, respectively (illustrated in Figure 4l). Three regions of the film are evaluated using this method (regions are marked in Figure 4a), and the results are displayed in Figure 4b–d. The Mn-ion displacement map (Figure 4b–d) superimposed on the STEM–HAADF image shows the major alignment of Mn-ion displacements along the a axis. The corresponding out-of-plane (Δz) component of Mn-ion displacements of each region is presented in Figure 4e–f. Similarly, in Figure 4h–j, the in-plane (Δx) component of Mn-ion displacements is presented. The arrows indicate the direction of Mn-ion displacements relative to the center of La ions, and the corresponding color scale indicates the magnitude of the displacements. It can be noted that Mn-ion displacements near the surface region and in the middle of the LSMO film have random distribution, while an ordered displacement vectors can be measured at the region close to the interface. The Δz and Δx average magnitude values measured over each region are plotted in Figure 4k, which shows higher Δz and Δx values at the interfacial region compared to the rest of the film. The measured average value of Δz and Δx at the near-surface region of the LSMO film is 8 ± 6 pm, which can be attributed to some image distortions, including specimen drift, scanning noise. (68) By taking this value as a precision limit, the calculated average magnitude of the local Mn-ion displacement is Δx = 26 ± 10 pm and Δz = 10 ± 10 pm (see the Supporting Information) along the in-plane and out-of-plane direction. Interestingly, both the mean values of Δz and Δx reach their maximum off-center displacements close to the interface (Figure 4d,k), in the so-called dead layer region. This increase in the magnitude of the displacement of Mn ions is consistent with the increase in the population of oxygen vacancies at the dead layer of the LSMO film. It can be understood that the presence of oxygen vacancies generates locally additional positive charges, and the oxygen octahedron can be extended and rotated by applying the strain tensor to the oxygen vacancy. Even though intuitively an oxygen vacancy would isotropically affect the nearest Mn ions (i.e., moving them away from the vacancy position), (69) surprisingly STEM–HAADF analyses are conclusive in detecting a coherent displacement of Mn ions in one specific direction. The origin of unidirectional displacement will be the subject of more focused work. The structural modifications exemplified by the Mn-ion displacements accompanied by the presence of localized formation of oxygen vacancies at the interface region suggest a crucial role of possible residual strain effects in promoting oxygen vacancies, as also observed in other oxide systems such as CeO2. (67) This finding is in strong agreement with the above strain analysis, which shows that the out-of-plane strain near the interface is higher than in the remaining part of the LSMO thin film.
DFT modeling and multislice calculations were used to evaluate the impact of lattice distortions and atomic displacement in the presence of oxygen vacancies. Details of the calculation parameters are given in the Experimental and Calculation Details section. To this end, we examined four different test structures (presented in Figure 5a–d): (i) bulk LSMO unit cell (hereafter SI), (ii) supercell with oxygen vacancies (henceforth SII), (iii) supercell with a displacement of Mn ions from B-site (hereafter SIII), and (iv) supercell with oxygen vacancies and Mn-ion (B-site) displacement (onward SIV). Oxygen vacancies were generated randomly in these test structures. Structures SI, SII, and SIV were energetically relaxed by DFT calculations, and structure SIII was built from SI by introducing additional displacements to the Mn ions along the [110] direction by 29 pm (roughly equivalent to the corresponding displacements resulting from the experimental data). More details of the structural models and simulation parameters are given in the Supporting Information. The energetically relaxed structures show atomic rearrangements due to the presence of vacancies and Mn-ion displacements, which may contribute to the observed STEM contrast.

Figure 5

Figure 5. Atomistic simulation results. (a–d) Represent the 3D view of the energetically relaxed structures by DFT calculation for structures SI, SII, SIII, and SIV, respectively. The incomplete polyhedral shows the position of oxygen vacancies at the other end of the polyhedral (e–h) multislice simulated STEM–ADF images of the [010]-orientated LSMO structure with 15 nm thickness using the corresponding structures. The scale bar is the same for all the simulated images. (i) Intensity profile calculated from each simulated STEM–ADF image.

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Finally, based on the relaxed structures, multislice image simulations were performed to better visualize the structural changes giving rise to enhanced electron scattering to low angles, as observed in the experiments. The STEM images were simulated using QSTEM, (40) and simulation parameters were chosen to approximate the experimental conditions (refer to Experimental and Calculation Details section). Each energetically relaxed structure was repeated to create a supercell structure of 15 nm thickness along the electron beam direction (refer to the Supporting Information). The simulated STEM–ADF images are presented in Figure 5e–h for structures SI, SII, SIII, and SIV, respectively (simulated STEM–HAADF images are given in Figure S5 in the Supporting Information). By careful inspection, a variation in intensity can be recognized in Figure 5e,h. The corresponding intensity profiles across the simulated regions are plotted in Figure 5i. This demonstrates that the simulated STEM–ADF image intensities of structures SI, SII, and SIII are substantially comparable. On the other hand, the intensity profile of the simulated STEM–ADF image of structures SI and SIV differs significantly.
The calculated mean image intensities, Imean, of the corresponding simulated STEM–ADF images are as follows: (Imean = 138)SI > (Imean = 135)SII > (Imean = 133)SIII > (Imean = 122)SIV. The simulated ADF image of structure SIV shows a significant decrease in intensity with respect to that for SI. The calculated intensity ratios (1.13) of the simultaneous ADF signals of structure SI and structure SIV are remarkably equivalent to the computed intensity ratios (1.10) of the two sites (where the contrast fluctuates) of the experimental STEM–ADF image. These results clearly demonstrate that the cumulative effect of both O vacancies (including the accompanied lattice relaxations) and Mn-ion (B-site) displacements are contrasting in the experimental STEM–ADF image.

4. Conclusions

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To summarize, we provide a thorough structural characterization by resorting to (S)TEM imaging, GPA, EELS, and EDX analysis, as well as DFT and multislice calculations. The experimental data show the presence of a 7 nm-thick interfacial region in the LSMO film, commonly associated with the formation of a magnetic dead layer at the film/substrate interface, with structural and electronic properties that are clearly different from the rest of the film. STEM–ADF imaging and EELS analysis confirm the presence of oxygen vacancies in the LSMO films, with increasing concentration as nearing the film/substrate interface. In the same interfacial region, STEM–HAADF imaging also reveals that Mn ions are displaced by an amount of 29 pm ± 10 pm from their centrosymmetric positions. The existence of oxygen vacancies within the film region concurs to determine these lattice distortions, resulting in a lattice strain gradient, as also supported by GPA strain analysis performed on HRTEM images. We have emphasized the defect contrast due to the cumulative effect of Mn-ion displacement and oxygen vacancies within the interfacial layer using STEM–ADF imaging instead of only the use of STEM–HAADF image, which is most commonly used for thin-film characterization. Finally, DFT calculations and STEM image simulation support the experimental findings, enlightening that Mn-ion relocation is connected to the lowering of the Mn valence state at the film/substrate interface. Our findings result from the simultaneous acquisition of STEM–ADF and STEM–HAADF images, which has the key advantage of adding sensitivity to defects connected to light elements such as oxygen. We provide unprecedented evidence that the lowering of structural symmetry due to Mn-ion displacement is linked to the Mn valence state reduction at the interfacial region, thus opening new perspectives on the possibility to tune the magnetic anisotropy of LSMO thin films via a strain-driven thin-film technology.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.1c15599.

  • t/λ map using the EELS spectra; strain map calculated from the STEM–HAADF image; chemical composition analysis using EDXS; determination of the Mn valence state; quantification of Mn-ion (B-site) displacements; details of DFT calculations; and STEM image simulation (PDF)

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Author Information

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  • Corresponding Authors
  • Authors
    • Daniel Knez - Institute of Electron Microscopy and Nanoanalysis, Graz University of Technology, Steyrergasse 17, 8010 Graz, Austria;  Orcidhttps://orcid.org/0000-0003-0755-958X
    • Sandeep Kumar Chaluvadi - Istituto Officina dei Materiali-CNR, Area Science Park, S.S.14, km 163.5, 34149 Trieste, Italy;  Orcidhttps://orcid.org/0000-0002-3689-3336
    • Pasquale Orgiani - Istituto Officina dei Materiali-CNR, Area Science Park, S.S.14, km 163.5, 34149 Trieste, Italy;  CNR-SPIN, UOS Salerno, 84084 Fisciano, Salerno, Italy;  Orcidhttps://orcid.org/0000-0002-1082-9651
    • Giorgio Rossi - Istituto Officina dei Materiali-CNR, Area Science Park, S.S.14, km 163.5, 34149 Trieste, Italy;  Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy;  Orcidhttps://orcid.org/0000-0002-9330-7436
    • Laurence Méchin - Normandie University, UNICAEN, ENSICAEN, CNRS, GREYC, 14000 Caen, France;  Orcidhttps://orcid.org/0000-0002-6350-1801
  • Author Contributions

    P.R., D.K., and R.C. conceived the experiment. D.K., P.R., and R.C. performed the TEM experiments. D.K. performed the TEM/EELS experiments by an aberration-corrected microscope. P.R. carried out data analysis, GPA, and theoretical calculations. S.K.C., P.O., and L.M. grew the films and performed structural characterization by XRD. G.R. contributed to the discussion of the results and to review drafts. P.R. and R.C. wrote the paper with the contribution from all the authors.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work has received funding from the EU-H2020 research and innovation programme under grant agreement no. 654360 having benefitted from the access provided by CNR-IOM in Trieste within the framework of the NFFA-Europe Transnational Access Activity (proposal ID334) and performed in the framework of the Nanoscience Foundry and Fine Analysis (NFFA-MIUR Italy Progetti Internazionali) facility. Advanced TEM experiments have been performed under the support of the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 823717-ESTEEM3. E. Cociancich is gratefully acknowledged for the TEM specimen preparation. P.R. acknowledges the receipt of a fellowship from the ICTP Programme for Training and Research in Italian Laboratories, Trieste, Italy. D.K. acknowledges the financial support by the “Zukunftsfonds Steiermark” for funding of the K2 camera.

References

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    The authors have investigated the interface effect on magnetic and transport properties in the superlattices composed of ferromagnetic La1-xSrxMnO3 (LSMO) and non magnetic SrTiO3 (STO) (x = 0.2, 0.3, and 0.4). The interface of LSMO with STO was proved to show spin-canting, the degree of which critically depends on the doping level x and the layer thickness (tLSMO) of LSMO. Large magnetoresistance subsisting at low temp. appears in the superlattices with specific sets of (x, tLSMO) parameters that shows a metal-insulator crossover behavior with appreciable spin-canting. Implications of the present results in the tunneling magnetoresistance for the LSMO/STO/LSMO junction are also discussed.
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    Impact of interfacial coupling of oxygen octahedra on ferromagnetic order in La0.7Sr0.3MnO3/SrTiO3 heterostructures
    Li, Xiaoyan; Lindfors-Vrejoiu, Ionela; Ziese, Michael; Gloter, Alexandre; van Aken, Peter A.
    Scientific Reports (2017), 7 (), 40068CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
    La0.7Sr0.3MnO3, a half-metallic ferromagnet with full spin polarization, is generally used as a std. spin injector in heterostructures. However, the magnetism of La0.7Sr0.3MnO3 is strongly modified near interfaces, which was addressed as "dead-layer" phenomenon whose origin is still controversial. Here, both magnetic and structural properties of La0.7Sr0.3MnO3/SrTiO3 heterostructures were investigated, with emphasis on the quant. anal. of oxygen octahedral rotation (OOR) across interfaces using annular-bright-field imaging. OOR was found to be significantly altered near interface for both La0.7Sr0.3MnO3 and SrTiO3, as linked to the magnetism deterioration. Esp. in La0.7Sr0.3MnO3/SrTiO3 superlattices, the almost complete suppression of OOR in 4 unit-cell-thick La0.7Sr0.3MnO3 results in a canted ferromagnetism. Detailed comparisons between strain and OOR relaxation and esp. the observation of an unexpected La0.7Sr0.3MnO3 lattice c expansion near interfaces, prove the relevance of OOR for the magnetic properties. These results indicate the capability of tuning the magnetism by engineering OOR at the at. scale.
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  18. 18
    Liao, Z.; Huijben, M.; Zhong, Z.; Gauquelin, N.; Macke, S.; Green, R. J.; Van Aert, S.; Verbeeck, J.; Van Tendeloo, G.; Held, K.; Sawatzky, G. A.; Koster, G.; Rijnders, G. Controlled Lateral Anisotropy in Correlated Manganite Heterostructures by Interface-Engineered Oxygen Octahedral Coupling. Nat. Mater. 2016, 15, 425– 431,  DOI: 10.1038/nmat4579
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    18
    Controlled lateral anisotropy in correlated manganite heterostructures by interface-engineered oxygen octahedral coupling
    Liao, Z.; Huijben, M.; Zhong, Z.; Gauquelin, N.; Macke, S.; Green, R. J.; Van Aert, S.; Verbeeck, J.; Van Tendeloo, G.; Held, K.; Sawatzky, G. A.; Koster, G.; Rijnders, G.
    Nature Materials (2016), 15 (4), 425-431CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Controlled in-plane rotation of the magnetic easy axis in manganite heterostructures by tailoring the interface oxygen network could allow the development of correlated oxide-based magnetic tunnelling junctions with noncollinear magnetization, with possible practical applications as miniaturized high-switching-speed magnetic random access memory (MRAM) devices. Here, to manipulate magnetic and electronic anisotropic properties in manganite heterostructures by engineering the oxygen network on the unit-cell level. The strong oxygen octahedral coupling is found to transfer the octahedral rotation, present in the NdGaO3 (NGO) substrate, to the La2/3Sr1/3MnO3 (LSMO) film in the interface region. This causes an unexpected realignment of the magnetic easy axis along the short axis of the LSMO unit cell as well as the presence of a giant anisotropic transport in these ultrathin LSMO films. As a result the authors possess control of the lateral magnetic and electronic anisotropies by at.-scale design of the oxygen octahedral rotation.
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  19. 19
    Ishii, Y.; Yamada, H.; Sato, H.; Akoh, H.; Ogawa, Y.; Kawasaki, M.; Tokura, Y. Improved Tunneling Magnetoresistance in Interface Engineered (La,Sr)MnO3 Junctions. Appl. Phys. Lett. 2006, 89, 042509,  DOI: 10.1063/1.2245442
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    Improved tunneling magnetoresistance in interface engineered (La,Sr)MnO3 junctions
    Ishii, Y.; Yamada, H.; Sato, H.; Akoh, H.; Ogawa, Y.; Kawasaki, M.; Tokura, Y.
    Applied Physics Letters (2006), 89 (4), 042509/1-042509/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Spin tunnel junctions were fabricated and characterized with various insulating barriers and interface structures employing (La,Sr)MnO3 (LSMO) as the ferromagnetic electrodes. Junctions with LaAlO3 barriers exhibited systematically larger tunnel magnetoresistance (TMR) (TMR ratio ∼230% at 10 K) than those with SrTiO3 barriers (∼50%). When two unit cells of undoped LaMnO3 are inserted between LSMO and SrTiO3 at both interfaces in the SrTiO3-barrier junction, the TMR was also significantly enhanced to ∼170%. These results, including the temp. dependence of TMR, qual. agree with the characteristics of the local magnetization at the interface that was evaluated by magnetization-induced second harmonic generation (MSHG) for the corresponding interface structures. However, slight deviations appear as a systematic suppression of TMR for all the junctions at high temps. compared with MSHG results. Also, TMR results appear to be more degraded than MSHG results for the SrTiO3-barrier junctions. The barrier thickness dependence of the TMR response revealed that a spin-independent and inelastic conduction channel is a possible origin for the apparent suppression of TMR compared with MSHG.
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  20. 20
    Matou, T.; Takeshima, K.; Anh, L. D.; Seki, M.; Tabata, H.; Tanaka, M.; Ohya, S. Reduction of the Magnetic Dead Layer and Observation of Tunneling Magnetoresistance in La0.67 Sr0.33MnO3 -Based Heterostructures with a LaMnO3 Layer. Appl. Phys. Lett. 2017, 110, 212406,  DOI: 10.1063/1.4984297
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    20
    Reduction of the magnetic dead layer and observation of tunneling magnetoresistance in La0.67Sr0.33MnO3-based heterostructures with a LaMnO3 layer
    Matou, Tatsuya; Takeshima, Kento; Anh, Le Duc; Seki, Munetoshi; Tabata, Hitoshi; Tanaka, Masaaki; Ohya, Shinobu
    Applied Physics Letters (2017), 110 (21), 212406/1-212406/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    The formation of a magnetic dead layer at the interfaces of the perovskite oxide La0.67Sr0.33MnO3 (LSMO) is one of the crucial issues for its spintronic applications. The authors report the redn. of the dead layer by growing LSMO on a LaMnO3 (LMO) layer. Also, the authors detect tunneling magnetoresistance (TMR) in an LSMO/LMO/LSMO heterostructure. The obtained sign of the TMR was neg., but it changed to pos. after annealing. This unusual neg. TMR can be attributed to the intrinsic structural difference between the upper and lower interfaces of LMO and can be understood by a weak antiferromagnetic metallic thin layer formed at the upper LSMO/LMO interface. This layer probably is formed by diffused Sr atoms and oxygen vacancies in the LMO barrier. The authors' results indicate that control of intermixing of atoms at the interfaces is a key to controlling the TMR. (c) 2017 American Institute of Physics.
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  21. 21
    Boschker, H.; Verbeeck, J.; Egoavil, R.; Bals, S.; Van Tendeloo, G.; Huijben, M.; Houwman, E. P.; Koster, G.; Blank, D. H. A.; Rijnders, G. Preventing the Reconstruction of the Polar Discontinuity at Oxide Heterointerfaces. Adv. Funct. Mater. 2012, 22, 2235– 2240,  DOI: 10.1002/adfm.201102763
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    21
    Preventing the Reconstruction of the Polar Discontinuity at Oxide Heterointerfaces
    Boschker, H.; Verbeeck, J.; Egoavil, R.; Bals, S.; van Tendeloo, G.; Huijben, M.; Houwman, E. P.; Koster, G.; Blank, D. H. A.; Rijnders, G.
    Advanced Functional Materials (2012), 22 (11), 2235-2240CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Perovskite oxide heteroepitaxy receives much attention because of the possibility to combine the diverse functionalities of perovskite oxide building blocks. A general boundary condition for the epitaxy is the presence of polar discontinuities at heterointerfaces. These polar discontinuities result in reconstructions, often creating new functionalities at the interface. However, for a significant no. of materials these reconstructions are unwanted as they alter the intrinsic materials properties at the interface. Therefore, a strategy to eliminate this reconstruction of the polar discontinuity at the interfaces is required. We show that the use of compositional interface engineering can prevent the reconstruction at the La0.67Sr0.33MnO3/SrTiO3 (LSMO/STO) interface. The polar discontinuity at this interface can be removed by the insertion of a single La0.33Sr0.67O layer, resulting in improved interface magnetization and elec. cond.
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    Peng, R.; Xu, H. C.; Xia, M.; Zhao, J. F.; Xie, X.; Xu, D. F.; Xie, B. P.; Feng, D. L. Tuning the Dead-Layer Behavior of La0.67Sr0.33MnO3/SrTiO3 via Interfacial Engineering. Appl. Phys. Lett. 2014, 104, 081606,  DOI: 10.1063/1.4866461
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    Tuning the dead-layer behavior of La0.67Sr0.33MnO3/SrTiO3 via interfacial engineering
    Peng, R.; Xu, H. C.; Xia, M.; Zhao, J. F.; Xie, X.; Xu, D. F.; Xie, B. P.; Feng, D. L.
    Applied Physics Letters (2014), 104 (8), 081606/1-081606/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    The dead-layer behavior, deterioration of the bulk properties in near-interface layers, restricts the applications of many oxide heterostructures. The authors present the systematic study of the dead-layer in La0.67Sr0.33MnO3/SrTiO3 grown by ozone-assisted MBE. Dead-layer behavior is systematically tuned by varying the interfacial doping, while unchanged with varied doping at any other at. layers. In situ photoemission and LEED measurements suggest intrinsic oxygen vacancies at the surface of ultra-thin La0.67Sr0.33MnO3, which are more concd. in thinner films. The authors' results show correlation between interfacial doping, oxygen vacancies, and the dead-layer, which can be explained by a simplified electrostatic model. (c) 2014 American Institute of Physics.
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    Huijben, M.; Liu, Y.; Boschker, H.; Lauter, V.; Egoavil, R.; Verbeeck, J.; te Velthuis, S. G. E.; Rijnders, G.; Koster, G. Enhanced Local Magnetization by Interface Engineering in Perovskite-Type Correlated Oxide Heterostructures. Adv. Mater. Interfaces 2015, 2, 1400416,  DOI: 10.1002/admi.201400416
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    Enhanced Local Magnetization by Interface Engineering in Perovskite-Type Correlated Oxide Heterostructures
    Huijben, Mark; Liu, Yaohua; Boschker, Hans; Lauter, Valeria; Egoavil, Ricardo; Verbeeck, Jo; te Velthuis, Suzanne G. E.; Rijnders, Guus; Koster, Gertjan
    Advanced Materials Interfaces (2015), 2 (3), 1400416/1-1400416/7CODEN: AMIDD2; ISSN:2196-7350. (Wiley-VCH Verlag GmbH & Co. KGaA)
    This article describes enhanced local magnetization by interface engineering in perovskite-type correlated oxide heterostructures. Interfacial phenomena, the capability of controlled growth of interfacial at. stacking, has to be combined with local probing of the interfacial properties to enable true interface engineering.
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    Glazer, A. M. The Classification of Tilted Octahedra in Perovskites. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1972, 28, 3384– 3392,  DOI: 10.1107/s0567740872007976
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    Classification of tilted octahedra in perovskites
    Glazer, A. M.
    Acta Crystallographica, Section B: Structural Crystallography and Crystal Chemistry (1972), 28 (Pt. 11), 3384-92CODEN: ACBCAR; ISSN:0567-7408.
    A simple method for describing and classifying octahedral tilting in perovskites is given, and it is shown how the tilts are related to the unit-cell geometries. Several examples from the literature are listed, and predictions about hitherto unknown structures of some materials are made.
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    Kan, D.; Aso, R.; Sato, R.; Haruta, M.; Kurata, H.; Shimakawa, Y. Tuning Magnetic Anisotropy by Interfacially Engineering the Oxygen Coordination Environment in a Transition Metal Oxide. Nat. Mater. 2016, 15, 432– 437,  DOI: 10.1038/nmat4580
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    Tuning magnetic anisotropy by interfacially engineering the oxygen coordination environment in a transition metal oxide
    Kan, Daisuke; Aso, Ryotaro; Sato, Riko; Haruta, Mitsutaka; Kurata, Hiroki; Shimakawa, Yuichi
    Nature Materials (2016), 15 (4), 432-437CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Strong correlations between electrons, spins and lattices-stemming from strong hybridization between transition metal d and oxygen p orbitals-are responsible for the functional properties of transition metal oxides. Artificial oxide heterostructures with chem. abrupt interfaces provide a platform for engineering bonding geometries that lead to emergent phenomena. Here the authors demonstrate the control of the oxygen coordination environment of the perovskite, SrRuO3, by heterostructuring it with Ca0.5Sr0.5TiO3 (0-4 monolayers thick) grown on a GdScO3 substrate. A Ru-O-Ti bond angle of the SrRuO3 /Ca0.5Sr0.5TiO3 interface can be engineered by layer-by-layer control of the Ca0.5Sr0.5TiO3 layer thickness, and the engineered Ru-O-Ti bond angle not only stabilizes a Ru-O-Ru bond angle never seen in bulk SrRuO3, but also tunes the magnetic anisotropy in the entire SrRuO3 layer. Interface engineering of the oxygen coordination environment allows one to control addnl. degrees of freedom in functional oxide heterostructures.
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  26. 26
    Adamo, C.; Ke, X.; Wang, H. Q.; Xin, H. L.; Heeg, T.; Hawley, M. E.; Zander, W.; Schubert, J.; Schiffer, P.; Muller, D. A.; Maritato, L.; Schlom, D. G. Effect of Biaxial Strain on the Electrical and Magnetic Properties of (001) La0.7Sr0.3MnO3 Thin Films. Appl. Phys. Lett. 2009, 95, 112504,  DOI: 10.1063/1.3213346
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    Effect of biaxial strain on the electrical and magnetic properties of (001) La0.75Sr0.3MnO3 thin films
    Adamo, C.; Ke, X.; Wang, H. Q.; Xin, H. L.; Heeg, T.; Hawley, M. E.; Zander, W.; Schubert, J.; Schiffer, P.; Muller, D. A.; Maritato, L.; Schlom, D. G.
    Applied Physics Letters (2009), 95 (11), 112504/1-112504/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    We have studied the effect of biaxial strain on thin films of (001) La0.7Sr0.3MnO3. We deposited films by reactive mol.-beam epitaxy on different single cryst. substrates, varying the substrate-induced biaxial strain from -2.3% to +3.2%. Magnetization and elec. transport measurements reveal that the dependence of the Curie temp. on biaxial strain is in very good agreement with the theor. predictions of A. Millis et al.(1998). (c) 2009 American Institute of Physics.
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    Boschker, H.; Mathews, M.; Houwman, E. P.; Nishikawa, H.; Vailionis, A.; Koster, G.; Rijnders, G.; Blank, D. H. A. Strong Uniaxial In-Plane Magnetic Anisotropy of (001)- and (011)-Oriented La0.67Sr0.33MnO3 Thin Films on NdGaO3 Substrates. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 214425,  DOI: 10.1103/physrevb.79.214425
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    27
    Strong uniaxial in-plane magnetic anisotropy of (001)- and (011)-oriented La0.67Sr0.33MnO3 thin films on NdGaO3 substrates
    Boschker, H.; Mathews, M.; Houwman, E. P.; Nishikawa, H.; Vailionis, A.; Koster, G.; Rijnders, G.; Blank, D. H. A.
    Physical Review B: Condensed Matter and Materials Physics (2009), 79 (21), 214425/1-214425/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    Epitaxial La0.67Sr0.33MnO3 (LSMO) ferromagnetic thin films were coherently grown on NdGaO3 (NGO) substrates with different crystal orientations of the surface plane. On the (110)o- and (001)o-oriented substrates, the film grows in the (001)pc orientation, and on the (100)o-, (010)o-, and (112)o-oriented substrates the film is (011)pc oriented (we will use subindices o and pc for the orthorhombic and pseudocubic crystal structures, resp.). The lattice parameters and pseudocube angles of the deformed LSMO pseudocube have been detd. from x-ray diffraction measurements. The in-plane magnetic easy and hard directions of these films have been detd. from the dependence of the remnant magnetization on the angle of the in-plane applied field. For all substrate orientations there is a strong in-plane uniaxial magnetic anisotropy, detd. by the crystal directions of the substrate surface. The easy and hard magnetic-anisotropy directions are explained consistently by the (bulk) inverse magnetostriction model, except for the film on NGO (112)o.
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    Cao, L.; Petracic, O.; Zakalek, P.; Weber, A.; Rücker, U.; Schubert, J.; Koutsioubas, A.; Mattauch, S.; Brückel, T. Reversible Control of Physical Properties via an Oxygen-Vacancy-Driven Topotactic Transition in Epitaxial La0.7Sr0.3MnO3– δ Thin Films. Adv. Mater. 2019, 31, 1806183,  DOI: 10.1002/adma.201806183
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    Tan, H.; Verbeeck, J.; Abakumov, A.; Van Tendeloo, G. Oxidation State and Chemical Shift Investigation in Transition Metal Oxides by EELS. Ultramicroscopy 2012, 116, 24– 33,  DOI: 10.1016/j.ultramic.2012.03.002
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    Oxidation state and chemical shift investigation in transition metal oxides by EELS
    Tan, Haiyan; Verbeeck, Jo; Abakumov, Artem; Van Tendeloo, Gustaaf
    Ultramicroscopy (2012), 116 (), 24-33CODEN: ULTRD6; ISSN:0304-3991. (Elsevier B.V.)
    Transition metal L2,3 electron energy-loss spectra for a wide range of V-, Mn- and Fe-based oxides were recorded and carefully analyzed for their correlation with the formal oxidn. states of the transition metal ions. Special attention is paid to obtain an accurate energy scale which provides abs. energy positions for all core-loss edges. The white-line ratio method, chem. shift method, ELNES fitting method, two-parameter method and other methods are compared and their validity is discussed. Both the ELNES fitting method and the chem. shift method have the advantage of a wide application range and good consistency but require special attention to accurately measure the core-loss edge position. The obtained conclusions are of fundamental importance, e.g., for obtaining at. resoln. oxidn. state information in modern expts.
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    Varela, M.; Oxley, M. P.; Luo, W.; Tao, J.; Watanabe, M.; Lupini, A. R.; Pantelides, S. T.; Pennycook, S. J. Atomic-Resolution Imaging of Oxidation States in Manganites. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 085117,  DOI: 10.1103/physrevb.79.085117
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    Atomic-resolution imaging of oxidation states in manganites
    Varela, M.; Oxley, M. P.; Luo, W.; Tao, J.; Watanabe, M.; Lupini, A. R.; Pantelides, S. T.; Pennycook, S. J.
    Physical Review B: Condensed Matter and Materials Physics (2009), 79 (8), 085117/1-085117/14CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    Aberration cor. electron optics allows routine acquisition of high spatial resoln. spectroscopic images in the scanning transmission electron microscope, which is important when trying to understand the physics of transition-metal oxides such as manganites. The phys. properties of these perovskites are intimately related to the occupancies of the partially filled 3d bands, which define their oxidn. state. The authors review procedures to obtain this electronic property in LaxCa1-xMnO3 from at.-column-resolved electron energy-loss spectra measured in the aberration cor. scanning transmission electron microscope. In bulk samples, several features of both the av. Mn L2,3 edge and the O K edge fine structure change linearly with Mn nominal valence. These linear correlations are extd. and used as a calibration to quantify oxidn. states from at. resoln. spectroscopic images. In such images, the same fine-structure features exhibit further changes, commensurate with the underlying at. lattice. Mn valence values calcd. from those images show unexpected oscillations. The combination of expt. with d.-functional theory and dynamical scattering simulations allows detailed interpretation of these maps, distinguishing dynamical scattering effects from actual changes in electronic properties related to the local at. structure. Specifically, in LaMnO3, the two nonequivalent O sites can be distinguished by these methods.
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    Li, Z.; Bosman, M.; Yang, Z.; Ren, P.; Wang, L.; Cao, L.; Yu, X.; Ke, C.; Breese, M. B. H.; Rusydi, A.; Zhu, W.; Dong, Z.; Foo, Y. L. Interface and Surface Cation Stoichiometry Modified by Oxygen Vacancies in Epitaxial Manganite Films. Adv. Funct. Mater. 2012, 22, 4312– 4321,  DOI: 10.1002/adfm.201200143
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    Interface and Surface Cation Stoichiometry Modified by Oxygen Vacancies in Epitaxial Manganite Films
    Li, Zhipeng; Bosman, Michel; Yang, Zhen; Ren, Peng; Wang, Lan; Cao, Liang; Yu, Xiaojiang; Ke, Chang; Breese, Mark B. H.; Rusydi, Andrivo; Zhu, Weiguang; Dong, Zhili; Foo, Yong Lim
    Advanced Functional Materials (2012), 22 (20), 4312-4321CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Perovskite manganites are viewed as one of the key building blocks of oxide spintronics devices due to their attractive phys. properties. However, cation off-stoichiometry at epitaxial interfaces between manganites and other materials can lead to interfacial dead layers, severely reducing the device performance. Here, TEM and synchrotron-based spectroscopy were used to demonstrate that oxygen vacancies during growth serve as a crit. factor for modifying the cation stoichiometry in pulsed laser deposited La0.8Sr0.2MnO3 films. Near the film/substrate (SrTiO3) interface, A-site cations (La/Sr) are in excess when oxygen vacancies are induced during film growth, partially substituting Mn. Simultaneously, Sr cations migrate towards the film surface and form a SrO rock-salt monolayer. Consequentially, a gradient of the Mn nominal valence is obsd. along the film growth direction, leading to anomalous magnetic properties. The results narrow the selection range of useful oxygen pressures during deposition and demonstrate that accurate cation stoichiometry can only be achieved after oxygen vacancies are eliminated during growth. Probably the oxygen pressure serves as a tuning parameter for the interfacial dead layers and, hence, for control over device properties.
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    Kurata, H.; Colliex, C. Electron-Energy-Loss Core-Edge Structures in Manganese Oxides. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 48, 2102– 2108,  DOI: 10.1103/physrevb.48.2102
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    Electron-energy-loss core-edge structures in manganese oxides
    Kurata, Hiroki; Colliex, Christian
    Physical Review B: Condensed Matter and Materials Physics (1993), 48 (4), 2102-8CODEN: PRBMDO; ISSN:0163-1829.
    Oxygen K and manganese L2,3 edges electron-energy-loss spectra have been measured for a series of manganese oxides with different nominal oxidn. states for the cation (MnO, Mn3O4, Mn2O3, MnO2, and BaMnO4). Spectra have been processed for quant. elemental anal. and for the evaluation of manganese L2,3 white-line intensity ratios and normalized total intensities. Prepeaks on the oxygen K edge are sensitive to the nature of local chem. bonds and their detailed substructures are interpreted with the support of a mol.-orbital picture. As for the cation point of view, the anal. of the white-line intensities provides a satisfactory measurement of the effective 3d orbital occupancy as well as a hint of the local magnetic properties. The present study also confirms that the hybridization between oxygen 2p and manganese 3d orbitals plays an important role when considering the electronic structures of these oxides.
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    Wang, Z. L.; Yin, J. S.; Jiang, Y. D.; Zhang, J. Studies of Mn Valence Conversion and Oxygen Vacancies in La1- XCaxMnO3-y Using Electron Energy-Loss Spectroscopy. Appl. Phys. Lett. 1997, 70, 3362– 3364,  DOI: 10.1063/1.119171
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    Studies of Mn valence conversion and oxygen vacancies in La1-xCaxMnO3-y using electron energy-loss spectroscopy
    Wang, Z. L.; Yin, J. S.; Jiang, Y. D.; Zhang, Jiming
    Applied Physics Letters (1997), 70 (25), 3362-3364CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Using the white line intensities, EELS in a transmission electron microscope was employed to characterize the valence conversion and O vacancies in La1-xCaxMnO3-y. For a nominal doping compn. x = 0.33, the ratio of Mn4+ to Mn3+ is detd. to be >0.25 but <0.5, and the content of O vacancy y is ≤0.065 (equiv. to 2.2 at.% of the O content). At ymax = 0.065, 60% of the residual charge introduced by Ca doping is balanced by the conversion of Mn3+ to Mn4+ and 40% by O vacancy.
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    Krivanek, O. L.; Disko, M. M.; Taftø, J.; Spence, J. C. H. Electron Energy Loss Spectroscopy as a Probe of the Local Atomic Environment. Ultramicroscopy 1982, 9, 249– 254,  DOI: 10.1016/0304-3991(82)90208-x
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    Electron energy loss spectroscopy as a probe of the local atomic environment
    Krivanek, O. L.; Disko, M. M.; Taftoe, J.; Spence, J. C. H.
    Ultramicroscopy (1982), 9 (3), 249-54CODEN: ULTRD6; ISSN:0304-3991.
    A review with 15 refs. on structure detns. by electron energy-loss spectroscopy.
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    Jin, L.; Jia, C.-L.; Lindfors-Vrejoiu, I.; Zhong, X.; Du, H.; Dunin-Borkowski, R. E. Direct Demonstration of a Magnetic Dead Layer Resulting from A-Site Cation Inhomogeneity in a (La,Sr)MnO3 Epitaxial Film System. Adv. Mater. Interfaces 2016, 3, 1600414,  DOI: 10.1002/admi.201600414
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    Hÿtch, M. J.; Snoeck, E.; Kilaas, R. Quantitative Measurement of Displacement and Strain Fields from HREM Micrographs. Ultramicroscopy 1998, 74, 131– 146,  DOI: 10.1016/s0304-3991(98)00035-7
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    Quantitative measurement of displacement and strain fields from HREM micrographs
    Hytch, M. J.; Snoeck, E.; Kilaas, R.
    Ultramicroscopy (1998), 74 (3), 131-146CODEN: ULTRD6; ISSN:0304-3991. (Elsevier Science B.V.)
    A method for measuring and mapping displacement fields and strain fields from high-resoln. electron microscope (HREM) images has been developed. The method is based upon centering a small aperture around a strong reflection in the Fourier transform of an HREM lattice image and performing an inverse Fourier transform. The phase component of the resulting complex image is shown to give information about local displacements of at. planes and the two-dimensional displacement field can be derived by applying the method to two non-colinear Fourier components. Local strain components can be found by analyzing the deriv. of the displacement field. The details of the technique are outlined and applied to an exptl. HREM image of a domain wall in ferroelec.-ferroelastic PbTiO3.
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    Chaluvadi, S. K.; Ajejas, F.; Orgiani, P.; Lebargy, S.; Minj, A.; Flament, S.; Camarero, J.; Perna, P.; Méchin, L. Epitaxial Strain and Thickness Dependent Structural, Electrical and Magnetic Properties of La0.67Sr0.33MnO3 Films. J. Phys. D: Appl. Phys. 2020, 53, 375005,  DOI: 10.1088/1361-6463/ab8e7b
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    Epitaxial strain and thickness dependent structural, electrical and magnetic properties of La0.67Sr0.33MnO3 films
    Chaluvadi, Sandeep Kumar; Ajejas, Fernando; Orgiani, Pasquale; Lebargy, Sylvain; Minj, Albert; Flament, Stephane; Camarero, Julio; Perna, Paolo; Mechin, Laurence
    Journal of Physics D: Applied Physics (2020), 53 (37), 375005CODEN: JPAPBE; ISSN:0022-3727. (IOP Publishing Ltd.)
    The crystal structural quality and the strain induced by the substrate strictly impose the magnetic and transport properties of La0.67Sr0.33MnO3 (LSMO) films. In particular, the magnetic anisotropy (MA) of epitaxial LSMO can be finely tuned by varying its thickness and by choosing single crystal substrates with a suitable lattice mismatch with the film. Here, we have deposited LSMO films with thicknesses in the 12-50 nm range by pulsed laser deposition on different single crystal substrates inducing either compressive or tensile in-plane strain on the manganites. The epitaxial quality of films was quantified by ω-scans around a (002) peak with full-width half-max. values as low as 0.08#x00B0; for films on the nearly matched NGO (110) substrate to 1.4° films on the high mismatched MgO (001) substrate. As the epitaxial strain in a thin-film increases, a significant redn. in the metal-insulation transition temp. (Tp) was obsd. The magnetic properties of the films probed by Kerr magnetometry show that the symmetry of the room temp. MA varies significantly as a function of both strain and thickness. Specifically, we obsd. pure uniaxial MA on NGO (110) and pure biaxial MA on STO buffered MgO (001), whereas a spin reorientation from uniaxial in-plane to out-of-plane on LSAT (001) and uniaxial to nearly isotropic in-plane on STO (001) substrate as the film thickness is increased. We provide an efficient tool to tune the MA according to the specific spintronic application targeted.
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    Egerton, R. F. Electron Energy-Loss Spectroscopy in the Electron Microscope, 2nd ed.; Plenum Press: New York, 1996.
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    De Backer, A.; van den Bos, K. H. W.; Van den Broek, W.; Sijbers, J.; Van Aert, S. StatSTEM: An Efficient Approach for Accurate and Precise Model-Based Quantification of Atomic Resolution Electron Microscopy Images. Ultramicroscopy 2016, 171, 104– 116,  DOI: 10.1016/j.ultramic.2016.08.018
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    StatSTEM: An efficient approach for accurate and precise model-based quantification of atomic resolution electron microscopy images
    De Backer, A.; van den Bos, K. H. W.; Van den Broek, W.; Sijbers, J.; Van Aert, S.
    Ultramicroscopy (2016), 171 (), 104-116CODEN: ULTRD6; ISSN:0304-3991. (Elsevier B.V.)
    An efficient model-based estn. algorithm is introduced to quantify the at. column positions and intensities from at. resoln. (scanning) transmission electron microscopy ((S)TEM) images. This algorithm uses the least squares estimator on image segments contg. individual columns fully accounting for overlap between neighboring columns, enabling the anal. of a large field of view. For this algorithm, the accuracy and precision with which measurements for the at. column positions and scattering cross-sections from annular dark field (ADF) STEM images can be estd., has been investigated. The highest attainable precision is reached even for low dose images. Furthermore, the advantages of the model-based approach taking into account overlap between neighboring columns are highlighted. This is done for the estn. of the distance between two neighboring columns as a function of their distance and for the estn. of the scattering cross-section which is compared to the integrated intensity from a Voronoi cell. To provide end-users this well-established quantification method, a user friendly program, StatSTEM, is developed which is freely available under a GNU public license.
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    Koch, C. T. Determination of Core Structure Periodicity and Point Defect Density along Dislocations; Arizona State University, 2002.
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    Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; Dal Corso, A.; De Gironcoli, S.; Fabris, S.; Fratesi, G.; Gebauer, R.; Gerstmann, U.; Gougoussis, C.; Kokalj, A.; Lazzeri, M.; Martin-Samos, L.; Marzari, N.; Mauri, F.; Mazzarello, R.; Paolini, S.; Pasquarello, A.; Paulatto, L.; Sbraccia, C.; Scandolo, S.; Sclauzero, G.; Seitsonen, A. P.; Smogunov, A.; Umari, P.; Wentzcovitch, R. M. QUANTUM ESPRESSO: A Modular and Open-Source Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter 2009, 21, 395502,  DOI: 10.1088/0953-8984/21/39/395502
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    QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials
    Giannozzi Paolo; Baroni Stefano; Bonini Nicola; Calandra Matteo; Car Roberto; Cavazzoni Carlo; Ceresoli Davide; Chiarotti Guido L; Cococcioni Matteo; Dabo Ismaila; Dal Corso Andrea; de Gironcoli Stefano; Fabris Stefano; Fratesi Guido; Gebauer Ralph; Gerstmann Uwe; Gougoussis Christos; Kokalj Anton; Lazzeri Michele; Martin-Samos Layla; Marzari Nicola; Mauri Francesco; Mazzarello Riccardo; Paolini Stefano; Pasquarello Alfredo; Paulatto Lorenzo; Sbraccia Carlo; Scandolo Sandro; Sclauzero Gabriele; Seitsonen Ari P; Smogunov Alexander; Umari Paolo; Wentzcovitch Renata M
    Journal of physics. Condensed matter : an Institute of Physics journal (2009), 21 (39), 395502 ISSN:.
    QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.
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    Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865– 3868,  DOI: 10.1103/physrevlett.77.3865
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    Generalized gradient approximation made simple
    Perdew, John P.; Burke, Kieron; Ernzerhof, Matthias
    Physical Review Letters (1996), 77 (18), 3865-3868CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Generalized gradient approxns. (GGA's) for the exchange-correlation energy improve upon the local spin d. (LSD) description of atoms, mols., and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental consts. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential.
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    Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758– 1775,  DOI: 10.1103/physrevb.59.1758
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    From ultrasoft pseudopotentials to the projector augmented-wave method
    Kresse, G.; Joubert, D.
    Physical Review B: Condensed Matter and Materials Physics (1999), 59 (3), 1758-1775CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blochl's projector augmented wave (PAW) method is derived. The total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addn., crit. tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed-core all-electron methods. These tests include small mols. (H2, H2O, Li2, N2, F2, BF3, SiF4) and several bulk systems (diamond, Si, V, Li, Ca, CaF2, Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
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    Guzmán, R.; Maurel, L.; Langenberg, E.; Lupini, A. R.; Algarabel, P. A.; Pardo, J. A.; Magén, C. Polar-Graded Multiferroic SrMnO3 Thin Films. Nano Lett. 2016, 16, 2221– 2227,  DOI: 10.1021/acs.nanolett.5b04455
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    Polar-Graded Multiferroic SrMnO3 Thin Films
    Guzman, Roger; Maurel, Laura; Langenberg, Eric; Lupini, Andrew R.; Algarabel, Pedro A.; Pardo, Jose A.; Magen, Cesar
    Nano Letters (2016), 16 (4), 2221-2227CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Engineering defects and strains in oxides provides a promising route for the quest of thin film materials with coexisting ferroic orders, multiferroics, with efficient magnetoelec. coupling at room temp. Precise control of the strain gradient would enable custom tailoring of the multiferroic properties but presently remains challenging. The existence of a polar-graded state in epitaxially strained antiferromagnetic SrMnO3 thin films is reported, whose polar nature was predicted theor. and recently demonstrated exptl. By means of aberration-cor. scanning transmission electron microscopy we map the polar rotation of the ferroelec. polarization with at. resoln., both far from and near the domain walls, and find flexoelectricity resulting from vertical strain gradients. The origin of this particular strain state is a gradual distribution of oxygen vacancies across the film thickness, according to electron energy loss spectroscopy. Herein we present a chem.-mediated route to induce polar rotations in oxygen-deficient multiferroic films, resulting in flexoelec. polar rotations and with potentially enhanced piezoelectricity.
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    Radaelli, P. G.; Iannone, G.; Marezio, M.; Hwang, H. Y.; Cheong, S.-W.; Jorgensen, J. D.; Argyriou, D. N. Structural Effects on the Magnetic and Transport Properties of Perovskite A1-XA’xMnO3 (X=0.25, 0.30). Phys. Rev. B: Condens. Matter Mater. Phys. 1997, 56, 8265– 8276,  DOI: 10.1103/physrevb.56.8265
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    Vailionis, A.; Boschker, H.; Siemons, W.; Houwman, E. P.; Blank, D. H. A.; Rijnders, G.; Koster, G. Misfit Strain Accommodation in Epitaxial ABO3 Perovskites: Lattice Rotations and Lattice Modulations. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 1– 10,  DOI: 10.1103/physrevb.83.064101
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    Vailionis, A.; Boschker, H.; Liao, Z.; Smit, J. R. A.; Rijnders, G.; Huijben, M.; Koster, G. Symmetry and Lattice Mismatch Induced Strain Accommodation near and Away from Correlated Perovskite Interfaces. Appl. Phys. Lett. 2014, 105, 131906,  DOI: 10.1063/1.4896969
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    Symmetry and lattice mismatch induced strain accommodation near and away from correlated perovskite interfaces
    Vailionis, A.; Boschker, H.; Liao, Z.; Smit, J. R. A.; Rijnders, G.; Huijben, M.; Koster, G.
    Applied Physics Letters (2014), 105 (13), 131906/1-131906/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Distinct MnO6 octahedral distortions near and away from the La0.67Sr0.33MnO3/SrTiO3(001) (LSMO/STO) interface are quantified using synchrotron x-ray diffraction and dynamical x-ray diffraction simulations. 3 Structural regions of stress accommodation throughout the film thickness were resolved: near the LSMO/STO interface, intermediate region farther from the interface, and the main layer away from the interface. Within the first 2 unit cells stress is accommodated by the suppression of octahedral rotations in the film, leading to the expansion of the c-axis lattice parameter. Farther from the interface film structure acquires octahedral tilts similar to thicker perovskite films under tensile stress, leading to a reduced c-axis parameter. These regions are related to 2 different strain coupling mechanisms: symmetry mismatch at the interface and lattice mismatch in the rest of the film. The findings suggest new routes for strain engineering in correlated perovskite heterostructures. (c) 2014 American Institute of Physics.
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    Kirkland, E. J.; Loane, R. F.; Silcox, J. Simulation of Annular Dark Field STEM Images Using a Modified Multislice Method. Ultramicroscopy 1987, 23, 77– 96,  DOI: 10.1016/0304-3991(87)90229-4
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    Muller, D. A.; Nakagawa, N.; Ohtomo, A.; Grazul, J. L.; Hwang, H. Y. Atomic-Scale Imaging of Nanoengineered Oxygen Vacancy Profiles in SrTiO3. Nature 2004, 430, 657– 661,  DOI: 10.1038/nature02756
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    Atomic-scale imaging of nanoengineered oxygen vacancy profiles in SrTiO3
    Muller, David A.; Nakagawa, Naoyuki; Ohtomo, Akira; Grazul, John L.; Hwang, Harold Y.
    Nature (London, United Kingdom) (2004), 430 (7000), 657-661CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    At the heart of modern oxide chem. lies the recognition that beneficial (as well as deleterious) materials properties can be obtained by deliberate deviations of oxygen atom occupancy from the ideal stoichiometry. Conversely, the capability to control and confine oxygen vacancies will be important to realize the full potential of perovskite ferroelec. materials, varistors and field-effect devices. In transition metal oxides, oxygen vacancies are generally electron donors, and in strontium titanate (SrTiO3) thin films, oxygen vacancies (unlike impurity dopants) are particularly important because they tend to retain high carrier mobilities, even at high carrier densities. Here the authors report the successful fabrication, using a pulsed laser deposition technique, of SrTiO3 superlattice films with oxygen doping profiles that exhibit subnanometre abruptness. The authors profile the vacancy concns. on an at. scale using annular-dark-field electron microscopy and core-level spectroscopy, and demonstrate abs. detection sensitivities of one to four oxygen vacancies. The authors' findings open a pathway to the microscopic study of individual vacancies and their clustering, not only in oxides, but in cryst. materials more generally.
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    Johnston-Peck, A. C.; Winterstein, J. P.; Roberts, A. D.; DuChene, J. S.; Qian, K.; Sweeny, B. C.; Wei, W. D.; Sharma, R.; Stach, E. A.; Herzing, A. A. Oxidation-State Sensitive Imaging of Cerium Dioxide by Atomic-Resolution Low-Angle Annular Dark Field Scanning Transmission Electron Microscopy. Ultramicroscopy 2016, 162, 52– 60,  DOI: 10.1016/j.ultramic.2015.12.004
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    Oxidation-state sensitive imaging of cerium dioxide by atomic-resolution low-angle annular dark field scanning transmission electron microscopy
    Johnston-Peck, Aaron C.; Winterstein, Jonathan P.; Roberts, Alan D.; DuChene, Joseph S.; Qian, Kun; Sweeny, Brendan C.; Wei, Wei David; Sharma, Renu; Stach, Eric A.; Herzing, Andrew A.
    Ultramicroscopy (2016), 162 (), 52-60CODEN: ULTRD6; ISSN:0304-3991. (Elsevier B.V.)
    Low-angle annular dark field (LAADF) scanning transmission electron microscopy (STEM) imaging is presented as a method that is sensitive to the oxidn. state of cerium ions in CeO2 nanoparticles. This relationship was validated through electron energy loss spectroscopy (EELS), in situ measurements, as well as multislice image simulations. Static displacements caused by the increased ionic radius of Ce3+ influence the electron channeling process and increase electron scattering to low angles while reducing scatter to high angles. This process manifests itself by reducing the high-angle annular dark field (HAADF) signal intensity while increasing the LAADF signal intensity in close proximity to Ce3+ ions. This technique can supplement STEM-EELS and in so doing, relax the exptl. challenges assocd. with acquiring oxidn. state information at high spatial resolns.
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    Angeloni, M.; Balestrino, G.; Boggio, N. G.; Medaglia, P. G.; Orgiani, P.; Tebano, A. Suppression of the Metal-Insulator Transition Temperature in Thin La0.7Sr0.3MnO3 Films. J. Appl. Phys. 2004, 96, 6387– 6392,  DOI: 10.1063/1.1812599
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    Journal of Applied Physics (2004), 96 (11), 6387-6392CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    In this paper, we illustrate an approach to discriminate between epitaxial strain and other factors responsible for the decrease of the metal-insulator transition temp. (TP) in thin La0.7Sr0.3MnO3 films grown by pulsed laser deposition. Using this approach, we have estd. the effect of the biaxial strain on TP. Ultrathin films, independent of epitaxial strain, do not show any metal-insulator transition over the full temp. range. This finding confirms the existence of an interface dead layer. The strain-independent decrease in TP, relative to its bulk value, obsd. for a much wider thickness range (up to about 1000 A) can most likely be attributed to oxygen deficiency.
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    Hillyard, S.; Silcox, J. Detector Geometry, Thermal Diffuse Scattering and Strain Effects in ADF STEM Imaging. Ultramicroscopy 1995, 58, 6– 17,  DOI: 10.1016/0304-3991(94)00173-k
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    Detector geometry, thermal diffuse scattering and strain effects in ADF STEM imaging
    Hillyard, Sean; Silcox, John
    Ultramicroscopy (1995), 58 (1), 6-17CODEN: ULTRD6; ISSN:0304-3991. (Elsevier)
    Intensities of at.-resoln. Annular Dark Field Scanning TEM (ADF STEM) images of zone-axis-oriented specimens change with defocus at rates that depend on lattice spacing. Thickness and strain effects on the intensities were demonstrated. Image simulations (with some exptl. basis) are presented that consider the dimensions of the ADF detector. Changing the inner radius of the detector seems to have relatively small effect on the image except to lower the detected intensity. Probe size was explored and a case identified where multiple scattering was important in the image. Thermal diffuse scattering (TDS) is important in high-angle scattering at room temps. but it does not seem to alter the image appearance markedly. Finally, the image arising from the strain field around a single B atom was simulated and the results suggest increased scattering in agreement with observations. This mechanism may be adequate for single impurity atom detection at low temps. and with special detector angles.
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    Perovic, D. D.; Rossouw, C. J.; Howie, A. Imaging Elastic Strains in High-Angle Annular Dark Field Scanning Transmission Electron Microscopy. Ultramicroscopy 1993, 52, 353– 359,  DOI: 10.1016/0304-3991(93)90046-z
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    Imaging elastic strains in high-angle annular dark field scanning transmission electron microscopy
    Perovic, D. D.; Rossouw, C. J.; Howie, A.
    Ultramicroscopy (1993), 52 (3-4), 353-9CODEN: ULTRD6; ISSN:0304-3991.
    High-angle annular dark field (HAADF) imaging in a dedicated scanning transmission electron microscope (STEM) was applied to the study of imperfect crystals. Firstly, a study of B-doped layers in Si revealed significantly stronger contrast and of opposite sign relative to simple at. no. contrast (Z-contrast) predictions. Misfitting substitutional B atoms act as point defect sites in a Si matrix which enhance scattering to high angles via a static Debye-Waller effect. Multi-beam Bloch-wave theory was used to quant. predict exptl. contrast levels. Secondly, HAADF-STEM imaging of inclined dislocation segments revealed a no. of novel contrast effects which depend on the specific position of the dislocation in the foil. Unlike conventional diffraction contrast from dislocations, HAADF dislocation contrast is neither similar nor complementary at the entrant and exit surfaces of the specimen. A qual. Bloch-wave scattering description was developed consistently to describe the dislocation contrast features.
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  • Abstract

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    Figure 1

    Figure 1. Cross-sectional HRTEM image and corresponding strain analysis: (a) HRTEM image of the [001]-oriented LSMO thin film grown on the LSAT substrate, (b) SAED pattern from the imaged region, (c) experimental HRTEM image used for strain analysis, and (d,e) symmetric strain components: in-plane (εxx) and out-of-plane (εzz) respectively. A color bar for the strain map is shown. (f) Line profiles of the strain components, averaged over the width of the strain maps in the [100] direction of the film. All strain values indicated in this figure are in percentage.

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    Figure 2

    Figure 2. Experimental high-resolution STEM images. (a) STEM–HAADF image showing the cross-sectional geometry of the LSMO/LSAT heterostructure along the [010] zone axis. The inset in the blue box shows that the interface is atomically flat and defect free. The inset in the red box shows a close-up image of the LSMO thin film. The scale bar of the insets corresponds to 1 nm. The upper right inset shows the projection of the LSMO unit cell along the [010] direction where green, purple, and red-colored circles represent La/Sr, Mn, and O atomic column positions, respectively. (b) STEM–ADF image showing the contrast variation at around 7 nm distance from the interface.

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    Figure 3

    Figure 3. EELS analysis of the Mn oxidation state in the film region. (a) Fine structures of O–K and Mn-L edges. Three prominent peaks for O–K edges are labeled “A”, “B”, and “C”. Peaks “A” and “C” correspond to O 2p–Mn 3d and 4sp hybrid states, respectively, while peak “B” is characteristic of O 2p Sr 4d or La 5d hybrid states. The energy difference ΔE between peaks “A” and “B” was measured to determine the Mn valency. (b) STEM–HAADF image of the LSMO/LSAT structure and the line shows the area from where EELS line profiles have been collected. The numbers indicate the area across which the average spectra were collected. (c) Estimated Mn valence in the LSMO film as a function of distance from the interface with the substrate.

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    Figure 4

    Figure 4. Quantitative analysis of the Mn-ion (B-site) displacements over the LSMO film. (a) Boxes marked in a different color on the STEM–HAADF image represent the regions used to map the displacement of Mn ions (B-site). (b–d) Map of atomic displacement vectors showing the displacement of Mn atoms (arrows) from the center of the projected La atoms superposed on the STEM–HAADF image [010] zone axis. (e–g) Out-of-plane (Δz) component and (h–j) in-plane (Δx) component of Mn displacements calculated from the corresponding regions. The scale bar on the images represents 1 nm. (k) Plot shows the variation in the magnitude of Δz and Δx from three different regions. (l) Projection of the LSMO unit cell along the [010] direction, where Δx and Δz denote the shift of Mn-ion (B-site) (blue) atoms along the in-plane and out-of-plane directions, respectively, concerning the centrosymmetric position (open circle).

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    Figure 5

    Figure 5. Atomistic simulation results. (a–d) Represent the 3D view of the energetically relaxed structures by DFT calculation for structures SI, SII, SIII, and SIV, respectively. The incomplete polyhedral shows the position of oxygen vacancies at the other end of the polyhedral (e–h) multislice simulated STEM–ADF images of the [010]-orientated LSMO structure with 15 nm thickness using the corresponding structures. The scale bar is the same for all the simulated images. (i) Intensity profile calculated from each simulated STEM–ADF image.

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      Origin of the metal-insulator transition in ultrathin films of La2/3Sr1/3MnO3
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      Many ultrathin films of transition metal oxides exhibit nonmetallic behavior, in contrast to their metallic bulk counterpart, thus displaying a metal-insulator transition (MIT) as the film thickness is reduced. The nature of this MIT has been a long-standing issue in the epitaxial oxide research community. Here, we report the processing dependence of the crit. thickness (tc) of MIT and the origin of the insulating phase in La2/3Sr1/3MnO3 (LSMO) films. A tc of 3 unit cells (u.c.) is achieved by minimizing oxygen vacancies under optimal growth conditions, diminishing the epitaxial strain with a tunable buffer layer and suppressing surface strain by film capping. The elec. transport measurements demonstrate that the nonmetallic behavior in LSMO thin films is an unavoidable result of localization initiated by inherent disorder but amplified by the redn. in dimensionality.
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      Kourkoutis, L. F.; Song, J. H.; Hwang, H. Y.; Muller, D. A. Microscopic Origins for Stabilizing Room-Temperature Ferromagnetism in Ultrathin Manganite Layers. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 11682– 11685,  DOI: 10.1073/pnas.1005693107
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      14
      Microscopic origins for stabilizing room-temperature ferromagnetism in ultrathin manganite layers
      Kourkoutis, L. Fitting; Song, J. H.; Hwang, H. Y.; Muller, D. A.
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      La0.7Sr0.3MnO3 is a conducting ferromagnet at room temp. Combined with thin SrTiO3 layers, the resulting heterostructures could be used as highly spin-polarized magnetic-tunnel-junction memories. However, when shrunk to dimensions below an apparent crit. thickness, the structures become insulating and ferromagnetic ordering is suppressed. Interface spin and charge modulations are thought to create an interfacial dead layer, thus fundamentally limiting the use of this material in at.-scale devices. The thickness of this dead layer, and whether it is intrinsic, is still controversial. Here we use at.-resoln. electron spectroscopy to demonstrate that the degrdn. of the magnetic and transport properties of La0.7Sr0.3MnO3/SrTiO3 multilayers correlates with at. intermixing at the interfaces, and the presence of extended two-dimensional cation defects in the La0.7Sr0.3MnO3 layers (in contrast to three-dimensional ppts. in thick films). When these extrinsic defects are eliminated, metallic ferromagnetism at room temp. can be stabilized in five-unit-cell-thick manganite layers in superlattices, placing the upper limit for any intrinsic dead layer at two unit cells per interface.
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    15. 15
      Izumi, M.; Ogimoto, Y.; Manako, T.; Kawasaki, M.; Tokura, Y. Interface Effect and Its Doping Dependence in La1- XSrXMnO3/SrTiO3 Superlattices. J. Phys. Soc. Jpn. 2002, 71, 2621– 2624,  DOI: 10.1143/jpsj.71.2621
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      15
      Interface effect and its doping dependence in La1-xSrxMnO3/SrTiO3 superlattices
      Izumi, Makoto; Ogimoto, Yasushi; Manako, Takashi; Kawasaki, Masashi; Tokura, Yoshinori
      Journal of the Physical Society of Japan (2002), 71 (11), 2621-2624CODEN: JUPSAU; ISSN:0031-9015. (Physical Society of Japan)
      The authors have investigated the interface effect on magnetic and transport properties in the superlattices composed of ferromagnetic La1-xSrxMnO3 (LSMO) and non magnetic SrTiO3 (STO) (x = 0.2, 0.3, and 0.4). The interface of LSMO with STO was proved to show spin-canting, the degree of which critically depends on the doping level x and the layer thickness (tLSMO) of LSMO. Large magnetoresistance subsisting at low temp. appears in the superlattices with specific sets of (x, tLSMO) parameters that shows a metal-insulator crossover behavior with appreciable spin-canting. Implications of the present results in the tunneling magnetoresistance for the LSMO/STO/LSMO junction are also discussed.
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      Tebano, A.; Aruta, C.; Sanna, S.; Medaglia, P. G.; Balestrino, G.; Sidorenko, A. A.; De Renzi, R.; Ghiringhelli, G.; Braicovich, L.; Bisogni, V.; Brookes, N. B. Evidence of Orbital Reconstruction at Interfaces in Ultrathin La0.7Sr0.3MnO3 Films. Phys. Rev. Lett. 2008, 100, 137401,  DOI: 10.1103/physrevlett.100.137401
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      16
      Evidence of Orbital Reconstruction at Interfaces in Ultrathin La0.67Sr0.33MnO3 Films
      Tebano, A.; Aruta, C.; Sanna, S.; Medaglia, P. G.; Balestrino, G.; Sidorenko, A. A.; De Renzi, R.; Ghiringhelli, G.; Braicovich, L.; Bisogni, V.; Brookes, N. B.
      Physical Review Letters (2008), 100 (13), 137401/1-137401/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Linear dichroism (LD) in x-ray absorption, diffraction, transport, and magnetization measurements on thin La0.7Sr0.3MnO3 films grown on different substrates, allow identification of a peculiar interface effect, related just to the presence of the interface. We report the LD signature of preferential 3d-eg(3z2-r2) occupation at the interface, suppressing the double exchange mechanism. This surface orbital reconstruction is opposite to that favored by residual strain and is independent of dipolar fields, the chem. nature of the substrate and the presence of capping layers.
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      Li, X.; Lindfors-Vrejoiu, I.; Ziese, M.; Gloter, A.; van Aken, P. A. Impact of Interfacial Coupling of Oxygen Octahedra on Ferromagnetic Order in La0.7Sr0.3MnO3/SrTiO3 Heterostructures. Sci. Rep. 2017, 7, 40068,  DOI: 10.1038/srep40068
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      Impact of interfacial coupling of oxygen octahedra on ferromagnetic order in La0.7Sr0.3MnO3/SrTiO3 heterostructures
      Li, Xiaoyan; Lindfors-Vrejoiu, Ionela; Ziese, Michael; Gloter, Alexandre; van Aken, Peter A.
      Scientific Reports (2017), 7 (), 40068CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
      La0.7Sr0.3MnO3, a half-metallic ferromagnet with full spin polarization, is generally used as a std. spin injector in heterostructures. However, the magnetism of La0.7Sr0.3MnO3 is strongly modified near interfaces, which was addressed as "dead-layer" phenomenon whose origin is still controversial. Here, both magnetic and structural properties of La0.7Sr0.3MnO3/SrTiO3 heterostructures were investigated, with emphasis on the quant. anal. of oxygen octahedral rotation (OOR) across interfaces using annular-bright-field imaging. OOR was found to be significantly altered near interface for both La0.7Sr0.3MnO3 and SrTiO3, as linked to the magnetism deterioration. Esp. in La0.7Sr0.3MnO3/SrTiO3 superlattices, the almost complete suppression of OOR in 4 unit-cell-thick La0.7Sr0.3MnO3 results in a canted ferromagnetism. Detailed comparisons between strain and OOR relaxation and esp. the observation of an unexpected La0.7Sr0.3MnO3 lattice c expansion near interfaces, prove the relevance of OOR for the magnetic properties. These results indicate the capability of tuning the magnetism by engineering OOR at the at. scale.
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    18. 18
      Liao, Z.; Huijben, M.; Zhong, Z.; Gauquelin, N.; Macke, S.; Green, R. J.; Van Aert, S.; Verbeeck, J.; Van Tendeloo, G.; Held, K.; Sawatzky, G. A.; Koster, G.; Rijnders, G. Controlled Lateral Anisotropy in Correlated Manganite Heterostructures by Interface-Engineered Oxygen Octahedral Coupling. Nat. Mater. 2016, 15, 425– 431,  DOI: 10.1038/nmat4579
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      18
      Controlled lateral anisotropy in correlated manganite heterostructures by interface-engineered oxygen octahedral coupling
      Liao, Z.; Huijben, M.; Zhong, Z.; Gauquelin, N.; Macke, S.; Green, R. J.; Van Aert, S.; Verbeeck, J.; Van Tendeloo, G.; Held, K.; Sawatzky, G. A.; Koster, G.; Rijnders, G.
      Nature Materials (2016), 15 (4), 425-431CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      Controlled in-plane rotation of the magnetic easy axis in manganite heterostructures by tailoring the interface oxygen network could allow the development of correlated oxide-based magnetic tunnelling junctions with noncollinear magnetization, with possible practical applications as miniaturized high-switching-speed magnetic random access memory (MRAM) devices. Here, to manipulate magnetic and electronic anisotropic properties in manganite heterostructures by engineering the oxygen network on the unit-cell level. The strong oxygen octahedral coupling is found to transfer the octahedral rotation, present in the NdGaO3 (NGO) substrate, to the La2/3Sr1/3MnO3 (LSMO) film in the interface region. This causes an unexpected realignment of the magnetic easy axis along the short axis of the LSMO unit cell as well as the presence of a giant anisotropic transport in these ultrathin LSMO films. As a result the authors possess control of the lateral magnetic and electronic anisotropies by at.-scale design of the oxygen octahedral rotation.
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    19. 19
      Ishii, Y.; Yamada, H.; Sato, H.; Akoh, H.; Ogawa, Y.; Kawasaki, M.; Tokura, Y. Improved Tunneling Magnetoresistance in Interface Engineered (La,Sr)MnO3 Junctions. Appl. Phys. Lett. 2006, 89, 042509,  DOI: 10.1063/1.2245442
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      Improved tunneling magnetoresistance in interface engineered (La,Sr)MnO3 junctions
      Ishii, Y.; Yamada, H.; Sato, H.; Akoh, H.; Ogawa, Y.; Kawasaki, M.; Tokura, Y.
      Applied Physics Letters (2006), 89 (4), 042509/1-042509/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Spin tunnel junctions were fabricated and characterized with various insulating barriers and interface structures employing (La,Sr)MnO3 (LSMO) as the ferromagnetic electrodes. Junctions with LaAlO3 barriers exhibited systematically larger tunnel magnetoresistance (TMR) (TMR ratio ∼230% at 10 K) than those with SrTiO3 barriers (∼50%). When two unit cells of undoped LaMnO3 are inserted between LSMO and SrTiO3 at both interfaces in the SrTiO3-barrier junction, the TMR was also significantly enhanced to ∼170%. These results, including the temp. dependence of TMR, qual. agree with the characteristics of the local magnetization at the interface that was evaluated by magnetization-induced second harmonic generation (MSHG) for the corresponding interface structures. However, slight deviations appear as a systematic suppression of TMR for all the junctions at high temps. compared with MSHG results. Also, TMR results appear to be more degraded than MSHG results for the SrTiO3-barrier junctions. The barrier thickness dependence of the TMR response revealed that a spin-independent and inelastic conduction channel is a possible origin for the apparent suppression of TMR compared with MSHG.
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    20. 20
      Matou, T.; Takeshima, K.; Anh, L. D.; Seki, M.; Tabata, H.; Tanaka, M.; Ohya, S. Reduction of the Magnetic Dead Layer and Observation of Tunneling Magnetoresistance in La0.67 Sr0.33MnO3 -Based Heterostructures with a LaMnO3 Layer. Appl. Phys. Lett. 2017, 110, 212406,  DOI: 10.1063/1.4984297
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      20
      Reduction of the magnetic dead layer and observation of tunneling magnetoresistance in La0.67Sr0.33MnO3-based heterostructures with a LaMnO3 layer
      Matou, Tatsuya; Takeshima, Kento; Anh, Le Duc; Seki, Munetoshi; Tabata, Hitoshi; Tanaka, Masaaki; Ohya, Shinobu
      Applied Physics Letters (2017), 110 (21), 212406/1-212406/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      The formation of a magnetic dead layer at the interfaces of the perovskite oxide La0.67Sr0.33MnO3 (LSMO) is one of the crucial issues for its spintronic applications. The authors report the redn. of the dead layer by growing LSMO on a LaMnO3 (LMO) layer. Also, the authors detect tunneling magnetoresistance (TMR) in an LSMO/LMO/LSMO heterostructure. The obtained sign of the TMR was neg., but it changed to pos. after annealing. This unusual neg. TMR can be attributed to the intrinsic structural difference between the upper and lower interfaces of LMO and can be understood by a weak antiferromagnetic metallic thin layer formed at the upper LSMO/LMO interface. This layer probably is formed by diffused Sr atoms and oxygen vacancies in the LMO barrier. The authors' results indicate that control of intermixing of atoms at the interfaces is a key to controlling the TMR. (c) 2017 American Institute of Physics.
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    21. 21
      Boschker, H.; Verbeeck, J.; Egoavil, R.; Bals, S.; Van Tendeloo, G.; Huijben, M.; Houwman, E. P.; Koster, G.; Blank, D. H. A.; Rijnders, G. Preventing the Reconstruction of the Polar Discontinuity at Oxide Heterointerfaces. Adv. Funct. Mater. 2012, 22, 2235– 2240,  DOI: 10.1002/adfm.201102763
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      21
      Preventing the Reconstruction of the Polar Discontinuity at Oxide Heterointerfaces
      Boschker, H.; Verbeeck, J.; Egoavil, R.; Bals, S.; van Tendeloo, G.; Huijben, M.; Houwman, E. P.; Koster, G.; Blank, D. H. A.; Rijnders, G.
      Advanced Functional Materials (2012), 22 (11), 2235-2240CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Perovskite oxide heteroepitaxy receives much attention because of the possibility to combine the diverse functionalities of perovskite oxide building blocks. A general boundary condition for the epitaxy is the presence of polar discontinuities at heterointerfaces. These polar discontinuities result in reconstructions, often creating new functionalities at the interface. However, for a significant no. of materials these reconstructions are unwanted as they alter the intrinsic materials properties at the interface. Therefore, a strategy to eliminate this reconstruction of the polar discontinuity at the interfaces is required. We show that the use of compositional interface engineering can prevent the reconstruction at the La0.67Sr0.33MnO3/SrTiO3 (LSMO/STO) interface. The polar discontinuity at this interface can be removed by the insertion of a single La0.33Sr0.67O layer, resulting in improved interface magnetization and elec. cond.
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    22. 22
      Peng, R.; Xu, H. C.; Xia, M.; Zhao, J. F.; Xie, X.; Xu, D. F.; Xie, B. P.; Feng, D. L. Tuning the Dead-Layer Behavior of La0.67Sr0.33MnO3/SrTiO3 via Interfacial Engineering. Appl. Phys. Lett. 2014, 104, 081606,  DOI: 10.1063/1.4866461
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      22
      Tuning the dead-layer behavior of La0.67Sr0.33MnO3/SrTiO3 via interfacial engineering
      Peng, R.; Xu, H. C.; Xia, M.; Zhao, J. F.; Xie, X.; Xu, D. F.; Xie, B. P.; Feng, D. L.
      Applied Physics Letters (2014), 104 (8), 081606/1-081606/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      The dead-layer behavior, deterioration of the bulk properties in near-interface layers, restricts the applications of many oxide heterostructures. The authors present the systematic study of the dead-layer in La0.67Sr0.33MnO3/SrTiO3 grown by ozone-assisted MBE. Dead-layer behavior is systematically tuned by varying the interfacial doping, while unchanged with varied doping at any other at. layers. In situ photoemission and LEED measurements suggest intrinsic oxygen vacancies at the surface of ultra-thin La0.67Sr0.33MnO3, which are more concd. in thinner films. The authors' results show correlation between interfacial doping, oxygen vacancies, and the dead-layer, which can be explained by a simplified electrostatic model. (c) 2014 American Institute of Physics.
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      Huijben, M.; Liu, Y.; Boschker, H.; Lauter, V.; Egoavil, R.; Verbeeck, J.; te Velthuis, S. G. E.; Rijnders, G.; Koster, G. Enhanced Local Magnetization by Interface Engineering in Perovskite-Type Correlated Oxide Heterostructures. Adv. Mater. Interfaces 2015, 2, 1400416,  DOI: 10.1002/admi.201400416
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      23
      Enhanced Local Magnetization by Interface Engineering in Perovskite-Type Correlated Oxide Heterostructures
      Huijben, Mark; Liu, Yaohua; Boschker, Hans; Lauter, Valeria; Egoavil, Ricardo; Verbeeck, Jo; te Velthuis, Suzanne G. E.; Rijnders, Guus; Koster, Gertjan
      Advanced Materials Interfaces (2015), 2 (3), 1400416/1-1400416/7CODEN: AMIDD2; ISSN:2196-7350. (Wiley-VCH Verlag GmbH & Co. KGaA)
      This article describes enhanced local magnetization by interface engineering in perovskite-type correlated oxide heterostructures. Interfacial phenomena, the capability of controlled growth of interfacial at. stacking, has to be combined with local probing of the interfacial properties to enable true interface engineering.
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      Glazer, A. M. The Classification of Tilted Octahedra in Perovskites. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1972, 28, 3384– 3392,  DOI: 10.1107/s0567740872007976
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      Classification of tilted octahedra in perovskites
      Glazer, A. M.
      Acta Crystallographica, Section B: Structural Crystallography and Crystal Chemistry (1972), 28 (Pt. 11), 3384-92CODEN: ACBCAR; ISSN:0567-7408.
      A simple method for describing and classifying octahedral tilting in perovskites is given, and it is shown how the tilts are related to the unit-cell geometries. Several examples from the literature are listed, and predictions about hitherto unknown structures of some materials are made.
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      Kan, D.; Aso, R.; Sato, R.; Haruta, M.; Kurata, H.; Shimakawa, Y. Tuning Magnetic Anisotropy by Interfacially Engineering the Oxygen Coordination Environment in a Transition Metal Oxide. Nat. Mater. 2016, 15, 432– 437,  DOI: 10.1038/nmat4580
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      Tuning magnetic anisotropy by interfacially engineering the oxygen coordination environment in a transition metal oxide
      Kan, Daisuke; Aso, Ryotaro; Sato, Riko; Haruta, Mitsutaka; Kurata, Hiroki; Shimakawa, Yuichi
      Nature Materials (2016), 15 (4), 432-437CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      Strong correlations between electrons, spins and lattices-stemming from strong hybridization between transition metal d and oxygen p orbitals-are responsible for the functional properties of transition metal oxides. Artificial oxide heterostructures with chem. abrupt interfaces provide a platform for engineering bonding geometries that lead to emergent phenomena. Here the authors demonstrate the control of the oxygen coordination environment of the perovskite, SrRuO3, by heterostructuring it with Ca0.5Sr0.5TiO3 (0-4 monolayers thick) grown on a GdScO3 substrate. A Ru-O-Ti bond angle of the SrRuO3 /Ca0.5Sr0.5TiO3 interface can be engineered by layer-by-layer control of the Ca0.5Sr0.5TiO3 layer thickness, and the engineered Ru-O-Ti bond angle not only stabilizes a Ru-O-Ru bond angle never seen in bulk SrRuO3, but also tunes the magnetic anisotropy in the entire SrRuO3 layer. Interface engineering of the oxygen coordination environment allows one to control addnl. degrees of freedom in functional oxide heterostructures.
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    26. 26
      Adamo, C.; Ke, X.; Wang, H. Q.; Xin, H. L.; Heeg, T.; Hawley, M. E.; Zander, W.; Schubert, J.; Schiffer, P.; Muller, D. A.; Maritato, L.; Schlom, D. G. Effect of Biaxial Strain on the Electrical and Magnetic Properties of (001) La0.7Sr0.3MnO3 Thin Films. Appl. Phys. Lett. 2009, 95, 112504,  DOI: 10.1063/1.3213346
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      26
      Effect of biaxial strain on the electrical and magnetic properties of (001) La0.75Sr0.3MnO3 thin films
      Adamo, C.; Ke, X.; Wang, H. Q.; Xin, H. L.; Heeg, T.; Hawley, M. E.; Zander, W.; Schubert, J.; Schiffer, P.; Muller, D. A.; Maritato, L.; Schlom, D. G.
      Applied Physics Letters (2009), 95 (11), 112504/1-112504/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      We have studied the effect of biaxial strain on thin films of (001) La0.7Sr0.3MnO3. We deposited films by reactive mol.-beam epitaxy on different single cryst. substrates, varying the substrate-induced biaxial strain from -2.3% to +3.2%. Magnetization and elec. transport measurements reveal that the dependence of the Curie temp. on biaxial strain is in very good agreement with the theor. predictions of A. Millis et al.(1998). (c) 2009 American Institute of Physics.
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    27. 27
      Boschker, H.; Mathews, M.; Houwman, E. P.; Nishikawa, H.; Vailionis, A.; Koster, G.; Rijnders, G.; Blank, D. H. A. Strong Uniaxial In-Plane Magnetic Anisotropy of (001)- and (011)-Oriented La0.67Sr0.33MnO3 Thin Films on NdGaO3 Substrates. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 214425,  DOI: 10.1103/physrevb.79.214425
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      27
      Strong uniaxial in-plane magnetic anisotropy of (001)- and (011)-oriented La0.67Sr0.33MnO3 thin films on NdGaO3 substrates
      Boschker, H.; Mathews, M.; Houwman, E. P.; Nishikawa, H.; Vailionis, A.; Koster, G.; Rijnders, G.; Blank, D. H. A.
      Physical Review B: Condensed Matter and Materials Physics (2009), 79 (21), 214425/1-214425/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Epitaxial La0.67Sr0.33MnO3 (LSMO) ferromagnetic thin films were coherently grown on NdGaO3 (NGO) substrates with different crystal orientations of the surface plane. On the (110)o- and (001)o-oriented substrates, the film grows in the (001)pc orientation, and on the (100)o-, (010)o-, and (112)o-oriented substrates the film is (011)pc oriented (we will use subindices o and pc for the orthorhombic and pseudocubic crystal structures, resp.). The lattice parameters and pseudocube angles of the deformed LSMO pseudocube have been detd. from x-ray diffraction measurements. The in-plane magnetic easy and hard directions of these films have been detd. from the dependence of the remnant magnetization on the angle of the in-plane applied field. For all substrate orientations there is a strong in-plane uniaxial magnetic anisotropy, detd. by the crystal directions of the substrate surface. The easy and hard magnetic-anisotropy directions are explained consistently by the (bulk) inverse magnetostriction model, except for the film on NGO (112)o.
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    28. 28
      Cao, L.; Petracic, O.; Zakalek, P.; Weber, A.; Rücker, U.; Schubert, J.; Koutsioubas, A.; Mattauch, S.; Brückel, T. Reversible Control of Physical Properties via an Oxygen-Vacancy-Driven Topotactic Transition in Epitaxial La0.7Sr0.3MnO3– δ Thin Films. Adv. Mater. 2019, 31, 1806183,  DOI: 10.1002/adma.201806183
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      Tan, H.; Verbeeck, J.; Abakumov, A.; Van Tendeloo, G. Oxidation State and Chemical Shift Investigation in Transition Metal Oxides by EELS. Ultramicroscopy 2012, 116, 24– 33,  DOI: 10.1016/j.ultramic.2012.03.002
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      Oxidation state and chemical shift investigation in transition metal oxides by EELS
      Tan, Haiyan; Verbeeck, Jo; Abakumov, Artem; Van Tendeloo, Gustaaf
      Ultramicroscopy (2012), 116 (), 24-33CODEN: ULTRD6; ISSN:0304-3991. (Elsevier B.V.)
      Transition metal L2,3 electron energy-loss spectra for a wide range of V-, Mn- and Fe-based oxides were recorded and carefully analyzed for their correlation with the formal oxidn. states of the transition metal ions. Special attention is paid to obtain an accurate energy scale which provides abs. energy positions for all core-loss edges. The white-line ratio method, chem. shift method, ELNES fitting method, two-parameter method and other methods are compared and their validity is discussed. Both the ELNES fitting method and the chem. shift method have the advantage of a wide application range and good consistency but require special attention to accurately measure the core-loss edge position. The obtained conclusions are of fundamental importance, e.g., for obtaining at. resoln. oxidn. state information in modern expts.
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      Varela, M.; Oxley, M. P.; Luo, W.; Tao, J.; Watanabe, M.; Lupini, A. R.; Pantelides, S. T.; Pennycook, S. J. Atomic-Resolution Imaging of Oxidation States in Manganites. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 085117,  DOI: 10.1103/physrevb.79.085117
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      Atomic-resolution imaging of oxidation states in manganites
      Varela, M.; Oxley, M. P.; Luo, W.; Tao, J.; Watanabe, M.; Lupini, A. R.; Pantelides, S. T.; Pennycook, S. J.
      Physical Review B: Condensed Matter and Materials Physics (2009), 79 (8), 085117/1-085117/14CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Aberration cor. electron optics allows routine acquisition of high spatial resoln. spectroscopic images in the scanning transmission electron microscope, which is important when trying to understand the physics of transition-metal oxides such as manganites. The phys. properties of these perovskites are intimately related to the occupancies of the partially filled 3d bands, which define their oxidn. state. The authors review procedures to obtain this electronic property in LaxCa1-xMnO3 from at.-column-resolved electron energy-loss spectra measured in the aberration cor. scanning transmission electron microscope. In bulk samples, several features of both the av. Mn L2,3 edge and the O K edge fine structure change linearly with Mn nominal valence. These linear correlations are extd. and used as a calibration to quantify oxidn. states from at. resoln. spectroscopic images. In such images, the same fine-structure features exhibit further changes, commensurate with the underlying at. lattice. Mn valence values calcd. from those images show unexpected oscillations. The combination of expt. with d.-functional theory and dynamical scattering simulations allows detailed interpretation of these maps, distinguishing dynamical scattering effects from actual changes in electronic properties related to the local at. structure. Specifically, in LaMnO3, the two nonequivalent O sites can be distinguished by these methods.
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    31. 31
      Li, Z.; Bosman, M.; Yang, Z.; Ren, P.; Wang, L.; Cao, L.; Yu, X.; Ke, C.; Breese, M. B. H.; Rusydi, A.; Zhu, W.; Dong, Z.; Foo, Y. L. Interface and Surface Cation Stoichiometry Modified by Oxygen Vacancies in Epitaxial Manganite Films. Adv. Funct. Mater. 2012, 22, 4312– 4321,  DOI: 10.1002/adfm.201200143
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      Interface and Surface Cation Stoichiometry Modified by Oxygen Vacancies in Epitaxial Manganite Films
      Li, Zhipeng; Bosman, Michel; Yang, Zhen; Ren, Peng; Wang, Lan; Cao, Liang; Yu, Xiaojiang; Ke, Chang; Breese, Mark B. H.; Rusydi, Andrivo; Zhu, Weiguang; Dong, Zhili; Foo, Yong Lim
      Advanced Functional Materials (2012), 22 (20), 4312-4321CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Perovskite manganites are viewed as one of the key building blocks of oxide spintronics devices due to their attractive phys. properties. However, cation off-stoichiometry at epitaxial interfaces between manganites and other materials can lead to interfacial dead layers, severely reducing the device performance. Here, TEM and synchrotron-based spectroscopy were used to demonstrate that oxygen vacancies during growth serve as a crit. factor for modifying the cation stoichiometry in pulsed laser deposited La0.8Sr0.2MnO3 films. Near the film/substrate (SrTiO3) interface, A-site cations (La/Sr) are in excess when oxygen vacancies are induced during film growth, partially substituting Mn. Simultaneously, Sr cations migrate towards the film surface and form a SrO rock-salt monolayer. Consequentially, a gradient of the Mn nominal valence is obsd. along the film growth direction, leading to anomalous magnetic properties. The results narrow the selection range of useful oxygen pressures during deposition and demonstrate that accurate cation stoichiometry can only be achieved after oxygen vacancies are eliminated during growth. Probably the oxygen pressure serves as a tuning parameter for the interfacial dead layers and, hence, for control over device properties.
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      Kurata, H.; Colliex, C. Electron-Energy-Loss Core-Edge Structures in Manganese Oxides. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 48, 2102– 2108,  DOI: 10.1103/physrevb.48.2102
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      Electron-energy-loss core-edge structures in manganese oxides
      Kurata, Hiroki; Colliex, Christian
      Physical Review B: Condensed Matter and Materials Physics (1993), 48 (4), 2102-8CODEN: PRBMDO; ISSN:0163-1829.
      Oxygen K and manganese L2,3 edges electron-energy-loss spectra have been measured for a series of manganese oxides with different nominal oxidn. states for the cation (MnO, Mn3O4, Mn2O3, MnO2, and BaMnO4). Spectra have been processed for quant. elemental anal. and for the evaluation of manganese L2,3 white-line intensity ratios and normalized total intensities. Prepeaks on the oxygen K edge are sensitive to the nature of local chem. bonds and their detailed substructures are interpreted with the support of a mol.-orbital picture. As for the cation point of view, the anal. of the white-line intensities provides a satisfactory measurement of the effective 3d orbital occupancy as well as a hint of the local magnetic properties. The present study also confirms that the hybridization between oxygen 2p and manganese 3d orbitals plays an important role when considering the electronic structures of these oxides.
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      Wang, Z. L.; Yin, J. S.; Jiang, Y. D.; Zhang, J. Studies of Mn Valence Conversion and Oxygen Vacancies in La1- XCaxMnO3-y Using Electron Energy-Loss Spectroscopy. Appl. Phys. Lett. 1997, 70, 3362– 3364,  DOI: 10.1063/1.119171
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      Studies of Mn valence conversion and oxygen vacancies in La1-xCaxMnO3-y using electron energy-loss spectroscopy
      Wang, Z. L.; Yin, J. S.; Jiang, Y. D.; Zhang, Jiming
      Applied Physics Letters (1997), 70 (25), 3362-3364CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Using the white line intensities, EELS in a transmission electron microscope was employed to characterize the valence conversion and O vacancies in La1-xCaxMnO3-y. For a nominal doping compn. x = 0.33, the ratio of Mn4+ to Mn3+ is detd. to be >0.25 but <0.5, and the content of O vacancy y is ≤0.065 (equiv. to 2.2 at.% of the O content). At ymax = 0.065, 60% of the residual charge introduced by Ca doping is balanced by the conversion of Mn3+ to Mn4+ and 40% by O vacancy.
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      Krivanek, O. L.; Disko, M. M.; Taftø, J.; Spence, J. C. H. Electron Energy Loss Spectroscopy as a Probe of the Local Atomic Environment. Ultramicroscopy 1982, 9, 249– 254,  DOI: 10.1016/0304-3991(82)90208-x
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      Electron energy loss spectroscopy as a probe of the local atomic environment
      Krivanek, O. L.; Disko, M. M.; Taftoe, J.; Spence, J. C. H.
      Ultramicroscopy (1982), 9 (3), 249-54CODEN: ULTRD6; ISSN:0304-3991.
      A review with 15 refs. on structure detns. by electron energy-loss spectroscopy.
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      Jin, L.; Jia, C.-L.; Lindfors-Vrejoiu, I.; Zhong, X.; Du, H.; Dunin-Borkowski, R. E. Direct Demonstration of a Magnetic Dead Layer Resulting from A-Site Cation Inhomogeneity in a (La,Sr)MnO3 Epitaxial Film System. Adv. Mater. Interfaces 2016, 3, 1600414,  DOI: 10.1002/admi.201600414
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      Hÿtch, M. J.; Snoeck, E.; Kilaas, R. Quantitative Measurement of Displacement and Strain Fields from HREM Micrographs. Ultramicroscopy 1998, 74, 131– 146,  DOI: 10.1016/s0304-3991(98)00035-7
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      Quantitative measurement of displacement and strain fields from HREM micrographs
      Hytch, M. J.; Snoeck, E.; Kilaas, R.
      Ultramicroscopy (1998), 74 (3), 131-146CODEN: ULTRD6; ISSN:0304-3991. (Elsevier Science B.V.)
      A method for measuring and mapping displacement fields and strain fields from high-resoln. electron microscope (HREM) images has been developed. The method is based upon centering a small aperture around a strong reflection in the Fourier transform of an HREM lattice image and performing an inverse Fourier transform. The phase component of the resulting complex image is shown to give information about local displacements of at. planes and the two-dimensional displacement field can be derived by applying the method to two non-colinear Fourier components. Local strain components can be found by analyzing the deriv. of the displacement field. The details of the technique are outlined and applied to an exptl. HREM image of a domain wall in ferroelec.-ferroelastic PbTiO3.
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      Chaluvadi, S. K.; Ajejas, F.; Orgiani, P.; Lebargy, S.; Minj, A.; Flament, S.; Camarero, J.; Perna, P.; Méchin, L. Epitaxial Strain and Thickness Dependent Structural, Electrical and Magnetic Properties of La0.67Sr0.33MnO3 Films. J. Phys. D: Appl. Phys. 2020, 53, 375005,  DOI: 10.1088/1361-6463/ab8e7b
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      Epitaxial strain and thickness dependent structural, electrical and magnetic properties of La0.67Sr0.33MnO3 films
      Chaluvadi, Sandeep Kumar; Ajejas, Fernando; Orgiani, Pasquale; Lebargy, Sylvain; Minj, Albert; Flament, Stephane; Camarero, Julio; Perna, Paolo; Mechin, Laurence
      Journal of Physics D: Applied Physics (2020), 53 (37), 375005CODEN: JPAPBE; ISSN:0022-3727. (IOP Publishing Ltd.)
      The crystal structural quality and the strain induced by the substrate strictly impose the magnetic and transport properties of La0.67Sr0.33MnO3 (LSMO) films. In particular, the magnetic anisotropy (MA) of epitaxial LSMO can be finely tuned by varying its thickness and by choosing single crystal substrates with a suitable lattice mismatch with the film. Here, we have deposited LSMO films with thicknesses in the 12-50 nm range by pulsed laser deposition on different single crystal substrates inducing either compressive or tensile in-plane strain on the manganites. The epitaxial quality of films was quantified by ω-scans around a (002) peak with full-width half-max. values as low as 0.08#x00B0; for films on the nearly matched NGO (110) substrate to 1.4° films on the high mismatched MgO (001) substrate. As the epitaxial strain in a thin-film increases, a significant redn. in the metal-insulation transition temp. (Tp) was obsd. The magnetic properties of the films probed by Kerr magnetometry show that the symmetry of the room temp. MA varies significantly as a function of both strain and thickness. Specifically, we obsd. pure uniaxial MA on NGO (110) and pure biaxial MA on STO buffered MgO (001), whereas a spin reorientation from uniaxial in-plane to out-of-plane on LSAT (001) and uniaxial to nearly isotropic in-plane on STO (001) substrate as the film thickness is increased. We provide an efficient tool to tune the MA according to the specific spintronic application targeted.
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      Egerton, R. F. Electron Energy-Loss Spectroscopy in the Electron Microscope, 2nd ed.; Plenum Press: New York, 1996.
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      De Backer, A.; van den Bos, K. H. W.; Van den Broek, W.; Sijbers, J.; Van Aert, S. StatSTEM: An Efficient Approach for Accurate and Precise Model-Based Quantification of Atomic Resolution Electron Microscopy Images. Ultramicroscopy 2016, 171, 104– 116,  DOI: 10.1016/j.ultramic.2016.08.018
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      StatSTEM: An efficient approach for accurate and precise model-based quantification of atomic resolution electron microscopy images
      De Backer, A.; van den Bos, K. H. W.; Van den Broek, W.; Sijbers, J.; Van Aert, S.
      Ultramicroscopy (2016), 171 (), 104-116CODEN: ULTRD6; ISSN:0304-3991. (Elsevier B.V.)
      An efficient model-based estn. algorithm is introduced to quantify the at. column positions and intensities from at. resoln. (scanning) transmission electron microscopy ((S)TEM) images. This algorithm uses the least squares estimator on image segments contg. individual columns fully accounting for overlap between neighboring columns, enabling the anal. of a large field of view. For this algorithm, the accuracy and precision with which measurements for the at. column positions and scattering cross-sections from annular dark field (ADF) STEM images can be estd., has been investigated. The highest attainable precision is reached even for low dose images. Furthermore, the advantages of the model-based approach taking into account overlap between neighboring columns are highlighted. This is done for the estn. of the distance between two neighboring columns as a function of their distance and for the estn. of the scattering cross-section which is compared to the integrated intensity from a Voronoi cell. To provide end-users this well-established quantification method, a user friendly program, StatSTEM, is developed which is freely available under a GNU public license.
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      Koch, C. T. Determination of Core Structure Periodicity and Point Defect Density along Dislocations; Arizona State University, 2002.
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      Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; Dal Corso, A.; De Gironcoli, S.; Fabris, S.; Fratesi, G.; Gebauer, R.; Gerstmann, U.; Gougoussis, C.; Kokalj, A.; Lazzeri, M.; Martin-Samos, L.; Marzari, N.; Mauri, F.; Mazzarello, R.; Paolini, S.; Pasquarello, A.; Paulatto, L.; Sbraccia, C.; Scandolo, S.; Sclauzero, G.; Seitsonen, A. P.; Smogunov, A.; Umari, P.; Wentzcovitch, R. M. QUANTUM ESPRESSO: A Modular and Open-Source Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter 2009, 21, 395502,  DOI: 10.1088/0953-8984/21/39/395502
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      QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials
      Giannozzi Paolo; Baroni Stefano; Bonini Nicola; Calandra Matteo; Car Roberto; Cavazzoni Carlo; Ceresoli Davide; Chiarotti Guido L; Cococcioni Matteo; Dabo Ismaila; Dal Corso Andrea; de Gironcoli Stefano; Fabris Stefano; Fratesi Guido; Gebauer Ralph; Gerstmann Uwe; Gougoussis Christos; Kokalj Anton; Lazzeri Michele; Martin-Samos Layla; Marzari Nicola; Mauri Francesco; Mazzarello Riccardo; Paolini Stefano; Pasquarello Alfredo; Paulatto Lorenzo; Sbraccia Carlo; Scandolo Sandro; Sclauzero Gabriele; Seitsonen Ari P; Smogunov Alexander; Umari Paolo; Wentzcovitch Renata M
      Journal of physics. Condensed matter : an Institute of Physics journal (2009), 21 (39), 395502 ISSN:.
      QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.
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      Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865– 3868,  DOI: 10.1103/physrevlett.77.3865
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      Generalized gradient approximation made simple
      Perdew, John P.; Burke, Kieron; Ernzerhof, Matthias
      Physical Review Letters (1996), 77 (18), 3865-3868CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Generalized gradient approxns. (GGA's) for the exchange-correlation energy improve upon the local spin d. (LSD) description of atoms, mols., and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental consts. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential.
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      Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758– 1775,  DOI: 10.1103/physrevb.59.1758
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      From ultrasoft pseudopotentials to the projector augmented-wave method
      Kresse, G.; Joubert, D.
      Physical Review B: Condensed Matter and Materials Physics (1999), 59 (3), 1758-1775CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blochl's projector augmented wave (PAW) method is derived. The total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addn., crit. tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed-core all-electron methods. These tests include small mols. (H2, H2O, Li2, N2, F2, BF3, SiF4) and several bulk systems (diamond, Si, V, Li, Ca, CaF2, Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
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      Guzmán, R.; Maurel, L.; Langenberg, E.; Lupini, A. R.; Algarabel, P. A.; Pardo, J. A.; Magén, C. Polar-Graded Multiferroic SrMnO3 Thin Films. Nano Lett. 2016, 16, 2221– 2227,  DOI: 10.1021/acs.nanolett.5b04455
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      Polar-Graded Multiferroic SrMnO3 Thin Films
      Guzman, Roger; Maurel, Laura; Langenberg, Eric; Lupini, Andrew R.; Algarabel, Pedro A.; Pardo, Jose A.; Magen, Cesar
      Nano Letters (2016), 16 (4), 2221-2227CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Engineering defects and strains in oxides provides a promising route for the quest of thin film materials with coexisting ferroic orders, multiferroics, with efficient magnetoelec. coupling at room temp. Precise control of the strain gradient would enable custom tailoring of the multiferroic properties but presently remains challenging. The existence of a polar-graded state in epitaxially strained antiferromagnetic SrMnO3 thin films is reported, whose polar nature was predicted theor. and recently demonstrated exptl. By means of aberration-cor. scanning transmission electron microscopy we map the polar rotation of the ferroelec. polarization with at. resoln., both far from and near the domain walls, and find flexoelectricity resulting from vertical strain gradients. The origin of this particular strain state is a gradual distribution of oxygen vacancies across the film thickness, according to electron energy loss spectroscopy. Herein we present a chem.-mediated route to induce polar rotations in oxygen-deficient multiferroic films, resulting in flexoelec. polar rotations and with potentially enhanced piezoelectricity.
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      Radaelli, P. G.; Iannone, G.; Marezio, M.; Hwang, H. Y.; Cheong, S.-W.; Jorgensen, J. D.; Argyriou, D. N. Structural Effects on the Magnetic and Transport Properties of Perovskite A1-XA’xMnO3 (X=0.25, 0.30). Phys. Rev. B: Condens. Matter Mater. Phys. 1997, 56, 8265– 8276,  DOI: 10.1103/physrevb.56.8265
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      Vailionis, A.; Boschker, H.; Siemons, W.; Houwman, E. P.; Blank, D. H. A.; Rijnders, G.; Koster, G. Misfit Strain Accommodation in Epitaxial ABO3 Perovskites: Lattice Rotations and Lattice Modulations. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 1– 10,  DOI: 10.1103/physrevb.83.064101
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      Vailionis, A.; Boschker, H.; Liao, Z.; Smit, J. R. A.; Rijnders, G.; Huijben, M.; Koster, G. Symmetry and Lattice Mismatch Induced Strain Accommodation near and Away from Correlated Perovskite Interfaces. Appl. Phys. Lett. 2014, 105, 131906,  DOI: 10.1063/1.4896969
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      Symmetry and lattice mismatch induced strain accommodation near and away from correlated perovskite interfaces
      Vailionis, A.; Boschker, H.; Liao, Z.; Smit, J. R. A.; Rijnders, G.; Huijben, M.; Koster, G.
      Applied Physics Letters (2014), 105 (13), 131906/1-131906/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Distinct MnO6 octahedral distortions near and away from the La0.67Sr0.33MnO3/SrTiO3(001) (LSMO/STO) interface are quantified using synchrotron x-ray diffraction and dynamical x-ray diffraction simulations. 3 Structural regions of stress accommodation throughout the film thickness were resolved: near the LSMO/STO interface, intermediate region farther from the interface, and the main layer away from the interface. Within the first 2 unit cells stress is accommodated by the suppression of octahedral rotations in the film, leading to the expansion of the c-axis lattice parameter. Farther from the interface film structure acquires octahedral tilts similar to thicker perovskite films under tensile stress, leading to a reduced c-axis parameter. These regions are related to 2 different strain coupling mechanisms: symmetry mismatch at the interface and lattice mismatch in the rest of the film. The findings suggest new routes for strain engineering in correlated perovskite heterostructures. (c) 2014 American Institute of Physics.
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      Howie, A. Image Contrast and Localized Signal Selection Techniques. J. Microsc. 1979, 117, 11– 23,  DOI: 10.1111/j.1365-2818.1979.tb00228.x
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      Kirkland, E. J.; Loane, R. F.; Silcox, J. Simulation of Annular Dark Field STEM Images Using a Modified Multislice Method. Ultramicroscopy 1987, 23, 77– 96,  DOI: 10.1016/0304-3991(87)90229-4
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      Muller, D. A.; Nakagawa, N.; Ohtomo, A.; Grazul, J. L.; Hwang, H. Y. Atomic-Scale Imaging of Nanoengineered Oxygen Vacancy Profiles in SrTiO3. Nature 2004, 430, 657– 661,  DOI: 10.1038/nature02756
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      Atomic-scale imaging of nanoengineered oxygen vacancy profiles in SrTiO3
      Muller, David A.; Nakagawa, Naoyuki; Ohtomo, Akira; Grazul, John L.; Hwang, Harold Y.
      Nature (London, United Kingdom) (2004), 430 (7000), 657-661CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      At the heart of modern oxide chem. lies the recognition that beneficial (as well as deleterious) materials properties can be obtained by deliberate deviations of oxygen atom occupancy from the ideal stoichiometry. Conversely, the capability to control and confine oxygen vacancies will be important to realize the full potential of perovskite ferroelec. materials, varistors and field-effect devices. In transition metal oxides, oxygen vacancies are generally electron donors, and in strontium titanate (SrTiO3) thin films, oxygen vacancies (unlike impurity dopants) are particularly important because they tend to retain high carrier mobilities, even at high carrier densities. Here the authors report the successful fabrication, using a pulsed laser deposition technique, of SrTiO3 superlattice films with oxygen doping profiles that exhibit subnanometre abruptness. The authors profile the vacancy concns. on an at. scale using annular-dark-field electron microscopy and core-level spectroscopy, and demonstrate abs. detection sensitivities of one to four oxygen vacancies. The authors' findings open a pathway to the microscopic study of individual vacancies and their clustering, not only in oxides, but in cryst. materials more generally.
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      Johnston-Peck, A. C.; Winterstein, J. P.; Roberts, A. D.; DuChene, J. S.; Qian, K.; Sweeny, B. C.; Wei, W. D.; Sharma, R.; Stach, E. A.; Herzing, A. A. Oxidation-State Sensitive Imaging of Cerium Dioxide by Atomic-Resolution Low-Angle Annular Dark Field Scanning Transmission Electron Microscopy. Ultramicroscopy 2016, 162, 52– 60,  DOI: 10.1016/j.ultramic.2015.12.004
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      Oxidation-state sensitive imaging of cerium dioxide by atomic-resolution low-angle annular dark field scanning transmission electron microscopy
      Johnston-Peck, Aaron C.; Winterstein, Jonathan P.; Roberts, Alan D.; DuChene, Joseph S.; Qian, Kun; Sweeny, Brendan C.; Wei, Wei David; Sharma, Renu; Stach, Eric A.; Herzing, Andrew A.
      Ultramicroscopy (2016), 162 (), 52-60CODEN: ULTRD6; ISSN:0304-3991. (Elsevier B.V.)
      Low-angle annular dark field (LAADF) scanning transmission electron microscopy (STEM) imaging is presented as a method that is sensitive to the oxidn. state of cerium ions in CeO2 nanoparticles. This relationship was validated through electron energy loss spectroscopy (EELS), in situ measurements, as well as multislice image simulations. Static displacements caused by the increased ionic radius of Ce3+ influence the electron channeling process and increase electron scattering to low angles while reducing scatter to high angles. This process manifests itself by reducing the high-angle annular dark field (HAADF) signal intensity while increasing the LAADF signal intensity in close proximity to Ce3+ ions. This technique can supplement STEM-EELS and in so doing, relax the exptl. challenges assocd. with acquiring oxidn. state information at high spatial resolns.
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      Angeloni, M.; Balestrino, G.; Boggio, N. G.; Medaglia, P. G.; Orgiani, P.; Tebano, A. Suppression of the Metal-Insulator Transition Temperature in Thin La0.7Sr0.3MnO3 Films. J. Appl. Phys. 2004, 96, 6387– 6392,  DOI: 10.1063/1.1812599
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      Suppression of the metal-insulator transition temperature in thin La0.7Sr0.3MnO3 films
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      Journal of Applied Physics (2004), 96 (11), 6387-6392CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      In this paper, we illustrate an approach to discriminate between epitaxial strain and other factors responsible for the decrease of the metal-insulator transition temp. (TP) in thin La0.7Sr0.3MnO3 films grown by pulsed laser deposition. Using this approach, we have estd. the effect of the biaxial strain on TP. Ultrathin films, independent of epitaxial strain, do not show any metal-insulator transition over the full temp. range. This finding confirms the existence of an interface dead layer. The strain-independent decrease in TP, relative to its bulk value, obsd. for a much wider thickness range (up to about 1000 A) can most likely be attributed to oxygen deficiency.
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      Hillyard, S.; Silcox, J. Detector Geometry, Thermal Diffuse Scattering and Strain Effects in ADF STEM Imaging. Ultramicroscopy 1995, 58, 6– 17,  DOI: 10.1016/0304-3991(94)00173-k
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      Detector geometry, thermal diffuse scattering and strain effects in ADF STEM imaging
      Hillyard, Sean; Silcox, John
      Ultramicroscopy (1995), 58 (1), 6-17CODEN: ULTRD6; ISSN:0304-3991. (Elsevier)
      Intensities of at.-resoln. Annular Dark Field Scanning TEM (ADF STEM) images of zone-axis-oriented specimens change with defocus at rates that depend on lattice spacing. Thickness and strain effects on the intensities were demonstrated. Image simulations (with some exptl. basis) are presented that consider the dimensions of the ADF detector. Changing the inner radius of the detector seems to have relatively small effect on the image except to lower the detected intensity. Probe size was explored and a case identified where multiple scattering was important in the image. Thermal diffuse scattering (TDS) is important in high-angle scattering at room temps. but it does not seem to alter the image appearance markedly. Finally, the image arising from the strain field around a single B atom was simulated and the results suggest increased scattering in agreement with observations. This mechanism may be adequate for single impurity atom detection at low temps. and with special detector angles.
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      Perovic, D. D.; Rossouw, C. J.; Howie, A. Imaging Elastic Strains in High-Angle Annular Dark Field Scanning Transmission Electron Microscopy. Ultramicroscopy 1993, 52, 353– 359,  DOI: 10.1016/0304-3991(93)90046-z
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      Imaging elastic strains in high-angle annular dark field scanning transmission electron microscopy
      Perovic, D. D.; Rossouw, C. J.; Howie, A.
      Ultramicroscopy (1993), 52 (3-4), 353-9CODEN: ULTRD6; ISSN:0304-3991.
      High-angle annular dark field (HAADF) imaging in a dedicated scanning transmission electron microscope (STEM) was applied to the study of imperfect crystals. Firstly, a study of B-doped layers in Si revealed significantly stronger contrast and of opposite sign relative to simple at. no. contrast (Z-contrast) predictions. Misfitting substitutional B atoms act as point defect sites in a Si matrix which enhance scattering to high angles via a static Debye-Waller effect. Multi-beam Bloch-wave theory was used to quant. predict exptl. contrast levels. Secondly, HAADF-STEM imaging of inclined dislocation segments revealed a no. of novel contrast effects which depend on the specific position of the dislocation in the foil. Unlike conventional diffraction contrast from dislocations, HAADF dislocation contrast is neither similar nor complementary at the entrant and exit surfaces of the specimen. A qual. Bloch-wave scattering description was developed consistently to describe the dislocation contrast features.
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      Phillips, P. J.; De Graef, M.; Kovarik, L.; Agrawal, A.; Windl, W.; Mills, M. J. Atomic-Resolution Defect Contrast in Low Angle Annular Dark-Field STEM. Ultramicroscopy 2012, 116, 47– 55,  DOI: 10.1016/j.ultramic.2012.03.013
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      Atomic-resolution defect contrast in low angle annular dark-field STEM
      Phillips, P. J.; De Graef, M.; Kovarik, L.; Agrawal, A.; Windl, W.; Mills, M. J.
      Ultramicroscopy (2012), 116 (), 47-55CODEN: ULTRD6; ISSN:0304-3991. (Elsevier B.V.)
      A review. While traditional high-resoln. STEM is performed by exclusively collecting electrons which have been scattered to high angles (i.e., HAADF), the present contribution will focus on small-angle scattered electrons, as in low angle annular dark-field (LAADF) STEM. This unique imaging mode allows one to image defect contrast while maintaining directly interpretable at. resoln. By simply adjusting the microscope camera length, and thus the acceptance angle of the annular detector, it is possible to transition between Z-contrast and defect contrast. Both LAADF and HAADF exptl. and computational results are discussed in regards to zone axis imaging of a γ/γ' Ni-superalloy; various length scales are explored. Electron de-channeling is obsd. while the probe is placed over defected regions of crystal.
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      Plamann, T.; Hÿtch, M. J. Tests on the Validity of the Atomic Column Approximation for STEM Probe Propagation. Ultramicroscopy 1999, 78, 153– 161,  DOI: 10.1016/s0304-3991(99)00019-4
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      Tests on the validity of the atomic column approximation for STEM probe propagation
      Plamann, T.; Hytch, M. J.
      Ultramicroscopy (1999), 78 (1-4), 153-161CODEN: ULTRD6; ISSN:0304-3991. (Elsevier Science B.V.)
      In this contribution the authors examine the channeling properties of electron probes with a diam. of a few Angstroms to det. limits to at. resoln. anal. as they are caused by the interaction between the electron probe and the specimen. If the probe size is not considerably smaller than the interat. spacings the light at. columns act as electron traps. The authors examine the effects of small misplacements of the probe from the at. column positions and of small specimen tilts on the shape of the channeling peak. Finally, if the columns are not perfectly straight but have a correlated displacement as a function of z (as often occurs close to an interface), the channeling characteristics of the probe can be seriously affected. Depending on the form of the at. column, and the at. no. of the element occupying the column, a strong dispersion of the probe may occur even at small thicknesses. This has serious consequences for the anal. signal, but also almost certainly for the intensity distribution in high annular dark-field images.
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      Cowley, J. M.; Huang, Y. De-Channelling Contrast in Annular Dark-Field STEM. Ultramicroscopy 1992, 40, 171– 180,  DOI: 10.1016/0304-3991(92)90058-r
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      Yao, L.; Majumdar, S.; Äkäslompolo, L.; Inkinen, S.; Qin, Q. H.; Van Dijken, S. Electron-Beam-Induced Perovskite-Brownmillerite-Perovskite Structural Phase Transitions in Epitaxial La2/3Sr1/3MnO3 Films. Adv. Mater. 2014, 26, 2789– 2793,  DOI: 10.1002/adma.201305656
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      Electron-Beam-Induced Perovskite-Brownmillerite-Perovskite Structural Phase Transitions in Epitaxial La2/3Sr1/3MnO3 Films
      Yao, Lide; Majumdar, Sayani; Aekaeslompolo, Laura; Inkinen, Sampo; Qin, Qi Hang; van Dijken, Sebastiaan
      Advanced Materials (Weinheim, Germany) (2014), 26 (18), 2789-2793CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The authors demonstrate that the evolution of the perovskite-brownmillerite phase transition can be fully controlled and monitored in epitaxial La2/3Sr1/3MnO3 (LSMO) films using electron-beam irradn. in a transmission electron microscope(TEM). Real time TEM imaging with at. resoln. reveals that the structural transition si driven by an incessant ordering of electron beam induced oxygen vacancies. The demonstrated ability to simultaneously induce and characterize oxygen deficient structural phases in a continuous and controllable manner opens up new pathways for at. scale studies on ionic transport dynamics.
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      Hemberger, J.; Krimmel, A.; Kurz, T.; Krug von Nidda, H. A.; Ivanov, V. Y.; Mukhin, A. A.; Balbashov, A. M.; Loidl, A. Structural, Magnetic, and Electrical Properties of Single-Crystalline La1-XSrxMnO3 (0.4<x<0.85). Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 66, 094410,  DOI: 10.1103/physrevb.66.094410
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      Structural, magnetic, and electrical properties of single-crystalline La1-xSrxMnO3 (0.4<x<0.85)
      Hemberger, J.; Krimmel, A.; Kurz, T.; Krug von Nidda, H.-A.; Ivanov, V. Yu.; Mukhin, A. A.; Balbashov, A. M.; Loidl, A.
      Physical Review B: Condensed Matter and Materials Physics (2002), 66 (9), 094410/1-094410/8CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      The authors report on structural, magnetic, and elec. properties of Sr-doped LaMnO3 single crystals for doping levels 0.4≤x≤0.85. The complex structural and magnetic phase diagram can only be explained assuming significant contributions from the orbital degrees of freedom. Close to x = 0.6 a ferromagnetic metal is followed by an antiferromagnetic metallic phase <200 K This antiferromagnetic metallic phase exists in a monoclinic crystallog. structure. Following theor. predictions this metallic antiferromagnet is expected to reveal an (x2-y2)-type orbital order. For higher Sr concns. an antiferromagnetic insulator is established below room temp.
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      Liao, Z.; Zhang, J. Metal-to-Insulator Transition in Ultrathin Manganite Heterostructures. Appl. Sci. 2019, 9, 144,  DOI: 10.3390/app9010144
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      Metal-to-insulator transition in ultrathin manganite heterostructures
      Liao, Zhaoliang; Zhang, Jiandi
      Applied Sciences (2019), 9 (1), 144CODEN: ASPCC7; ISSN:2076-3417. (MDPI AG)
      Thickness-driven phase transitions have been widely obsd. in many correlated transition metal oxides materials. One of the important topics is the thickness-driven metal to insulator transition in half-metal La2/3Sr1/3MnO3 (LSMO) thin films, which has attracted great attention in the past few decades. In this article, we review research on the nature of the metal-to-insulator (MIT) transition in LSMO ultrathin films. We discuss in detail the proposed mechanisms, the progress made up to date, and the key issues existing in understanding the related MIT.We also discuss MIT in other correlated oxide materials as a comparison that also has some implications for understanding the origin of MIT.
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      Kobrinskii, A. L.; Goldman, A. M.; Varela, M.; Pennycook, S. J. Thickness Dependence of the Exchange Bias in Epitaxial Manganite Bilayers. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 094405,  DOI: 10.1103/physrevb.79.094405
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      Thickness dependence of the exchange bias in epitaxial manganite bilayers
      Kobrinskii, A. L.; Goldman, A. M.; Varela, Maria; Pennycook, S. J.
      Physical Review B: Condensed Matter and Materials Physics (2009), 79 (9), 094405/1-094405/7CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Exchange bias has been studied in a series of La2/3Ca1/3MnO3/La1/3Ca2/3MnO3 bilayers grown on (001) SrTiO3 substrates by ozone-assisted mol.-beam epitaxy. The high cryst. quality of the samples and interfaces has been verified using high-resoln. x-ray diffractometry and Z-contrast scanning transmission electron microscopy with electron-energy-loss spectroscopy. The dependence of exchange bias on the thickness of the antiferromagnetic layer has been investigated. A crit. value for the onset of the hysteresis loop shift has been detd. An antiferromagnetic anisotropy const. has been obtained by fitting the results to the generalized Meiklejohn-Bean model.
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      Venkatesan, S.; Döblinger, M.; Daumont, C.; Kooi, B.; Noheda, B.; De Hosson, J. T. M.; Scheu, C. Influence of Strain on the Electronic Structure of the TbMnO3/SrTiO3 Epitaxial Interface. Appl. Phys. Lett. 2011, 99, 222902,  DOI: 10.1063/1.3663218
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      Influence of strain on the electronic structure of the TbMnO3/SrTiO3 epitaxial interface
      Venkatesan, S.; Doeblinger, M.; Daumont, C.; Kooi, B.; Noheda, B.; De Hosson, J. T. M.; Scheu, C.
      Applied Physics Letters (2011), 99 (22), 222902/1-222902/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Understanding the magnetotransport properties of epitaxial strained thin films requires knowledge of the chem. at the interface. We report on the change in Mn electronic structure at the epitaxially strained TbMnO3/SrTiO3 interface. Scanning transmission electron microscopy shows an abrupt interface with a bright contrast, indicating the presence of misfit strain. Electron energy loss spectroscopy displays a chem. shift of the Mn L2,3 edge together with a high white line intensity ratio revealing a redn. in the nominal Mn oxidn. state in the first 3-4 monolayers. These observations indicate misfit strain significantly changes the electronic structure at the interface. (c) 2011 American Institute of Physics.
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      Orgiani, P.; Galdi, A.; Aruta, C.; Cataudella, V.; De Filippis, G.; Perroni, C. A.; Marigliano Ramaglia, V.; Ciancio, R.; Brookes, N. B.; Moretti Sala, M.; Ghiringhelli, G.; Maritato, L. Multiple Double-Exchange Mechanism by Mn2+ Doping in Manganite Compounds. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 205122,  DOI: 10.1103/physrevb.82.205122
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      Multiple double-exchange mechanism by Mn2+ doping in manganite compounds
      Orgiani, P.; Galdi, A.; Aruta, C.; Cataudella, V.; De Filippis, G.; Perroni, C. A.; Marigliano Ramaglia, V.; Ciancio, R.; Brookes, N. B.; Moretti Sala, M.; Ghiringhelli, G.; Maritato, L.
      Physical Review B: Condensed Matter and Materials Physics (2010), 82 (20), 205122/1-205122/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Double-exchange (DE) mechanisms in RE1-xAExMnO3 manganites (where RE is a trivalent rare-earth ion and AE is a divalent alkali-earth ion) relies on the strong exchange interaction between two Mn3+ and Mn4+ ions through interfiling oxygen 2p states. Nevertheless, the role of RE and AE ions has ever been considered "silent" with respect to the DE conducting mechanisms. Here we show that a new path for DE mechanism is indeed possible by partially replacing the RE-AE elements by Mn2+ ions, in La-deficient LaxMnO3-δ thin films. X-ray absorption spectroscopy demonstrated the relevant presence of Mn2+ ions, which is unambiguously proved to be substituted at La site by resonant inelastic x-ray scattering. Mn2+ is proved to be directly correlated with the enhanced magnetotransport properties because of an addnl. hopping mechanism trough interfiling Mn2+ ions. Such a scenario has been theor. confirmed by calcns. within the effective single-band model. The use of Mn2+ both as a doping element and an ion electronically involved in the conduction mechanism reveals an unconventional phenomenon in transport properties of manganites. More important, such a strategy might be also pursed in other strongly correlated materials.
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    64. 64
      Penn, A. N.; Koohfar, S.; Kumah, D. P.; Lebeau, J. M. On the Redistribution of Charge in La0.7Sr0.3CrO3/La0.7Sr0.3MnO3 Multilayer Thin Films. AIP Adv. 2020, 10, 045133,  DOI: 10.1063/1.5140352
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      Ko, E. K.; Mun, J.; Lee, H. G.; Kim, J.; Song, J.; Chang, S. H.; Kim, T. H.; Chung, S. B.; Kim, M.; Wang, L.; Noh, T. W. Oxygen Vacancy Engineering for Highly Tunable Ferromagnetic Properties: A Case of SrRuO3 Ultrathin Film with a SrTiO3 Capping Layer. Adv. Funct. Mater. 2020, 30, 2001486,  DOI: 10.1002/adfm.202001486
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      Oxygen Vacancy Engineering for Highly Tunable Ferromagnetic Properties: A Case of SrRuO3 Ultrathin Film with a SrTiO3 Capping Layer
      Ko, Eun Kyo; Mun, Junsik; Lee, Han Gyeol; Kim, Jinkwon; Song, Jeongkeun; Chang, Seo Hyoung; Kim, Tae Heon; Chung, Suk Bum; Kim, Miyoung; Wang, Lingfei; Noh, Tae Won
      Advanced Functional Materials (2020), 30 (50), 2001486CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Oxide heterostructures have great potential for spintronics applications due to their well-defined heterointerfaces and vast functionalities. To integrate such compelling features into practical spintronics devices, effective control of the magnetic switching behavior is key. Here, continuous control of the magnetic coercive field in SrTiO3/SrRuO3 ultrathin heterostructures is achieved by oxygen vacancy (VO) engineering. Pulsed laser deposition of an oxygen-deficient SrTiO3 capping layer can trigger VO migration into the SrRuO3 layer while avoiding the formation of Ru vacancies. Moreover, by varying the thickness and growth conditions of the SrTiO3 capping layer, the value of the coercive field (HC) in the ferromagnetic SrRuO3 layer can be continuously tuned. The max. enhancement of HC at 5 K is 3.2 T. Such a wide-range tunability of HC may originate from a VO-induced enhancement of perpendicular magnetic anisotropy and domain wall pinning. This study offers effective approaches for controlling phys. properties of oxide heterostructures via VO engineering, which may facilitate the development of oxide-based functional devices.
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    66. 66
      Lu, J.; Si, L.; Yao, X.; Tian, C.; Wang, J.; Zhang, Q.; Lai, Z.; Malik, I. A.; Liu, X.; Jiang, P.; Zhu, K.; Shi, Y.; Luo, Z.; Gu, L.; Held, K.; Mi, W.; Zhong, Z.; Nan, C.-W.; Zhang, J. Electric Field Controllable High-Spin SrRuO3 Driven by a Solid Ionic Junction. Phys. Rev. B 2020, 101, 214401,  DOI: 10.1103/physrevb.101.214401
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      Electric field controllable high-spin SrRuO3 driven by a solid ionic junction
      Lu, Jingdi; Si, Liang; Yao, Xiefei; Tian, Chengfeng; Wang, Jing; Zhang, Qinghua; Lai, Zhengxun; Malik, Iftikhar Ahmed; Liu, Xin; Jiang, Peiheng; Zhu, Kejia; Shi, Youguo; Luo, Zhenlin; Gu, Lin; Held, Karsten; Mi, Wenbo; Zhong, Zhicheng; Nan, Ce-Wen; Zhang, Jinxing
      Physical Review B (2020), 101 (21), 214401CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      Controlling magnetism and spin structures in strongly correlated systems by using elec. fields is of fundamental importance but challenging. Here, a high-spin ruthenate phase is achieved via a solid ionic chem. junction at the SrRuO3/SrTiO3 interface with distinct formation energies and diffusion barriers of oxygen vacancies, an analog to electronic band alignment in the semiconductor heterojunction. Oxygen vacancies trapped within this interfacial SrRuO3 reconstruct the Ru-4d electronic structure and orbital occupancy, leading to an enhanced magnetic moment. Furthermore, this emergent interfacial magnetic phase can be switched reversibly by elec.-field-rectifying oxygen migration in a solid-state ionic gating device, providing a framework for the at. design of functionalities in strongly correlated oxides using a method of solid chem.
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    67. 67
      Yang, N.; Orgiani, P.; Di Bartolomeo, E.; Foglietti, V.; Torelli, P.; Ievlev, A. V.; Rossi, G.; Licoccia, S.; Balestrino, G.; Kalinin, S. V.; Aruta, C. Effects of Dopant Ionic Radius on Cerium Reduction in Epitaxial Cerium Oxide Thin Films. J. Phys. Chem. C 2017, 121, 8841– 8849,  DOI: 10.1021/acs.jpcc.7b00386
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      Effects of Dopant Ionic Radius on Cerium Reduction in Epitaxial Cerium Oxide Thin Films
      Yang, Nan; Orgiani, Pasquale; Di Bartolomeo, Elisabetta; Foglietti, Vittorio; Torelli, Piero; Ievlev, Anton V.; Rossi, Giorgio; Licoccia, Silvia; Balestrino, Giuseppe; Kalinin, Sergei V.; Aruta, Carmela
      Journal of Physical Chemistry C (2017), 121 (16), 8841-8849CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The role of trivalent rare-earth dopants on the cerium oxidn. state was systematically studied by in situ photoemission spectroscopy with synchrotron radiation for 10 mol % rare-earth doped epitaxial ceria films. Dopant rare-earths with smaller ionic radius foster the formation of Ce3+ by releasing the stress strength induced by the cation substitution. With a decrease of the dopant ionic radius from La3+ to Yb3+, the out-of-plane axis parameter of the crystal lattice decreases without introducing macroscopic defects. The high crystal quality of the authors' films allowed the authors to comparatively study both the ionic cond. and surface reactivity ruling out the influence of structural defects. The measured increase in the activation energy of films and their enhanced surface reactivity can be explained in terms of the dopant ionic radius effects on the Ce4+ → Ce3+ redn. as a result of lattice relaxation. Such findings open new perspectives in designing ceria-based materials with tailored properties by choosing suitable cation substitution.
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      von Harrach, H. S. Instrumental Factors in High-Resolution FEG STEM. Ultramicroscopy 1995, 58, 1– 5,  DOI: 10.1016/0304-3991(94)00172-j
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      Instrumental factors in high-resolution FEG STEM
      von Harrach, H. S.
      Ultramicroscopy (1995), 58 (1), 1-5CODEN: ULTRD6; ISSN:0304-3991. (Elsevier)
      The instrumental factors affecting the resoln. of a scanning transmission electron microscope (STEM) differ in many respects from those affecting the transmission electron microscope (TEM). The requirements for reducing the instabilities affecting a scanned electron probe to 10 pm are discussed with ref. to a 300 kV field-emission STEM of 0.13 nm resoln.
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    69. 69
      Qiao, Q.; Zhang, Y.; Contreras-Guerrero, R.; Droopad, R.; Pantelides, S. T.; Pennycook, S. J.; Ogut, S.; Klie, R. F. Direct Observation of Oxygen-Vacancy-Enhanced Polarization in a SrTiO3-Buffered Ferroelectric BaTiO3 Film on GaAs. Appl. Phys. Lett. 2015, 107, 201604,  DOI: 10.1063/1.4936159
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      69
      Direct observation of oxygen-vacancy-enhanced polarization in a SrTiO3-buffered ferroelectric BaTiO3 film on GaAs
      Qiao, Qiao; Zhang, Yuyang; Contreras-Guerrero, Rocio; Droopad, Ravi; Pantelides, Sokrates T.; Pennycook, Stephen J.; Ogut, Serdar; Klie, Robert F.
      Applied Physics Letters (2015), 107 (20), 201604/1-201604/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      The integration of functional oxide thin-films on compd. semiconductors can lead to a class of reconfigurable spin-based optoelectronic devices if defect-free, fully reversible active layers are stabilized. However, previous first-principles calcns. predicted that SrTiO3 thin films grown on Si exhibit pinned ferroelec. behavior that is not switchable, due to the presence of interfacial vacancies. Meanwhile, piezoresponse force microscopy measurements have demonstrated ferroelectricity in BaTiO3 grown on semiconductor substrates. The presence of interfacial oxygen vacancies in such complex-oxide/semiconductor systems remains unexplored, and their effect on ferroelectricity is controversial. Here, we use a combination of aberration-cor. scanning transmission electron microscopy and first-principles d. functional theory modeling to examine the role of interfacial oxygen vacancies on the ferroelec. polarization of a BaTiO3 thin film grown on GaAs. We demonstrate that interfacial oxygen vacancies enhance the polar discontinuity (and thus the single domain, out-of-plane polarization pinning in BaTiO3), and propose that the presence of surface charge screening allows the formation of switchable domains. (c) 2015 American Institute of Physics.
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    • t/λ map using the EELS spectra; strain map calculated from the STEM–HAADF image; chemical composition analysis using EDXS; determination of the Mn valence state; quantification of Mn-ion (B-site) displacements; details of DFT calculations; and STEM image simulation (PDF)


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