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Influence of Magnetic Sublattice Ordering on Skyrmion Bubble Stability in 2D Magnet Fe5GeTe2
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Cite this: ACS Nano 2024, 18, 28, 18246–18256
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https://doi.org/10.1021/acsnano.4c00853
Published July 8, 2024

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Abstract

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The realization of above room-temperature ferromagnetism in the two-dimensional (2D) magnet Fe5GeTe2 represents a major advance for the use of van der Waals (vdW) materials in practical spintronic applications. In particular, observations of magnetic skyrmions and related states within exfoliated flakes of this material provide a pathway to the fine-tuning of topological spin textures via 2D material heterostructure engineering. However, there are conflicting reports as to the nature of the magnetic structures in Fe5GeTe2. The matter is further complicated by the study of two types of Fe5GeTe2 crystals with markedly different structural and magnetic properties, distinguished by their specific fabrication procedure: whether they are slowly cooled or rapidly quenched from the growth temperature. In this work, we combine X-ray and electron microscopy to observe the formation of magnetic stripe domains, skyrmion-like type-I, and topologically trivial type-II bubbles, within exfoliated flakes of Fe5GeTe2. The results reveal the influence of the magnetic ordering of the Fe1 sublattice below 150 K, which dramatically alters the magnetocrystalline anisotropy and leads to a complex magnetic phase diagram and a sudden change of the stability of the magnetic textures. In addition, we highlight the significant differences in the magnetic structures intrinsic to slow-cooled and quenched Fe5GeTe2 flakes.

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Since the observation of magnetic ordering in CrI3 and Cr2Ge2Te6, (1,2) two-dimensional (2D) van der Waals (vdW) magnets have seen widespread research interest. Due to their well-defined atomically flat layers, which typically lack dangling bonds, they are well-suited for being stacked into heterostructures and combined with the growing catalogue of other 2D materials. (3) This enables exploitation of all manner of interfacial and proximity effects, which can be expected to give rise to a rich variety of physical phenomena with potentially wide-ranging technological applications. (4) In particular, the discovery of 2D magnets offers a diverse material platform for spintronics, which aims to utilize the spin degree of freedom of electrons in storage and logic devices. (5) Along these lines, a number of prototype spintronic devices have already been demonstrated. (6,7)
The envisioned applications depend crucially on the Curie temperature, TC, of the magnetic material: it should be above room temperature for convenient operation of proposed devices. Most 2D magnets discovered so far have ordering temperatures well below 300 K─for example, CrI3 and Cr2Ge2Te6 both have a TC around 60 K in bulk, which is reduced in the monolayer limit. A notable exception to this trend is the family of materials FenGeTe2 (FGT), where crystal structures with 2.5 < n < 5 have been synthesized, (8−10) and those with n = 6, 7 have been proposed. (11) The most widely studied of these materials is Fe3–xGeTe2 (F3GT, where x indicates some Fe deficiency), which initially stood out for having a TC of up to 230 K. (12−14) However, recently, TC values approaching room temperature have been demonstrated for Fe4–xGeTe2 (F4GT, 270 K) (15) and Fe5–xGeTe2 (F5GT, 310–330 K). (16) For all FGT materials, this TC is tunable by both chemical substitution (17−19) or electrostatic and ionic gating. (20,21)
One potential avenue of spintronics application research involves the exploitation of topological spin textures, such as magnetic skyrmions, for racetrack memory or neuromorphic devices. (22,23) Magnetic skyrmions, nanoscale quasi-particles with a unitary topological charge, (24,25) have been found in a range of magnetic materials and are typically stabilized by either the Dzyaloshinskii–Moriya interaction (DMI) or dipolar interaction. (26,27) Here, it is crucial to draw attention to the distinction between magnetic skyrmions in comparison to magnetic bubbles, (27) which remains a point of contention within the wider literature. For the purposes of this paper, we will define skyrmions as magnetic objects with topological charge Q = 1, which exhibit a single chirality throughout the host material, typically due to the presence of DMI. On the other hand, magnetic bubbles are stabilized by the dipolar interaction and come in two flavors: type-I bubbles (often called skyrmionic bubbles), which possess the same Q = 1 topology as skyrmions but are typically found with both left- and right-handed chirality within a single sample; and type-II bubbles, which have a Q = 0 and are composed of two domain walls joined by a pair of Bloch lines.
In the case of van der Waals magnets, magnetic skyrmions or bubbles have now been observed by microscopy in a range of systems, (28,29) and in particular in F3GT. (30−36) Recently, investigations have extended to F5GT, with various reports of real-space spin textures in thin flake or lamella samples. (37,38) There are claims that there may be some form of DMI present in F5GT due to supercells within the crystal structure breaking inversion symmetry. (39,40) On the other hand, there exist several reports of F5GT exhibiting skyrmionic type-I bubbles of both chiralities and type-II bubbles, which indicates, at best, a weak DMI. (40,41) Recent studies of few-layer F5GT flakes appear to agree with this apparent lack of significant DMI, showing Bloch-type stripe and bubble states with a strong layer number dependence. (42,43) However, there are reports showing DMI-induced Néel-type skyrmions in the partially substituted (Fe0.5Co0.5)5GeTe2, attributed to a noncentrosymmetric structure. (44−46) The matter is further complicated by the study of two types of F5GT─defined by whether the single crystals are quenched or slow-cooled after their high-temperature growth, which appear to show significantly different structural and magnetic properties. (9,47) Therefore, we saw a need for a comprehensive study investigating the potential formation and stability of skyrmions and related real-space spin textures in F5GT.
In this work, we investigate the spin textures hosted in exfoliated flakes of F5GT prepared from single crystals of both the slow-cooled and quenched varieties. Using a combination of magnetometry, scanning transmission X-ray microscopy (STXM), and Lorentz transmission electron microscopy (LTEM), in the slow-cooled type F5GT, we observe the formation of Bloch-type stripe domains, alongside type-I bubbles with Q = 1 and type-II bubbles Q = 0. We demonstrate a significant alteration of the stability of these spin textures below 150 K, which appears to coincide with a change in the uniaxial anisotropy due to the ordering of the Fe1 sublattice around this temperature. Furthermore, we show that the size and shape of the flake sample have a considerable effect on the spin texture formation due to the delicate balance of magnetic energy terms. Finally, we highlight the significant differences in the spin textures observed in the slow-cooled and quenched F5GT flakes.

Results

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Two Types of Fe5GeTe2 Crystals

The crystal structure of F5GT is shown in Figure 1a, revealing the position of three different Fe species within the lattice: Fe1, Fe2, and Fe3. Previous X-ray diffraction and scanning transmission electron microscopy studies have shown that the Fe2 and Fe3 sites have a fixed position, while the Fe1 site is only partially occupied and the Ge site is located on a split position, with the split site occupation depending on the location of Fe1. (16) Accordingly, the in-plane crystal lattice can exhibit interesting supercell structures, characterized by different local ordering of the Fe1 and Ge lattice sites. (39) These local orderings are thought to break inversion symmetry─while the average crystal structure with rhombohedral space group R3̅m is centrosymmetric, the locally ordered regions could possess the noncentrosymmetric space group R3m. (41) Similar to the arguments concerning local vacancies breaking symmetry in F3GT, (35) this broken inversion symmetry in F5GT may result in a local DMI and thus DMI-stabilized helimagnetic spin textures. (39)

Figure 1

Figure 1. Structure and characterization of the slow-cooled and quenched Fe5GeTe2 samples. (a) Crystal structure of Fe5GeTe2, with the positions of Fe1, Fe2, Fe3, Ge, and Te indicated. The half-colored Fe1 and Ge atoms indicate split vacancy sites. (b–e) Magnetization M of the bulk slow-cooled (b,c) and the quenched (d,e) Fe5GeTe2 crystals. Measurements were performed as a function of temperature T following both field-cooling (FC) and field-warming (FW) processes, under a field of 30 mT applied either perpendicular (B⊥c) or parallel (B∥c) to the c crystal axis. (f,g) Optical images of the two investigated slow-cooled flakes, stamped on Si3N4 membranes. Flake 1 was capped with an hBN flake and investigated in the STXM measurements. Flake 2 was investigated in the LTEM measurements. The labeled regions of interest (ROI, red dashed lines) indicate the areas investigated in the imaging experiments. The ROI of flake 1 has a thickness of ∼120 nm, while flake 2 has a thickness of ∼150 nm.

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The crystal structure of F5GT is significantly altered depending on the cooling method utilized after the growth procedure. Bulk F5GT crystals are typically characterized by numerous stacking faults, manifesting in elongated diffraction peaks (streaking) along the c axis. (47) However, this disorder is absent in crystals rapidly quenched from high temperature. (16) This quenching is thought to stabilize a more ordered structural phase which exists above 550 K, resulting in a metastable structure at room temperature. The magnetic properties of crystals fabricated by the two processes have also been reported to be quite different. (9) To explore this, we synthesized bulk F5GT crystals of both varieties (see Materials and Methods).
Magnetometry measurements were first performed on the synthesized bulk crystals (see Materials and Methods). The magnetization M(T) of the slow-cooled bulk F5GT crystal is plotted as a function of temperature in Figure 1b,c, with a 30 mT magnetic field applied either perpendicular or parallel to the c axis, respectively, for both field-cooling (FC) and field-warming (FW) processes. The data reveals that the sample has a TC of 286 K. Figure 1b demonstrates the complex behavior of the crystal for fields applied in-plane (perpendicular to the c axis), where M(T) initially increases with decreasing T, up to maximum value at approximately 160 K, and displays a further cusp at around 110 K. In contrast, for the out-of-plane applied field (parallel to the c axis), M(T) is mostly flat below TC, as shown in Figure 1c. Previous studies have linked this temperature-dependent M to the behavior of the moments on the partially occupied Fe1 sites, which only order at lower temperatures (discussed in more detail later). (9) It has been argued that the decrease in M at low temperatures could be evidence for canting of the moments or even ferrimagnetic ordering. (48)
The M(T) behavior of the quenched sample is considerably different, as shown in Figure 1d,e. Looking at the first FC data in Figure 1d, M(T) initially behaves similarly to the slow-cooled sample, ordering at TC of ∼276 K and showing a steady increase up to a maximum at ∼180 K. However, below 140 K, there is a sudden and dramatic change in M(T) (marked by the dashed red circle). A similar transition is seen beginning at 175 K in the out-of-plane data in Figure 1e (separate crystals were used for each field orientation to reproduce the irreversible behavior). Upon finishing the FC process, and performing further FW and FC protocols, the subsequent data shows that the magnetic behavior of the sample has irreversibly changed: the sample no longer displays the complex M behavior for in-plane fields and exhibits a much higher TC of ∼310 K. Previous work has linked this sudden change to a magneto-structural transition in the quenched crystal: the ordering of the Fe1 moments appears to be linked to a reappearance of the stacking faults within the crystal structure, presumably due to some magnetoelastic behavior. (16) The significant differences in the bulk magnetic behavior naturally raise questions over the nature of the spin textures hosted in exfoliated flakes of each of these crystals.
For the subsequent imaging experiments, flakes of F5GT were stamped onto Si3N4 membranes. In the case of X-ray microscopy, both slow-cooled and quenched F5GT flakes were capped with an exfoliated flake of hexagonal boron nitride (hBN) to prevent further oxidation (see Materials and Methods). For the LTEM measurements, slow-cooled material was utilized, and the flakes were left uncapped to improve the electron transmission through the stack. In the following, we present microscopy data of both types of F5GT flakes. However, contrary to expectations in the literature, we observed significant structural defects and disordered magnetic textures in the quenched flakes, which made their thorough investigation challenging. Thus, we first primarily focus on the well-characterized and reproducible behavior observed in the slow-cooled flake samples and return to the quenched F5GT at the end of the article. Optical micrographs of the two investigated slow-cooled flakes (denoted flake 1 and flake 2) are shown in Figure 1f,g where the region of interest (ROI) examined in subsequent STXM and LTEM images is highlighted.

Microscopy of Slow-Cooled Flakes

STXM acquires magnetic contrast images by measuring the X-ray transmission through the sample and exploiting the effects of X-ray magnetic circular dichroism (XMCD), yielding a signal scaling with the out-of-plane magnetization, mz (see Figure S1 and Materials and Methods). In contrast, LTEM exploits the Lorentz force induced by the local in-plane magnetic fields produced by the sample magnetization to achieve magnetic contrast (see Materials and Methods). The techniques therefore give complementary information, and the in-plane field sensitivity allows LTEM to determine the domain wall type (Bloch or Néel) and therefore distinguish the nature of the spin textures in the sample.
To investigate the stability of spin textures within flakes of the slow-cooled sample, we first performed STXM imaging of the ROI of flake 1. Images were acquired as a function of increasing out-of-plane applied magnetic field, starting from −250 mT [which we call the field sweep (FS) procedure]. The results for a range of temperatures are shown in Figure 2a–d, revealing the formation of stripe and skyrmion states. We defined the TC of the flake sample as the temperature at which we could no longer observe stripe domain contrast at 0 mT, which was found to be ∼275 K, similar to that seen in the bulk. Close to TC, the flake exhibits the formation of dense arrays of bubbles at both negative and positive applied fields, with a stripe state forming around 0 mT, as shown in the 260 K data in Figure 2d. At slightly lower temperatures, such as the 180 K data in Figure 2c, the sample primarily exhibits stripe domains, with only a few isolated bubbles forming at high positive applied fields.

Figure 2

Figure 2. X-ray and LTEM imaging of slow-cooled Fe5GeTe2 under an out-of-plane applied magnetic field. (a–d) X-ray micrographs of the ROI of the exfoliated slow-cooled Fe5GeTe2 flake 1, acquired at a range of temperatures and applied out-of-plane magnetic fields. The images were taken following the field-sweep protocol: at each temperature, the out-of-plane applied magnetic field was increased stepwise after being initialized at −250 mT. The color map scales with the out-of-plane magnetization of the flake, mz (light is down, dark is up). (e) Selected LTEM images of the slow-cooled Fe5GeTe2 flake 2 acquired at 98 K. The upper two panels show the raw contrast achieved when defocusing the electron beam, revealing magnetic contrast of stripe domains and, at higher applied fields, magnetic bubble states. The lower three panels show the in-plane magnetic induction reconstructed using the transport-of-intensity equation (TIE) of the different observed bubble objects: type-II bubbles and type-I bubbles of left- and right-handed chirality. The color wheel indicates the direction of the in-plane magnetic induction.

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However, below 160 K, we observe the onset of a second temperature range of bubble formation, where once again dense arrays of bubbles are nucleated at both negative and positive applied fields, such as in the 120 K data in Figure 3b. Finally, at yet lower temperatures below 100 K, once again, only the formation of stripe domains is observed, as shown in the 40 K data set in Figure 3a. Taken together, the low-temperature behavior of the F5GT flake therefore contrasts starkly with the monodomain switching seen at low temperatures in F3GT. (36) Moreover, the formation of two thermally separated regimes of bubble formation is different from most other skyrmion or bubble-hosting materials, with only one exception, (49) and thus, the interactions responsible for this behavior deserve attention. We shall return to this point in the next section.

Figure 3

Figure 3. X-ray imaging of slow-cooled Fe5GeTe2 under an in-plane applied magnetic field. (a–c) X-ray micrographs of the ROI of the exfoliated Fe5GeTe2 flake 1, acquired at a range of temperatures and applied in-plane magnetic fields. The images were taken following the field-sweep protocol: at each temperature, the in-plane applied magnetic field was increased stepwise after being initialized at −250 mT. The color map is scaled with the out-of-plane magnetization of the flake, mz (light is down, dark is up). Dashed green lines indicate example locations of the linescans used to evaluate the stripe domain spacing. (d) Average stripe domain size d at 0 mT as a function of the sample temperature. Error bars indicate the standard error acquired when averaging the results of multiple line scans across the domains in each image. TK indicates the temperature at which the domain size suddenly changes, which we argue is due to a sudden alteration of the uniaxial anisotropy by the Fe1 sublattice ordering.

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To determine the in-plane structure of the observed magnetic states, we performed LTEM imaging of slow-cooled flake 2, with a summary of the results shown in Figure 2e (further LTEM results shown in Figures S2–S5). The first panel reveals the formation of Bloch-type stripe domains at 0 mT, while the second panel shows the simultaneous formation of three types of bubble states realized at higher applied magnetic fields─a type-II bubble (labeled i) and both left- and right-handed chiralities of type-I bubble (labeled ii and iii, respectively). The next three panels show a local map of the in-plane magnetic induction of a type-II bubble, characterized by two Bloch domain walls connected by a pair of Bloch lines, and skyrmion-like type-I bubbles of both left and right chirality, respectively. Throughout the experiments, we predominantly observed type-II bubbles. The presence a mixture of bubble states with different chiralities is strong evidence that the magnetic textures are primarily stabilized by the dipolar interaction, and indicates a lack of significant DMI, which would otherwise favor one chirality of type-I bubble over the other objects.
We performed further X-ray microscopy measurements of the same slow-cooled flake 1, performing the FS procedure while applying an in-plane magnetic field. Figure 3a–c shows a selection of the resulting X-ray micrographs across three temperatures. The F5GT flake only exhibited stripe domains or in-plane uniformly magnetized states for this field configuration. Notably, we observed that the field required to saturate the flake was less than 50 mT at higher temperatures. However, below 160 K, this saturation field rapidly increased, and at 120 K, we were no longer able to saturate the sample in-plane with the 250 mT maximum field available in the X-ray microscope. Another significant result is the characteristic stripe domain size d at each temperature, as plotted in Figure 3d. This was determined by acquiring line profiles of the stripe domains parallel to the winding direction of the Bloch walls and defined as the peak-to-peak distance in these line profiles. The results reveal an interesting temperature dependence, where d initially increases with decreasing temperature, until there is an abrupt change around 150 K, where there is a dramatic step decrease in d, which then continues to increase at lower temperatures. An explanation for this result will be given in the next section.

Stability of Magnetic Phases

The behavior seen in the X-ray microscopy data described so far is summarized by the magnetic phase diagrams for both out-of-plane and in-plane applied magnetic fields, plotted in Figure 4a,b, respectively. Different markers are utilized to identify the observed magnetic state in each acquired image. In Figure 4a, the formation of dense skyrmion array states is indicated by the bold star markers, highlighting the two temperature regimes where significant bubble formation is observed. The sudden step change in the saturation field for the in-plane field phase diagram can be seen in Figure 4b. We note that the full phase diagram at low temperature could not be explored due to the upper limit of the applied magnetic field in the X-ray microscope (±250 mT). The field asymmetry of the phase diagrams is due only to the field-sweep direction. For the opposite direction of the field sweep, the phase diagram would appear mirrored in the x axis.

Figure 4

Figure 4. Magnetic phase diagrams of slow-cooled Fe5GeTe2. (a,b) Magnetic phase diagrams of the slow-cooled Fe5GeTe2 flake 1, acquired by the field-sweep (FS) process for out-of-plane and in-plane applied magnetic fields, respectively. The presence of the uniformly magnetized (UM, purple crosses), stripe domain (SD, orange circles), dense bubble array (B, green complete stars), isolated bubble (iB, green empty stars), and combined stripe and bubble (B + SD, green squares) states is indicated. Each point corresponds to an acquired X-ray micrograph. The direction of the field sweep, initialized at −250 mT, is indicated by the red arrow. (c) Measured saturation magnetization, MS, of the bulk slow-cooled F5GT crystal, plotted as a function of temperature. (d) Measured uniaxial anisotropy of the bulk slow-cooled F5GT crystal, Keff (orange circles) plotted as a function of temperature, determined from integrating the difference between magnetization loops of the bulk crystal measured with the field parallel and perpendicular to the c axis. Also plotted is the estimated shape anisotropy contribution, Ksh (purple crosses), and the resulting estimated uniaxial anisotropy, KU (red stars), obtained from Keff – Ksh. In all plots, the value of TC and the characteristic transition temperature TK is indicated by vertical gray lines.

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As alluded to above, there is one previous example of a material exhibiting thermally separated skyrmion formation regimes: the multiferroic skyrmion host Cu2OSeO3. (49) In this material, one thermal fluctuation-induced skyrmion phase is found close to TC, which is typical for bulk chiral magnet materials. However, the combination of increasing cubic anisotropy and anisotropic exchange also drives skyrmion formation in a second, low-temperature region. (50) In F5GT, we have observed several interesting behaviors emerging at around 150–160 K, which we label TK, including pronounced changes in the in-plane saturation field, the characteristic stripe domain size, and the stability of bubble states. Inspired by this intriguing result, we investigated the uniaxial anisotropy in our slow-cooled F5GT sample by performing extensive magnetometry measurements.
In Figure 4c,d, we plot both the saturation magnetization MS and the anisotropy of the bulk slow-cooled F5GT crystal as a function of temperature. These values were extracted from magnetization M versus applied field magnetometry measurements (see Materials and Methods, Figure S6). By integrating over hysteresis loops obtained with the field applied perpendicular and parallel to the c axis, the work done, W, to magnetize the sample along each axis can be estimated from (51)
W=μ0∫0MSHdM
(1)
Thus, the effective anisotropy of the sample can be calculated as Keff = WH∥c – WH⊥c. The result is plotted by the orange circles in Figure 4d, showing negative values, and a steady decrease with decreasing temperature until about 160 K, where the trend levels out. The measured Keff contains the shape anisotropy of the single crystal, which can be expected to be in-plane, hence the negative value of Keff. Using the experimentally acquired values of MS, we estimated the contribution of this shape anisotropy following the calculation Ksh=−12μ0NdMS2, which is plotted by the purple crosses in Figure 4d. Note that Nd is defined by the shape of the sample and was estimated to be 0.57. (49) From this, the estimated uniaxial anisotropy KU of the sample is calculated by subtracting Ksh from Keff, with the result also plotted by the red stars in Figure 4d.
While we would stress that this method yields only an estimation of the magnitude of the anisotropy value, the relative values between temperatures are reliable. Thus, the result shows that at higher temperatures, there is likely very little uniaxial anisotropy within the sample. At these temperatures, the flake sample will be dominated by the shape and surface anisotropy contributions. In particular, the observation of out-of-plane domains in the flake indicates that out-of-plane anisotropy remains dominant. However, below the characteristic temperature TK of 150 K, there is a sudden change of KU, corresponding to an increasingly out-of-plane easy-axis. This temperature coincides with the sudden decrease in stripe domain size observed in Figure 3d, the onset of the second region of skyrmion formation at lower T shown in Figure 4a, and the observed increase in the IP saturation field below TK shown in Figure 4b. While we do not have direct evidence from the current work, based on the previous studies utilizing neutron diffraction and Mössbauer spectroscopy, we suggest that all of these behaviors can be ascribed to the ordering of the FeI sublattice. (16) In that work, this ordering also seemed to be associated with an increase in the uniaxial anisotropy of the F5GT sample at lower temperatures, as observed here. In the previous work, the sublattice ordering was shown to occur at 100–120 K, while in our sample, these effects are seen at 150 K, which could be due to differences in the stoichiometry, as reflected by the differing TC values.
Since the stripe domain states are presumably stabilized by the dipolar interaction, their size should be determined by Kittel’s law. (52) The increase of uniaxial anisotropy below TK would result in larger domains since the domain walls become more energetically costly. However, in Figure 3d, the typical domain size showed a sudden decrease below TK. Thus, the observed behavior can not only be described by the change in anisotropy, but also we speculate that the sublattice ordering is also altering the saturation magnetization and/or exchange interaction, although the detail requires further study.
Concerning the thermally separated regimes of bubble formation, further details were acquired during the LTEM imaging experiments. In particular, we observed different behavior of the skyrmion stability at the edge and center of the larger flake 2 sample [labeled as ROI1 (edge) and ROI2 (center) in Figure 1g]. This is illustrated in Figure 5, where the observed density of magnetic bubbles for each applied magnetic field at a range of temperatures is plotted for each measurement technique (once again following the FS procedure from negative to positive field). The data in Figure 5a–c, acquired from the STXM data of flake 1, shows the behavior described thus far: at high temperatures close to TC, a dense array of bubbles forms at both positive and negative applied fields. At 200 K, bubbles are only formed for positive applied fields. However, at temperatures below 150 K, bubble formation is seen across a wide field range. Near identical behavior was observed at similar temperatures at the edge of flake 2, as shown in Figure 5d–f. Note that the higher maximum applied field in the LTEM setup allows us to saturate the sample at all measured temperatures.

Figure 5

Figure 5. Bubble density in the slow-cooled Fe5GeTe2 flake 1 sample. (a–i) Bubble density ρ measured as a function of the applied magnetic field using STXM (a–c) and LTEM (d–i) at selected temperatures for the slow-cooled Fe5GeTe2 flakes. LTEM measurements were performed at both the sample edge (d–f) and center (g–i), as labeled.

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In contrast, different behavior is exhibited at the center of the larger flake 2 sample, as shown in Figure 5g–i: skyrmions are stable across most of the field range at all temperatures, which is reminiscent of typical bubble-hosting uniaxial ferromagnets. (53) This different behavior observed in two regions with the same thickness, but with different proximity to the sample edge, indicates that the direction of the stray field produced by the magnetization may have a dramatic effect on the stability of the magnetic objects within F5GT flakes. Specifically, in the center of the large flake 2, the stray field should be predominantly fixed in the OOP direction. In contrast, at the edge of flake 2, and for the generally smaller flake 1, the stray field will be significantly tilted away from the out-of-plane direction toward the sample boundaries. We speculate that this may suppress the stability of the bubbles for moderate temperatures, presumably by favoring domain walls pointing along the stray-field direction (toward the edge of the sample). However, below 150 K, we suggest that the increased uniaxial anisotropy increases the stability of bubble states in all regions once again. Similar geometry-dependent spin texture stability was observed in LTEM measurements of Fe1.9Ni0.9Pd0.2P. (54)

X-ray Microscopy of Quenched Flakes

Before summarizing the results of the slow-cooled samples, we will briefly discuss the STXM results for the quenched flakes. In general, the magnetic states and behaviors of these samples were difficult to characterize, with each flake displaying nonuniform magnetic states. Despite not reaching decisive conclusions, we nevertheless feel it is useful to highlight the significant differences we observed between these samples and the slow-cooled flakes. Some example X-ray micrographs of one quenched flake are shown in Figure 6a,b, measured at 300 K before any cooling procedure─thus, the ferromagnetic was not ordered at this temperature. In Figure 6b, the vein-like regions of light and dark contrast within a single flake thickness therefore indicate structural inhomogeneities, which seemed to exhibit different magnetic behavior to the surrounding material once cooled. We present two example images from field-sweeps performed at 75 K for both out-of-plane and in-plane applied magnetic fields in Figure 6c,d, respectively. In Figure 6c, lighter and darker magnetic domains appear within the lighter structural contrast regions. This more clearly visible for the in-plane field image shown in Figure 6d, where skyrmion/bubble-like objects filled the vein-like regions of lighter structural contrast. To check that these findings were not the result of an unfortunate selection of F5GT flake, we performed measurements on two additional quenched flakes, one of which was from a separately grown bulk crystal. In some cases, we observed regions of more typical stripe states, but in general, the results were inconsistent, and in all three cases showed inhomogeneous spin texture formation and structural inhomogeneities (see Figures S7 and S8).

Figure 6

Figure 6. X-ray microscopy of the quenched Fe5GeTe2 flake. (a) Overview X-ray micrograph of the quenched F5GT flake. The region of interest (ROI) of subsequent images is indicated. (b) X-ray micrograph of the ROI at 300 K before any cooling, revealing significant contrast indicating structural inhomogeneities within the flake sample. (c,d) Two representative X-ray micrographs of a region of the quenched Fe5GeTe2 flake at 75 K, measured following the field sweep procedure for both out-of-plane and in-plane field, respectively. The images are for a single X-ray polarization, and thus, the contrast is both structural and magnetic in origin, allowing the correlation between magnetic and structural features to be seen.

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The image in Figure 6d is reminiscent of a recently published LTEM study of quenched F5GT flakes, (55) where the authors observed large regions of primarily IP magnetization, together with smaller regions of OOP states hosting skyrmion or Meron states. The large-scale structures appear similar to the vein-like structural contrast features observed in the present data. However, here, we must highlight another study performed on the same quenched F5GT crystals utilized in the present work, which showed more ordered magnetic structures, such as clear stripe and skyrmion states, similar to the slow-cooled flakes studied here. (41) The difference between the two studies is the sample fabrication method: while we used exfoliation to create F5GT flakes, the previous study utilized focused ion beam milling to fabricate F5GT lamellae. Thus, the different behavior may be due to these processes, where the milling via Ga ions or the straining effect of the exfoliation and transfer method may play a role. In particular, the breakdown of the metastable phase in the quenched samples is associated with the FeI sublattice ordering and magneto-structural transition around TK. Thus, in our case, any straining of the flake during fabrication may influence these properties and perhaps lead to the inhomogeneous crystal structure within a single flake.
Within the literature, there is a general sentiment that quenched F5GT should display superior crystal quality in comparison to the slow-cooled F5GT due to possessing a sharper X-ray diffraction pattern, with a markedly reduced amount of stacking faults. (9,47) Thus, it seems many studies have since chosen to focus on quenched F5GT crystals. However, from the perspective of uniform skyrmion and stripe formation, the present data indicates that the slow-cooled F5GT flakes display more reproducible spin texture formation. However, for other experiments and applications, it may be that the properties of the quenched variety are favorable, in particular the increase of TC to above room temperature. Taken altogether, it is clear that the details of crystal structure, in particular the presence of defects and stacking faults, play a crucial role in the magnetic behavior of F5GT via the magnetoelastic coupling, and thus we suggest that both varieties of F5GT should be considered for future studies.

Conclusions

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Past studies of F5GT have reported that the Fe1 sublattice magnetically orders at a substantially lower temperature than TC. (16) Our measurements reveal the effect that this has on the spin texture formation in slow-cooled F5GT flakes, realized via an increase in the uniaxial magnetocrystalline anisotropy exhibited by the sample. This results in a significantly higher in-plane saturation field, a sudden decrease of the characteristic magnetic domain size, and an increase in the stability of type-I and type-II bubbles at low temperatures, leading to a secondary temperature range of skyrmion bubble formation. Moreover, we have shown that the local geometry of the sample also has a significant impact on the bubble stability, where the bubbles appear more stable toward the center of the large investigated flake. For moderate temperatures, the dipolar-induced stray field energy is large enough to suppress the bubble formation in small flakes and at the edges of larger flakes.
Due to their short exposure to air (∼10 min), the top and bottom few nm of our flakes are likely oxidized, which we have not characterized in the present measurements. However, the Bloch-type domain walls observed in the magnetic textures do not imply the presence of interfacial effects such as DMI, unlike in the existing discussion concerning Néel-type skyrmions in F3GT. (31,32,36) In addition, we did not see evidence for the short-scale helical ordering observed in a previous work on F5GT (39)─all observed stripe states consisted of larger regions of uniform magnetization separated by narrow Bloch-type domain walls (see Figure S9). The difference may be because the previous study utilized magnetic force microscopy, which is a surface-sensitive imaging technique, as opposed to the transmission techniques applied here.
The combination of strongly temperature-dependent anisotropy and saturation magnetization results in a complex magnetic phase diagram for slow-cooled F5GT, which is significantly different from thin film and bulk skyrmion systems, (56,57) as well as other 2D magnets. (36) The delicate balance of the magnetic energy terms should enable tuning of the stability of the hosted spin textures in heterostructure stacks, which we anticipate will be widely explored in future works. Finally, our comparison of the behavior of both slow-cooled and quenched F5GT crystals should encourage further investigations into the details of the defects, atomic vacancies, and layer ordering, with a view toward improving the suitability of F5GT for spintronic applications.

Materials and Methods

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Sample Fabrication and Characterization

Single crystals of Fe5GeTe2 were produced in quartz ampules in the presence of iodine as a mineralizer. Powders of the elements were utilized in a ratio of 6:1:2, with the Ge powder placed at the bottom of the ampule and Fe and Te pressed into pellets and placed on top of Ge. After vacuum sealing, the ampules were heated to 750 °C at 120 K/h and were left at this temperature for 2 weeks. For the quenched samples, the ampules were cooled from this high temperature in room-temperature water, while for the slow-cooled samples, the ampules were slowly returned to 21 °C. We performed EDX experiments in a multifunctional TEM (JEOL JEM2800) configured in STEM mode coupled with the NSS spectral imaging system for elemental mapping. This determined the final composition of the slow-cooled samples to be Fe5.3GeTe2.4 (see Figure S10), while previous results have shown the composition of the quenched samples to be Fe4.7GeTe1.9. (41) Magnetometry measurements were performed in a Quantum Design VSM3, with the crystals fixed to a quartz rod sample holder with GE varnish.
Flake samples of both crystal varieties were prepared using an all-dry exfoliation method. First, the FGT bulk crystals were mechanically cleaved and exfoliated onto a polydimethylsiloxane (PDMS) stamp. The PDMS was surveyed using an optical microscope, and flakes with a suitable thickness were selected by their optical contrast. Selected flakes were then stamped onto 100 nm thick Si3N4 membranes, and each was capped by an exfoliated hexagonal boron nitride (hBN) sheet with thickness on the order of 20 nm. The entire process was performed in ambient conditions, with each side of the flakes exposed to the atmosphere for roughly 15 min. Thus, the first few atomic layers of each flake are expected to be oxidized. The flake thicknesses were measured using atomic force microscopy and electron-filtered transmission electron microscopy, yielding values of 120 and 150 nm for the regions of interest in flakes 1 and 2, respectively.

Scanning Transmission X-ray Microscopy

X-ray microscopy was performed with the MAXYMUS endstation at the BESSY II electronic storage ring operated by the Helmholtz-Zentrum Berlin für Materialien and Energie. The Si3N4 membrane chips were mounted to a Cu sample holder and placed inside the microscope chamber. Cooling was achieved by a He flow cyrostat. By controlling the arrangement of four permanent magnets, a magnetic field could be applied in both out-of-plane and in-plane directions, with a maximum magnitude of 250 mT. The X-ray beam was focused to a spot size of approximately 20 nm using a Fresnel zone plate and order-selecting aperture, fixing the maximum spatial resolution. An image was acquired by rastering the sample through the beam spot and measuring the X-ray transmission pixel by pixel using a piezoelectric motor stage. Photons were counted by an avalanche photodiode placed behind the sample. A typical image consisted of a 5 × 5 μm area with a 20 nm pixel size and a dwell time of 1.5 ms, with a total acquisition time of 2–3 min. Magnetic contrast was achieved by exploiting the effects of X-ray magnetic circular dichroism (XMCD) at the Fe L3 edge, with a nominal energy of 708.0 eV. The resulting magnetic signal scales with the out-of-plane magnetization, mz (parallel to the X-ray beam). The presented images of the magnetic domain structure were acquired with a single circular X-ray polarization.

Lorentz Transmission Electron Microscopy

We performed LTEM experiments in a Thermo Fisher Scientific Talos F200x scanning transmission electron microscope (STEM) using the Gatan 636 TEM liquid nitrogen cryo-TEM holder. LTEM images are the summation of the electron–specimen interaction through the thickness projected onto a 2D detector image plane. We cooled the specimen with zero external magnetic field Bext = 0 mT. The objective lens was utilized to apply out-of-plane magnetic fields. When two LTEM images of a magnetic spin texture are acquired at over- and under-defocus values, respectively, the in-plane magnetic inductance of the sample may be reconstructed using the transport-of-intensity equation, assuming a flat electrostatic distribution (thickness).

Data Availability

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The data that support the findings of this study are available from the corresponding author upon request.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnano.4c00853.

  • X-ray absorption spectra of the F5GT flakes; additional LTEM images and accompanying phase diagrams; magnetometry data of the bulk slow-cooled F5GT crystal; additional STXM images of two further quenched F5GT flakes; EDX data; and analysis of the LTEM data concerning the helical state at zero applied field (PDF)

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Author Information

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  • Corresponding Authors
    • Max T. Birch - Max Planck Institute for Intelligent Systems, Heisenbergstraße 3, Stuttgart 70569, Germany;  RIKEN Center for Emergent Matter Science, Hirosawa 2-1, Wako 351-0198, Japan;  Orcidhttps://orcid.org/0000-0001-9320-8561;  Email: [email protected]
    • Marko Burghard - Max Planck Institute for Solid State Research, Heisenbergstraße 1, Stuttgart 70569, Germany;  Email: [email protected]
  • Authors
    • Fehmi S. Yasin - RIKEN Center for Emergent Matter Science, Hirosawa 2-1, Wako 351-0198, Japan;  Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, United States;  Orcidhttps://orcid.org/0000-0001-9382-7565
    • Kai Litzius - Max Planck Institute for Intelligent Systems, Heisenbergstraße 3, Stuttgart 70569, Germany
    • Lukas Powalla - Max Planck Institute for Solid State Research, Heisenbergstraße 1, Stuttgart 70569, Germany;  Orcidhttps://orcid.org/0000-0003-1364-9979
    • Sebastian Wintz - Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, Berlin 14109, Germany;  Orcidhttps://orcid.org/0000-0001-6138-8078
    • Frank Schulz - Max Planck Institute for Intelligent Systems, Heisenbergstraße 3, Stuttgart 70569, Germany;  Orcidhttps://orcid.org/0000-0003-0891-2314
    • Alexander E. Kossak - Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States;  Orcidhttps://orcid.org/0000-0001-9233-1445
    • Markus Weigand - Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, Berlin 14109, Germany
    • Tanja Scholz - Max Planck Institute for Solid State Research, Heisenbergstraße 1, Stuttgart 70569, Germany
    • Bettina V. Lotsch - Max Planck Institute for Solid State Research, Heisenbergstraße 1, Stuttgart 70569, Germany;  University of Munich (LMU), Butenandtstraße 5-13 (Haus D), München 81377, Germany;  Orcidhttps://orcid.org/0000-0002-3094-303X
    • Gisela Schütz - Max Planck Institute for Intelligent Systems, Heisenbergstraße 3, Stuttgart 70569, Germany
    • Xiuzhen Z. Yu - RIKEN Center for Emergent Matter Science, Hirosawa 2-1, Wako 351-0198, Japan;  Orcidhttps://orcid.org/0000-0003-3136-7289
  • Funding

    Open access funded by Max Planck Society.

  • Notes
    The authors declare no competing financial interest.

    This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript or allow others to do so for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (https://www.energy.gov/doe-public-access-plan).

Acknowledgments

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The authors thank the Helmholtz-Zentrum Berlin for the allocation of synchrotron radiation beamtime. We are grateful to the Nanostructuring Lab (NSL) at the Max Planck Institute for Solid State Research for technical support. M.B. is grateful for support by the Deutsche Forschungsgemeinschaft (DFG) through SPP-2244 “2D Materials─Physics of van der Waals [hetero]structures” via Grant BU 1125/12-1. X.Z.Y. acknowledges Grants-In-Aid for Scientific Research (A) (grant no. 19H00660) from the Japan Society for the Promotion of Science (JSPS) and the Japan Science and Technology Agency (JST) CREST program (grant no. JPMJCR20T1), Japan. Research sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the US Department of Energy.

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    Park, S. Y.; Kim, D. S.; Liu, Y.; Hwang, J.; Kim, Y.; Kim, W.; Kim, J. Y.; Petrovic, C.; Hwang, C.; Mo, S. K. Controlling the Magnetic Anisotropy of the van der Waals Ferromagnet Fe3GeTe2 through Hole Doping. Nano Lett. 2020, 20, 95– 100,  DOI: 10.1021/acs.nanolett.9b03316
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    13
    Controlling the Magnetic Anisotropy of the van der Waals Ferromagnet Fe3GeTe2 through Hole Doping
    Park, Se Young; Kim, Dong Seob; Liu, Yu; Hwang, Jinwoong; Kim, Younghak; Kim, Wondong; Kim, Jae-Young; Petrovic, Cedomir; Hwang, Choongyu; Mo, Sung-Kwan; Kim, Hyung-jun; Min, Byoung-Chul; Koo, Hyun Cheol; Chang, Joonyeon; Jang, Chaun; Choi, Jun Woo; Ryu, Hyejin
    Nano Letters (2020), 20 (1), 95-100CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Identifying material parameters affecting properties of ferromagnets is key to optimize materials better suited for spintronics. Magnetic anisotropy is of particular importance in van der Waals magnets, since it not only influences magnetic and spin transport properties, but also is essential to stabilizing magnetic order in the two dimensional limit. Here, we report that hole doping effectively modulates the magnetic anisotropy of a van der Waals ferromagnet, and explore the phys. origin of this effect. Fe3-xGeTe2 nanoflakes show a significant suppression of the magnetic anisotropy with hole doping. Electronic structure measurements and calcns. reveal that the chem. potential shift assocd. with hole doping is responsible for the reduced magnetic anisotropy by decreasing the energy gain from the spin-orbit induced band splitting. Our findings provide an understanding of the intricate connection between electronic structures and magnetic properties in two-dimensional magnets and propose a method to engineer magnetic properties through doping.
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  14. 14
    Birch, M. T.; Powalla, L.; Litzius, K.; Nehruji, V.; Hovorka, O.; Wintz, S.; Schulz, F.; Mayoh, D. A.; Balakrishnan, G.; Weigand, M.; Burghard, M.; Schütz, G. Control of Stripe, Skyrmion and Skyrmionium Formation in the 2D Magnet Fe3–xGeTe2 by Varying Composition. 2D Mater. 2024, 11, 025008,  DOI: 10.1088/2053-1583/ad1a6b
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    There is no corresponding record for this reference.
  15. 15
    Seo, J.; Kim, D. Y.; An, E. S.; Kim, K.; Kim, G. Y.; Hwang, S. Y.; Kim, D. W.; Jang, B. G.; Kim, H.; Eom, G. Nearly Room Temperature Ferromagnetism in a Magnetic Metal-Rich van der Waals Metal. Sci. Adv. 2020, 6, eaay8912  DOI: 10.1126/sciadv.aay8912
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    There is no corresponding record for this reference.
  16. 16
    May, A. F.; Ovchinnikov, D.; Zheng, Q.; Hermann, R.; Calder, S.; Huang, B.; Fei, Z.; Liu, Y.; Xu, X.; McGuire, M. A. Ferromagnetism Near Room Temperature in the Cleavable van der Waals Crystal Fe5GeTe2. ACS Nano 2019, 13, 4436– 4442,  DOI: 10.1021/acsnano.8b09660
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    16
    Ferromagnetism Near Room Temperature in the Cleavable van der Waals Crystal Fe5GeTe2
    May, Andrew F.; Ovchinnikov, Dmitry; Zheng, Qiang; Hermann, Raphael; Calder, Stuart; Huang, Bevin; Fei, Zaiyao; Liu, Yaohua; Xu, Xiaodong; McGuire, Michael A.
    ACS Nano (2019), 13 (4), 4436-4442CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Two-dimensional materials with intrinsic functionality are becoming increasingly important in exploring fundamental condensed matter science and for developing advanced technologies. Bulk crystals that can be exfoliated are particularly relevant to these pursuits as they provide the opportunity to study the role of phys. dimensionality and explore device physics in highly cryst. samples and designer heterostructures in a routine manner. Magnetism is a key element in these endeavors; however, relatively few cleavable materials are magnetic and none possess magnetic order at ambient conditions. Here, we introduce Fe5-xGeTe2 as a cleavable material with ferromagnetic behavior at room temp. We established intrinsic magnetic order at room temp. in bulk crystals (TC = 310 K) through magnetization measurements and in exfoliated, thin flakes (TC ≈ 280 K) using the anomalous Hall effect. Our work reveals Fe5GeTe2 as a prime candidate for incorporating intrinsic magnetism into functional van der Waals heterostructures and devices near room temp.
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  17. 17
    May, A. F.; Du, M.-H.; Cooper, V. R.; McGuire, M. A. Tuning Magnetic Order in the van der Waals Metal Fe5GeTe2 by Cobalt Substitution. Phys. Rev. Mater. 2020, 4, 074008,  DOI: 10.1103/PhysRevMaterials.4.074008
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    17
    Tuning magnetic order in the van der Waals metal Fe5GeTe2 by cobalt substitution
    May, Andrew F.; Du, Mao-Hua; Cooper, Valentino R.; McGuire, Michael A.
    Physical Review Materials (2020), 4 (7), 074008CODEN: PRMHBS; ISSN:2475-9953. (American Physical Society)
    This paper is a contribution to the joint Phys. Review Applied and Phys. Review Materials collection titledTwo-Dimensional Materials and Devices. Fe5-xGeTe2 is a van der Waals material with one of the highest reported bulk Curie temps., TC'310K. In this study, theor. calcns. and expts. are utilized to demonstrate that the magnetic ground state is highly sensitive to local at. arrangements and the interlayer stacking. Cobalt substitution is found to be an effective way to manipulate the magnetic properties while also increasing the ordering temp. In particular, cobalt substitution up to '30% enhances TC and changes the magnetic anisotropy, while '50% cobalt substitution yields an antiferromagnetic state. Single crystal x-ray diffraction evidences a structural change upon increasing the cobalt concn., with a rhombohedral cell obsd. in the parent material and a primitive cell obsd. for '46% cobalt content relative to iron. First-principles calcns. demonstrate that it is a combination of high cobalt content and the concomitant change to primitive layer stacking that produces antiferromagnetic order. These results illustrate the sensitivity of magnetism in Fe5-xGeTe2 to compn. and structure, and emphasize the important role of local structural order-disorder and layer stacking in cleavable magnetic materials.
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  18. 18
    Mayoh, D. A.; Wood, G. D. A.; Holt, S. J. R.; Beckett, G.; Dekker, E. J. L.; Lees, M. R.; Balakrishnan, G. Effects of Fe Deficiency and Co Substitution in Polycrystalline and Single Crystals of Fe3GeTe2. Cryst. Growth Des. 2021, 21, 6786– 6792,  DOI: 10.1021/acs.cgd.1c00684
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  19. 19
    May, A. F.; Yan, J.; Hermann, R.; Du, M.-H.; McGuire, M. A. Tuning the Room Temperature Ferromagnetism in Fe5GeTe2 by Arsenic Substitution. 2D Mater. 2022, 9, 015013,  DOI: 10.1088/2053-1583/ac34d9
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    19
    Tuning the room temperature ferromagnetism in Fe5GeTe2 by arsenic substitution
    May, Andrew F.; Yan, Jiaqiang; Hermann, Raphael; Du, Mao-Hua; McGuire, Michael A.
    2D Materials (2022), 9 (1), 015013CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
    In order to tune the magnetic properties of the cleavable high-Curie temp. ferromagnet Fe5-xGeTe2, the effect of increasing the electron count through arsenic substitution has been investigated. Small addns. of arsenic (2.5% and 5%) seemingly enhance ferromagnetic order in polycryst. samples by quenching fluctuations on one of the three magnetic sublattices, whereas larger As concns. decrease the ferromagnetic Curie temp. (TC) and satn. magnetization. This work also describes the growth and characterization of Fe4.8AsTe2 single crystals that are structurally analogous to Fe5-xGeTe2 but with some phase stability complications. Magnetization measurements reveal dominant antiferromagnetic behavior in Fe4.8AsTe2 with a N´eel temp. of TN ≈ 42 K. A field-induced spin-flop below TN results in a switch from neg. to pos. magnetoresistance, with significant hysteresis causing butterfly-shaped resistance loops. In addn. to reporting the properties of Fe4.8AsTe2, this work shows the importance of manipulating the individual magnetic sublattices in Fe5-xGeTe2 and motivates further efforts to control the magnetic properties in related materials by fine tuning of the Fermi energy or crystal chem.
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  20. 20
    Deng, Y.; Yu, Y.; Song, Y.; Zhang, J.; Wang, N. Z.; Sun, Z.; Yi, Y.; Wu, Y. Z.; Wu, S.; Zhu, J.; Wang, J.; Chen, X. H.; Zhang, Y. Gate-Tunable Room-Temperature Ferromagnetism in Two-Dimensional Fe3GeTe2. Nature 2018, 563, 94– 99,  DOI: 10.1038/s41586-018-0626-9
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    20
    Gate-tunable room-temperature ferromagnetism in two-dimensional Fe3GeTe2
    Deng, Yujun; Yu, Yijun; Song, Yichen; Zhang, Jingzhao; Wang, Nai Zhou; Sun, Zeyuan; Yi, Yangfan; Wu, Yi Zheng; Wu, Shiwei; Zhu, Junyi; Wang, Jing; Chen, Xian Hui; Zhang, Yuanbo
    Nature (London, United Kingdom) (2018), 563 (7729), 94-99CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Materials research has driven the development of modern nano-electronic devices. In particular, research in magnetic thin films has revolutionized the development of spintronic devices1,2 because identifying new magnetic materials is key to better device performance and design. Van der Waals crystals retain their chem. stability and structural integrity down to the monolayer and, being atomically thin, are readily tuned by various kinds of gate modulation3,4. Recent expts. have demonstrated that it is possible to obtain two-dimensional ferromagnetic order in insulating Cr2Ge2Te6 (ref. 5) and CrI3 (ref. 6) at low temps. Here we develop a device fabrication technique and isolate monolayers from the layered metallic magnet Fe3GeTe2 to study magnetotransport. We find that the itinerant ferromagnetism persists in Fe3GeTe2 down to the monolayer with an out-of-plane magnetocryst. anisotropy. The ferromagnetic transition temp., Tc, is suppressed relative to the bulk Tc of 205 K in pristine Fe3GeTe2 thin flakes. An ionic gate, however, raises Tc to room temp., much higher than the bulk Tc. The gate-tunable room-temp. ferromagnetism in two-dimensional Fe3GeTe2 opens up opportunities for potential voltage-controlled magnetoelectronics7-11 based on atomically thin van der Waals crystals.
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  21. 21
    Tan, C.; Xie, W.-Q.; Zheng, G.; Aloufi, N.; Albarakati, S.; Algarni, M.; Li, J.; Partridge, J.; Culcer, D.; Wang, X.; Yi, J. B.; Tian, M.; Xiong, Y.; Zhao, Y.-J.; Wang, L. Gate-Controlled Magnetic Phase Transition in a van der Waals Magnet Fe5GeTe2. Nano Lett. 2021, 21, 5599– 5605,  DOI: 10.1021/acs.nanolett.1c01108
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    21
    Gate-Controlled Magnetic Phase Transition in a van der Waals Magnet Fe5GeTe2
    Tan, Cheng; Xie, Wen-Qiang; Zheng, Guolin; Aloufi, Nuriyah; Albarakati, Sultan; Algarni, Meri; Li, Junbo; Partridge, James; Culcer, Dimitrie; Wang, Xiaolin; Yi, Jia Bao; Tian, Mingliang; Xiong, Yimin; Zhao, Yu-Jun; Wang, Lan
    Nano Letters (2021), 21 (13), 5599-5605CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Magnetic van der Waals (vdW) materials are poised to enable all-elec. control of magnetism in the two-dimensional limit. However, tuning the magnetic ground state in vdW itinerant ferromagnets by voltage-induced charge doping remains a significant challenge, due to the extremely large carrier densities in these materials. Here, by cleaving the vdW itinerant ferromagnet Fe5GeTe2 (F5GT) into 5.4 nm (around two unit cells), we find that the ferromagnetism (FM) in F5GT can be substantially tuned by the thickness. Moreover, by utilizing a solid protonic gate, an electron doping concn. of above 1021 cm-3 has been exhibited in F5GT nanosheets. Such a high carrier accumulation exceeds that possible in widely used elec. double-layer transistors (EDLTs) and surpasses the intrinsic carrier d. of F5GT. Importantly, it is accompanied by a magnetic phase transition from FM to antiferromagnetism (AFM). The realization of an antiferromagnetic phase in nanosheet F5GT suggests the promise of applications in high-temp. antiferromagnetic vdW devices and heterostructures.
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  22. 22
    Tomasello, R.; Martinez, E.; Zivieri, R.; Torres, L.; Carpentieri, M.; Finocchio, G. A Strategy for the Design of Skyrmion Racetrack Memories. Sci. Rep. 2014, 4, 6784,  DOI: 10.1038/srep06784
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    22
    A strategy for the design of skyrmion racetrack memories
    Tomasello, R.; Martinez, E.; Zivieri, R.; Torres, L.; Carpentieri, M.; Finocchio, G.
    Scientific Reports (2014), 4 (), 6784CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
    Magnetic storage based on racetrack memory is very promising for the design of ultra-dense, low-cost and low-power storage technol. Information can be coded in a magnetic region between two domain walls or, as predicted recently, in topol. magnetic objects known as skyrmions. Here, we show the technol. advantages and limitations of using Bloch and Ne´el skyrmions manipulated by spin current generated within the ferromagnet or via the spin-Hall effect arising from a non-magnetic heavy metal underlayer. We found that the Ne´el skyrmion moved by the spin-Hall effect is a very promising strategy for technol. implementation of the next generation of skyrmion racetrack memories (zero field, high thermal stability, and ultra-dense storage). We employed micromagnetics reinforced with an anal. formulation of skyrmion dynamics that we developed from the Thiele equation. We identified that the excitation, at high currents, of a breathing mode of the skyrmion limits the maximal velocity of the memory.
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  23. 23
    Grollier, J.; Querlioz, D.; Camsari, K.; Everschor-Sitte, K.; Fukami, S.; Stiles, M. D. Neuromorphic Spintronics. Nat. Electron. 2020, 3, 360– 370,  DOI: 10.1038/s41928-019-0360-9
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    Mühlbauer, S.; Binz, B.; Jonietz, F.; Pfleiderer, C.; Rosch, A.; Neubauer, A.; Georgii, R.; Böni, P. Skyrmion Lattice in a Chiral Magnet. Science 2009, 323, 915– 919,  DOI: 10.1126/science.1166767
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    24
    Skyrmion Lattice in a Chiral Magnet
    Muehlbauer, S.; Binz, B.; Jonietz, F.; Pfleiderer, C.; Rosch, A.; Neubauer, A.; Georgii, R.; Boeni, P.
    Science (Washington, DC, United States) (2009), 323 (5916), 915-919CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Skyrmions represent topol. stable field configurations with particle-like properties. We used neutron scattering to observe the spontaneous formation of a two-dimensional lattice of skyrmion lines, a type of magnetic vortex, in the chiral itinerant-electron magnet MnSi. The skyrmion lattice stabilizes at the border between paramagnetism and long-range helimagnetic order perpendicular to a small applied magnetic field regardless of the direction of the magnetic field relative to the at. lattice. Our study exptl. establishes magnetic materials lacking inversion symmetry as an arena for new forms of cryst. order composed of topol. stable spin states.
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  25. 25
    Yu, X. Z.; Onose, Y.; Kanazawa, N.; Park, J. H.; Han, J. H.; Matsui, Y.; Nagaosa, N.; Tokura, Y. Real-Space Observation of a Two-Dimensional Skyrmion Crystal. Nature 2010, 465, 901– 904,  DOI: 10.1038/nature09124
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    25
    Real-space observation of a two-dimensional skyrmion crystal
    Yu, X. Z.; Onose, Y.; Kanazawa, N.; Park, J. H.; Han, J. H.; Matsui, Y.; Nagaosa, N.; Tokura, Y.
    Nature (London, United Kingdom) (2010), 465 (7300), 901-904CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    Crystal order is not restricted to the periodic at. array, but can also be found in electronic systems such as the Wigner crystal or as orbital order, stripe order and magnetic order. In the case of magnetic order, spins align parallel to each other in ferromagnets and antiparallel in antiferromagnets. In other, less conventional, cases, spins can sometimes form highly nontrivial structures called spin textures. Among them is the unusual, topol. stable skyrmion spin texture, in which the spins point in all the directions wrapping a sphere. The skyrmion configuration in a magnetic solid is anticipated to produce unconventional spin-electronic phenomena such as the topol. Hall effect. The crystn. of skyrmions as driven by thermal fluctuations has recently been confirmed in a narrow region of the temp./magnetic field (T-B) phase diagram in neutron scattering studies of the three-dimensional helical magnets MnSi (ref. 17) and Fe1-xCoxSi (ref. 22). Here the authors report real-space imaging of a two-dimensional skyrmion lattice in a thin film of Fe0.5Co0.5Si using Lorentz TEM. With a magnetic field of 50-70 mT applied normal to the film, the authors observe skyrmions as a hexagonal arrangement of swirling spin textures, with a lattice spacing of 90 nm. The related T-B phase diagram is in good agreement with Monte Carlo simulations. In this two-dimensional case, the skyrmion crystal seems very stable and appears over a wide range of the phase diagram, including near zero temp. Such a controlled nanometer-scale spin topol. in a thin film may be useful in observing unconventional magneto-transport effects.
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  26. 26
    Back, C.; Cros, V.; Ebert, H.; Everschor-Sitte, K.; Fert, A.; Garst, M.; Ma, T.; Mankovsky, S.; Monchesky, T. L.; Mostovoy, M. The 2020 Skyrmionics Roadmap. J. Phys. D: Appl. Phys. 2020, 53, 363001,  DOI: 10.1088/1361-6463/ab8418
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    The 2020 skyrmionics roadmap
    Back, C.; Cros, V.; Ebert, H.; Everschor-Sitte, K.; Fert, A.; Garst, M.; Ma, Tianping; Mankovsky, S.; Monchesky, T. L.; Mostovoy, M.; Nagaosa, N.; Parkin, S. S. P.; Pfleiderer, C.; Reyren, N.; Rosch, A.; Taguchi, Y.; Tokura, Y.; von Bergmann, K.; Zang, Jiadong
    Journal of Physics D: Applied Physics (2020), 53 (36), 363001CODEN: JPAPBE; ISSN:0022-3727. (IOP Publishing Ltd.)
    The notion of non-trivial topol. winding in condensed matter systems represents a major area of present-day theor. and exptl. research. Magnetic materials offer a versatile platform that is particularly amenable for the exploration of topol. spin solitons in real space such as skyrmions. First identified in non-centrosym. bulk materials, the rapidly growing zool. of materials systems hosting skyrmions and related topol. spin solitons includes bulk compds., surfaces, thin films, heterostructures, nano-wires and nano-dots. This underscores an exceptional potential for major breakthroughs ranging from fundamental questions to applications as driven by an interdisciplinary exchange of ideas between areas in magnetism which traditionally have been pursued rather independently. The skyrmionics Roadmap provides a review of the present state of the art and the wide range of research directions and strategies currently under way. These are, for instance, motivated by the identification of the fundamental structural properties of skyrmions and related textures, processes of nucleation and annihilation in the presence of non-trivial topol. winding, an exceptionally efficient coupling to spin currents generating spin transfer torques at tiny current densities, as well as the capability to purpose-design broad-band spin dynamic and logic devices.
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  27. 27
    Büttner, F.; Lemesh, I.; Beach, G. S. D. Theory of Isolated Magnetic Skyrmions: From Fundamentals to Room Temperature Applications. Sci. Rep. 2018, 8, 4464,  DOI: 10.1038/s41598-018-22242-8
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    27
    Theory of isolated magnetic skyrmions: From fundamentals to room temperature applications
    Buttner Felix; Lemesh Ivan; Beach Geoffrey S D
    Scientific reports (2018), 8 (1), 4464 ISSN:.
    Magnetic skyrmions are topological quasiparticles of great interest for data storage applications because of their small size, high stability, and ease of manipulation via electric current. However, although models exist for some limiting cases, there is no universal theory capable of accurately describing the structure and energetics of all skyrmions. The main barrier is the complexity of non-local stray field interactions, which are usually included through crude approximations. Here we present an accurate analytical framework to treat isolated skyrmions in any material, assuming only a circularly-symmetric 360° domain wall profile and a homogeneous magnetization profile in the out-of-plane direction. We establish the first rigorous criteria to distinguish stray field from DMI skyrmions, resolving a major dispute in the community. We discover new phases, such as bi-stability, a phenomenon unknown in magnetism so far. We predict materials for sub-10 nm zero field room temperature stable skyrmions suitable for applications. Finally, we derive analytical equations to describe current-driven dynamics, find a topological damping, and show how to engineer materials in which compact skyrmions can be driven at velocities >1000 m/s.
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  28. 28
    Han, M.-G.; Garlow, J. A.; Liu, Y.; Zhang, H.; Li, J.; DiMarzio, D.; Knight, M. W.; Petrovic, C.; Jariwala, D.; Zhu, Y. Topological Magnetic-Spin Textures in Two-Dimensional van der Waals Cr2Ge2Te6. Nano Lett. 2019, 19, 7859– 7865,  DOI: 10.1021/acs.nanolett.9b02849
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    Topological Magnetic-Spin Textures in Two-Dimensional van der Waals Cr2Ge2Te6
    Han, Myung-Geun; Garlow, Joseph A.; Liu, Yu; Zhang, Huiqin; Li, Jun; DiMarzio, Donald; Knight, Mark W.; Petrovic, Cedomir; Jariwala, Deep; Zhu, Yimei
    Nano Letters (2019), 19 (11), 7859-7865CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Two-dimensional (2D) van der Waals (vdW) materials show a range of profound phys. properties that can be tailored through their incorporation in heterostructures and manipulated with external forces. The recent discovery of long-range ferromagnetic order down to at. layers provides an addnl. degree of freedom in engineering 2-dimensional materials and their heterostructure devices for spintronics, valleytronics, and magnetic tunnel junction switches. Here, using direct imaging by cryo-Lorentz TEM topol. nontrivial magnetic-spin states, skyrmionic bubbles, can be realized in exfoliated insulating 2-dimensional vdW Cr2Ge2Te6. Due to the competition between dipolar interactions and uniaxial magnetic anisotropy, hexagonally packed nanoscale bubble lattices emerge by field cooling with magnetic field applied along the out-of-plane direction. Despite a range of topol. spin textures in stripe domains arising due to pair formation and annihilation of Bloch lines, bubble lattices with single chirality are prevalent. Observation of topol. nontrivial homochiral skyrmionic bubbles in exfoliated vdW materials provides a new avenue for novel quantum states in atomically thin insulators for magneto-electronic and quantum devices.
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  29. 29
    Wu, Y.; Francisco, B.; Chen, Z.; Wang, W.; Zhang, Y.; Wan, C.; Han, X.; Chi, H.; Hou, Y.; Lodesani, A.; Yin, G.; Liu, K.; Cui, Y.-t.; Wang, K. L.; Moodera, J. S. A Van der Waals Interface Hosting Two Groups of Magnetic Skyrmions. Adv. Mater. 2022, 34, 2110583,  DOI: 10.1002/adma.202110583
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    29
    A Van der Waals Interface Hosting Two Groups of Magnetic Skyrmions
    Wu, Yingying; Francisco, Brian; Chen, Zhijie; Wang, Wei; Zhang, Yu; Wan, Caihua; Han, Xiufeng; Chi, Hang; Hou, Yasen; Lodesani, Alessandro; Yin, Gen; Liu, Kai; Cui, Yong-tao; Wang, Kang L.; Moodera, Jagadeesh S.
    Advanced Materials (Weinheim, Germany) (2022), 34 (16), 2110583CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Multiple magnetic skyrmion phases add an addnl. degree of freedom for skyrmion-based ultrahigh-d. spin memory devices. Extending the field to 2D van der Waals magnets is a rewarding challenge, where the realizable degree of freedoms (e.g., thickness, twist angle, and elec. gating) and high skyrmion d. result in intriguing new properties and enhanced functionality. In this work, a van der Waals interface, formed by two 2D ferromagnets Cr2Ge2Te6 and Fe3GeTe2 with a Curie temp. of ≈65 and ≈205 K, resp., hosting two groups of magnetic skyrmions, is reported. Two sets of topol. Hall effect signals are obsd. below 6s0 K when Cr2Ge2Te6 is magnetically ordered. These two groups of skyrmions are directly imaged using magnetic force microscopy, and supported by micromagnetic simulations. Interestingly, the magnetic skyrmions persist in the heterostructure with zero applied magnetic field. The results are promising for the realization of skyrmionic devices based on van der Waals heterostructures hosting multiple skyrmion phases.
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  30. 30
    Ding, B.; Li, Z.; Xu, G.; Li, H.; Hou, Z.; Liu, E.; Xi, X.; Xu, F.; Yao, Y.; Wang, W. Observation of Magnetic Skyrmion Bubbles in a van der Waals Ferromagnet Fe3GeTe2. Nano Lett. 2020, 20, 868– 873,  DOI: 10.1021/acs.nanolett.9b03453
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    30
    Observation of Magnetic Skyrmion Bubbles in a van der Waals Ferromagnet Fe3GeTe2
    Ding, Bei; Li, Zefang; Xu, Guizhou; Li, Hang; Hou, Zhipeng; Liu, Enke; Xi, Xuekui; Xu, Feng; Yao, Yuan; Wang, Wenhong
    Nano Letters (2020), 20 (2), 868-873CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Two-dimensional (2D) van der Waals (vdW) magnetic materials have recently been introduced as a new horizon in materials science, and they enable potential applications for next-generation spintronic devices. Here, the observations of stable Bloch-type magnetic skyrmions in single crystals of 2-dimensional vdW Fe3GeTe2 (FGT) are reported by using in situ Lorentz TEM. The ground-state magnetic stripe domains in FGT transform into skyrmion bubbles when an external magnetic field is applied perpendicularly to the (001) thin plate with temps. below the Curie temp. TC are found. Most interestingly, a hexagonal lattice of skyrmion bubbles is obtained via field-cooling manipulation with magnetic field applied along the [001] direction. Owing to their topol. stability, the skyrmion bubble lattices are stable to large field-cooling tilted angles and further reproduced by using the micromagnetic simulations. These observations directly demonstrate that the 2-dimensional vdW FGT possesses a rich variety of topol. spin textures, being of great promise for future applications in the field of spintronics.
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  31. 31
    Park, T.-E.; Peng, L.; Liang, J.; Hallal, A.; Yasin, F. S.; Zhang, X.; Song, K. M.; Kim, S. J.; Kim, K.; Weigand, M. Néel-Type Skyrmions and Their Current-Induced Motion in van der Waals Ferromagnet-Based Heterostructures. Phys. Rev. B 2021, 103, 104410,  DOI: 10.1103/physrevb.103.104410
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    Peng, L.; Yasin, F. S.; Park, T.-E.; Kim, S. J.; Zhang, X.; Nagai, T.; Kimoto, K.; Woo, S.; Yu, X. Tunable Néel–Bloch Magnetic Twists in Fe3GeTe2 with van der Waals Structure. Adv. Funct. Mater. 2021, 31, 2103583,  DOI: 10.1002/adfm.202103583
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    32
    Tunable Neel-Bloch Magnetic Twists in Fe3GeTe2 with van der Waals Structure
    Peng, Licong; Yasin, Fehmi S.; Park, Tae-Eon; Kim, Sung Jong; Zhang, Xichao; Nagai, Takuro; Kimoto, Koji; Woo, Seonghoon; Yu, Xiuzhen
    Advanced Functional Materials (2021), 31 (37), 2103583CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The advent of ferromagnetism in 2D van der Waals (vdW) magnets has stimulated high interest in exploring topol. magnetic textures, such as skyrmions for use in future skyrmion-based spintronic devices. To engineer skyrmions in vdW magnets by transforming Bloch-type magnetic bubbles into Neel-type skyrmions, a heavy metal/vdW magnetic thin film heterostructure has been made to induce interfacial Dzyaloshinskii-Moriya interaction (DMI). However, the unambiguous identification of the magnetic textures inherent to vdW magnets, for example, whether the magnetic twists (skyrmions/domain walls) are Neel- or Bloch-type, is unclear. Here we demonstrate that the magnetic twists can be tuned between Neel and Bloch-type in the vdW magnet Fe3GeTe2 (FGT) with/without interfacial DMI. We use an in-plane magnetic field to align the modulation wavevector q of the magnetizations in order to distinguish the Neel- or Bloch-type magnetic twists. We observe that q is perpendicular to the in-plane field in the heterostructure (Pt/oxidized-FGT/FGT/oxidized-FGT), while q aligns at a rotated angle with respect to the field direction in the FGT thin plate thinned from bulk. We find that the aligned domain wall twists hold fan-like modulations, coinciding qual. with our computational results.
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  33. 33
    Wu, Y.; Zhang, S.; Zhang, J.; Wang, W.; Zhu, Y. L.; Hu, J.; Yin, G.; Wong, K.; Fang, C.; Wan, C. Néel-Type Skyrmion in WTe2/Fe3GeTe2 van der Waals Heterostructure. Nat. Commun. 2020, 11, 3860,  DOI: 10.1038/s41467-020-17566-x
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    33
    Neel-type skyrmion in WTe2/Fe3GeTe2 van der Waals heterostructure
    Wu Yingying; Yin Gen; Wong Kin; Shao Qiming; Wang Kang L; Zhang Senfu; Zhang Junwei; Zhang Xixiang; Wang Wei; Zhu Yang Lin; Mao Zhiqiang; Hu Jin; Fang Chi; Wan Caihua; Han Xiufeng; Taniguchi Takashi; Watanabe Kenji; Zang Jiadong
    Nature communications (2020), 11 (1), 3860 ISSN:.
    The promise of high-density and low-energy-consumption devices motivates the search for layered structures that stabilize chiral spin textures such as topologically protected skyrmions. At the same time, recently discovered long-range intrinsic magnetic orders in the two-dimensional van der Waals materials provide a new platform for the discovery of novel physics and effects. Here we demonstrate the Dzyaloshinskii-Moriya interaction and Neel-type skyrmions are induced at the WTe2/Fe3GeTe2 interface. Transport measurements show the topological Hall effect in this heterostructure for temperatures below 100 K. Furthermore, Lorentz transmission electron microscopy is used to directly image Neel-type skyrmion lattice and the stripe-like magnetic domain structures as well. The interfacial coupling induced Dzyaloshinskii-Moriya interaction is estimated to have a large energy of 1.0 mJ m(-2). This work paves a path towards the skyrmionic devices based on van der Waals layered heterostructures.
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  34. 34
    Yang, M. Creation of Skyrmions in van der Waals Ferromagnet Fe3GeTe2 on (Co/Pd)n Superlattice. Sci. Adv. 2020, 6, eabb5157  DOI: 10.1126/sciadv.abb5157
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    Chakraborty, A.; Srivastava, A. K.; Sharma, A. K.; Gopi, A. K.; Mohseni, K.; Ernst, A.; Deniz, H.; Hazra, B. K.; Das, S.; Sessi, P.; Kostanovskiy, I.; Ma, T.; Meyerheim, H. L.; Parkin, S. S. P. Magnetic Skyrmions in a Thickness Tunable 2D Ferromagnet from a Defect Driven Dzyaloshinskii–Moriya Interaction. Adv. Mater. 2022, 34, 2108637,  DOI: 10.1002/adma.202108637
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    There is no corresponding record for this reference.
  36. 36
    Birch, M. T.; Powalla, L.; Wintz, S.; Hovorka, O.; Litzius, K.; Loudon, J.; Turnbull, L.; Nehruji, V.; Son, K.; Bubeck, C. History-Dependent Domain and Skyrmion Formation in 2D van der Waals Magnet Fe3GeTe2. Nat. Commun. 2022, 13, 3035,  DOI: 10.1038/s41467-022-30740-7
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    36
    History-dependent domain and skyrmion formation in 2D van der Waals magnet Fe3GeTe2
    Birch, M. T.; Powalla, L.; Wintz, S.; Hovorka, O.; Litzius, K.; Loudon, J. C.; Turnbull, L. A.; Nehruji, V.; Son, K.; Bubeck, C.; Rauch, T. G.; Weigand, M.; Goering, E.; Burghard, M.; Schuetz, G.
    Nature Communications (2022), 13 (1), 3035CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
    The discovery of two-dimensional magnets has initiated a new field of research, exploring both fundamental low-dimensional magnetism, and prospective spintronic applications. Recently, observations of magnetic skyrmions in the 2D ferromagnet Fe3GeTe2 (FGT) have been reported, introducing further application possibilities. However, controlling the exhibited magnetic state requires systematic knowledge of the history-dependence of the spin textures, which remains largely unexplored in 2D magnets. In this work, we utilize real-space imaging, and complementary simulations, to det. and explain the thickness-dependent magnetic phase diagrams of an exfoliated FGT flake, revealing a complex, history-dependent emergence of the uniformly magnetized, stripe domain and skyrmion states. The results show that the interplay of the dominant dipolar interaction and strongly temp. dependent out-of-plane anisotropy energy terms enables the selective stabilization of all three states at zero field, and at a single temp., while the Dzyaloshinksii-Moriya interaction must be present to realize the obsd. Neel-type domain walls. The findings open perspectives for 2D devices incorporating topol. spin textures.
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  37. 37
    Zhang, H.; Chen, R.; Zhai, K.; Chen, X.; Caretta, L.; Huang, X.; Chopdekar, R. V.; Cao, J.; Sun, J.; Yao, J.; Birgeneau, R.; Ramesh, R. Itinerant Ferromagnetism in van der Waals Fe5GeTe2 Crystals Above Room Temperature. Phys. Rev. B 2020, 102, 064417,  DOI: 10.1103/PhysRevB.102.064417
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  38. 38
    Lv, X.; Pei, K.; Yang, C.; Qin, G.; Liu, M.; Zhang, J.; Che, R. Controllable Topological Magnetic Transformations in the Thickness-Tunable van der Waals Ferromagnet Fe5GeTe2. ACS Nano 2022, 16, 19319– 19327,  DOI: 10.1021/acsnano.2c08844
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    There is no corresponding record for this reference.
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    Ly, T. T.; Park, J.; Kim, K.; Ahn, H.; Lee, N. J.; Kim, K.; Park, T.; Duvjir, G.; Lam, N. H.; Jang, K. Direct Observation of Fe-Ge Ordering in Fe5–xGeTe2 Crystals and Resultant Helimagnetism. Adv. Funct. Mater. 2021, 31, 2009758,  DOI: 10.1002/adfm.202009758
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    39
    Direct Observation of Fe-Ge Ordering in Fe5-xGeTe2 Crystals and Resultant Helimagnetism
    Ly, Trinh Thi; Park, Jungmin; Kim, Kyoo; Ahn, Hyo-Bin; Lee, Nyun Jong; Kim, Kwangsu; Park, Tae-Eon; Duvjir, Ganbat; Lam, Nguyen Huu; Jang, Kyuha; You, Chun-Yeol; Jo, Younghun; Kim, Se Kwon; Lee, Changgu; Kim, Sanghoon; Kim, Jungdae
    Advanced Functional Materials (2021), 31 (17), 2009758CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Microscopic structures and magnetic properties are investigated for Fe5-xGeTe2 single crystal, recently discovered as a promising van der Waals (vdW) ferromagnet. An Fe atom (Fe(1)) located in the outermost Fe5Ge sublayer has two possible split-sites which are either above or below the Ge atom. Scanning tunneling microscopy shows √3 × √3 superstructures which are attributed to the ordering of Fe(1) layer. The √3 × √3 superstructures have two different phases due to the symmetry of Fe(1) ordering. Intriguingly, the obsd. √3 × √3 ordering breaks the inversion symmetry of crystal, resulting in substantial antisym. exchange interaction. The temp. dependence of magnetization reveals a sharp magnetic anomaly suggesting helical magnetism of the Fe5-xGeTe2 due to its non-centrosymmetricity. Anal. study also supports that the obsd. ordering can give rise to the helimagnetism. The work will provide essential information to understand the complex magnetic properties and the origin of the new vdW ferromagnet, Fe5-xGeTe2 for future topol.-based spin devices.
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  40. 40
    Zhang, C.; Liu, C.; Zhang, S.; Zhou, B.; Guan, C.; Ma, Y.; Algaidi, H.; Zheng, D.; Li, Y.; He, X. Magnetic Skyrmions with Unconventional Helicity Polarization in a Van Der Waals Ferromagnet. Adv. Mater. 2022, 34, 2204163,  DOI: 10.1002/adma.202204163
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    40
    Magnetic Skyrmions with Unconventional Helicity Polarization in a Van Der Waals Ferromagnet
    Zhang, Chenhui; Liu, Chen; Zhang, Senfu; Zhou, Bojian; Guan, Chaoshuai; Ma, Yinchang; Algaidi, Hanin; Zheng, Dongxing; Li, Yan; He, Xin; Zhang, Junwei; Li, Peng; Hou, Zhipeng; Yin, Gen; Liu, Kai; Peng, Yong; Zhang, Xi-Xiang
    Advanced Materials (Weinheim, Germany) (2022), 34 (42), 2204163CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Skyrmion helicity, which defines the spin swirling direction, is a fundamental parameter that may be utilized to encode data bits in future memory devices. Generally, in centrosym. ferromagnets, dipole skyrmions with helicity of -π/2 and π/2 are degenerate in energy, leading to equal populations of both helicities. On the other hand, in chiral materials where the Dzyaloshinskii-Moriya interaction (DMI) prevails and the dipolar interaction is negligible, only a preferred helicity is selected by the type of DMI. However, whether there is a rigid boundary between these two regimes remains an open question. Herein, the observation of dipole skyrmions with unconventional helicity polarization in a van der Waals ferromagnet, Fe5-δGeTe2, is reported. Combining magnetometry, Lorentz transmission electron microscopy, elec. transport measurements, and micromagnetic simulations, the short-range superstructures in Fe5-δGeTe2 resulting in a localized DMI contribution, which breaks the degeneracy of the opposite helicities and leads to the helicity polarization, is demonstrated. Therefore, the helicity feature in Fe5-δGeTe2 is controlled by both the dipolar interaction and DMI that the former leads to Bloch-type skyrmions with helicity of ±π/2 whereas the latter breaks the helicity degeneracy. This work provides new insights into the skyrmion topol. in van der Waals materials.
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  41. 41
    Schmitt, M.; Denneulin, T.; Kovács, A.; Saunderson, T. G.; Rüßmann, P.; Shahee, A.; Scholz, T.; Tavabi, A. H.; Gradhand, M.; Mavropoulos, P.; Lotsch, B. V.; Dunin-Borkowski, R. E.; Mokrousov, Y.; Blügel, S.; Kläui, M. Skyrmionic Spin structures in Layered Fe5GeTe2 up to Room Temperature. Commun. Phys. 2022, 5, 254,  DOI: 10.1038/s42005-022-01031-w
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    41
    Skyrmionic spin structures in layered Fe5GeTe2 up to room temperature
    Schmitt, Maurice; Denneulin, Thibaud; Kovacs, Andras; Saunderson, Tom G.; Ruessmann, Philipp; Shahee, Aga; Scholz, Tanja; Tavabi, Amir H.; Gradhand, Martin; Mavropoulos, Phivos; Lotsch, Bettina V.; Dunin-Borkowski, Rafal E.; Mokrousov, Yuriy; Bluegel, Stefan; Klaeui, Mathias
    Communications Physics (2022), 5 (1), 254CODEN: CPOHDJ; ISSN:2399-3650. (Nature Portfolio)
    Abstr.: The role of the crystal lattice, temp. and magnetic field for the spin structure formation in the 2D van der Waals magnet Fe5GeTe2 with magnetic ordering up to room temp. is a key open question. Using Lorentz transmission electron microscopy, we exptl. observe topol. spin structures up to room temp. in the metastable pre-cooling and stable post-cooling phase of Fe5GeTe2. Over wide temp. and field ranges, skyrmionic magnetic bubbles form without preferred chirality, which is indicative of centrosymmetry. These skyrmions can be obsd. even in the absence of external fields. To understand the complex magnetic order in Fe5GeTe2, we compare macroscopic magnetometry characterization results with microscopic d. functional theory and spin-model calcns. Our results show that even up to room temp., topol. spin structures can be stabilized in centrosym. van der Waals magnets.
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  42. 42
    Fujita, R.; Bassirian, P.; Li, Z.; Guo, Y.; Mawass, M. A.; Kronast, F.; van der Laan, G.; Hesjedal, T. Layer-Dependent Magnetic Domains in Atomically Thin Fe5GeTe2. ACS Nano 2022, 16, 10545– 10553,  DOI: 10.1021/acsnano.2c01948
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    42
    Layer-Dependent Magnetic Domains in Atomically Thin Fe5GeTe2
    Fujita, Ryuji; Bassirian, Pedram; Li, Zhengxian; Guo, Yanfeng; Mawass, Mohamad A.; Kronast, Florian; van der Laan, Gerrit; Hesjedal, Thorsten
    ACS Nano (2022), 16 (7), 10545-10553CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Magnetic domain formation in two-dimensional (2D) materials gives perspectives into the fundamental origins of 2D magnetism and also motivates the development of advanced spintronics devices. However, the characterization of magnetic domains in atomically thin van der Waals (vdW) flakes remains challenging. Here, we employ X-ray photoemission electron microscopy (XPEEM) to perform layer-resolved imaging of the domain structures in the itinerant vdW ferromagnet Fe5GeTe2 which shows near room temp. bulk ferromagnetism and a weak perpendicular magnetic anisotropy (PMA). In the bulk limit, we observe the well-known labyrinth-type domains. Thinner flakes, on the other hand, are characterized by increasingly fragmented domains. While PMA is a characteristic property of Fe5GeTe2, we observe a spin-reorientation transition with the spins canting in-plane for flakes thinner than six layers. Notably, a bubble phase emerges in four-layer flakes. This thickness dependence, which clearly deviates from the single-domain behavior obsd. in other 2D magnetic materials, demonstrates the exciting prospect of stabilizing complex spin textures in 2D vdW magnets at relatively high temps.
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  43. 43
    Högen, M.; Fujita, R.; Tan, A. K. C.; Geim, A.; Pitts, M.; Li, Z.; Guo, Y.; Stefan, L.; Hesjedal, T.; Atatüre, M. Imaging Nucleation and Propagation of Pinned Domains in Few-Layer Fe5GeTe2. ACS Nano 2023, 17, 16879– 16885,  DOI: 10.1021/acsnano.3c03825
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    Zhang, H.; Raftrey, D.; Chan, Y. T.; Shao, Y. T.; Chen, R.; Chen, X.; Huang, X.; Reichanadter, J. T.; Dong, K.; Susarla, S. Room-Temperature Skyrmion Lattice in a Layered Magnet (Fe0.5Co0.5)5GeTe2. Sci. Adv. 2022, 8, eabm7103  DOI: 10.1126/sciadv.abm7103
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    There is no corresponding record for this reference.
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    Zhang, H.; Shao, Y. T.; Chen, R.; Chen, X.; Susarla, S.; Raftrey, D.; Reichanadter, J. T.; Caretta, L.; Huang, X.; Settineri, N. S. A Room Temperature Polar Magnetic Metal. Phys. Rev. Mater. 2022, 6, 044403,  DOI: 10.1103/PhysRevMaterials.6.044403
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    A room temperature polar magnetic metal
    Zhang, Hongrui; Shao, Yu-Tsun; Chen, Rui; Chen, Xiang; Susarla, Sandhya; Raftrey, David; Reichanadter, Jonathan T.; Caretta, Lucas; Huang, Xiaoxi; Settineri, Nicholas S.; Chen, Zhen; Zhou, Jingcheng; Bourret-Courchesne, Edith; Ercius, Peter; Yao, Jie; Fischer, Peter; Neaton, Jeffrey B.; Muller, David A.; Birgeneau, Robert J.; Ramesh, Ramamoorthy
    Physical Review Materials (2022), 6 (4), 044403CODEN: PRMHBS; ISSN:2475-9953. (American Physical Society)
    The emergence of long-range magnetic order in noncentrosym. compds. has stimulated interest in the possibility of exotic spin transport phenomena and topol. protected spin textures for applications in next-generation spintronics. Polar magnets, with broken symmetries of spatial inversion and time reversal, usually host chiral spin textures. This work reports on a wurtzite-structure polar magnetic metal, identified as AA'-stacked (Fe0.5Co0.5)5GeTe2, which exhibits a Neel-type skyrmion lattice as well as a Rashba-Edelstein effect at room temp. Atomic resoln. imaging of the structure reveals a structural transition as a function of Co-substitution, leading to the emergence of the polar phase at 50% Co. This discovery reveals an unprecedented layered polar magnetic system for investigating intriguing spin topologies, and it ushers in a promising new framework for spintronics.
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  46. 46
    Meisenheimer, P.; Zhang, H.; Raftrey, D.; Chen, X.; Shao, Y.-T.; Chan, Y.-T.; Yalisove, R.; Chen, R.; Yao, J.; Scott, M. C.; Wu, W.; Muller, D. A.; Fischer, P.; Birgeneau, R. J.; Ramesh, R. Ordering of Room-Temperature Magnetic Skyrmions in a Polar van der Waals Magnet. Nat. Commun. 2023, 14, 3744,  DOI: 10.1038/s41467-023-39442-0
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    Stahl, J.; Shlaen, E.; Johrendt, D. The van der Waals Ferromagnets Fe5−δGeTe2 and Fe5−δ−xNixGeTe2 – Crystal Structure, Stacking Faults, and Magnetic Properties. Z. Anorg. Allg. Chem. 2018, 644, 1923– 1929,  DOI: 10.1002/zaac.201800456
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    47
    The van der Waals Ferromagnets Fe5-δGeTe2 and Fe5-δ-xNixGeTe2 - Crystal Structure, Stacking Faults, and Magnetic Properties
    Stahl, Juliane; Shlaen, Evgeniya; Johrendt, Dirk
    Zeitschrift fuer Anorganische und Allgemeine Chemie (2018), 644 (24), 1923-1929CODEN: ZAACAB; ISSN:1521-3749. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Fe5-δGeTe2 was synthesized by heating the elements at 1050 K and characterized by single crystal and powder X-ray anal. The structure [R3m, a = 4.0376(4) Å, c = 29.194(6) Å] consists of Fe5-δGe layers sepd. by tellurium double layers forming a van der Waals gap. The pronounced two-dimensional character of Fe5-δGeTe2 causes stacking faults along the c direction. Simulations of different stacking variants using the DIFFaX software reveal disorder occurring in domains. Magnetic measurements of Fe5-δGeTe2 show ferromagnetism below 279 K with a satn. moment of 1.80 μB at 1.8 K. Nickel substitution of the iron sites has little influence on the structure but changes the satn. moment, which passes through a max. of 2.11 μB in Fe4.11Ni0.50GeTe2. This indicates that structural influences as well as the diln. of the magnetic iron atoms play a decisive role.
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  48. 48
    Alahmed, L.; Nepal, B.; Macy, J.; Zheng, W.; Casas, B.; Sapkota, A.; Jones, N.; Mazza, A. R.; Brahlek, M.; Jin, W. Magnetism and Spin Dynamics in Room-Temperature van der Waals Magnet Fe5GeTe2. 2D Mater. 2021, 8, 045030,  DOI: 10.1088/2053-1583/ac2028
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    48
    Magnetism and spin dynamics in room-temperature van der Waals magnet Fe5GeTe2
    Alahmed, Laith; Nepal, Bhuwan; Macy, Juan; Zheng, Wenkai; Casas, Brian; Sapkota, Arjun; Jones, Nicholas; Mazza, Alessandro R.; Brahlek, Matthew; Jin, Wencan; Mahjouri-Samani, Masoud; Zhang, Steven S.-L.; Mewes, Claudia; Balicas, Luis; Mewes, Tim; Li, Peng
    2D Materials (2021), 8 (4), 045030CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
    Two-dimensional van der Waals (vdWs) materials have gathered a lot of attention recently. However, the majority of these materials have Curie temps. that are well below room temp., making it challenging to incorporate them into device applications. In this work, we synthesized a room-temp. vdW magnetic crystal Fe5GeTe2 with a Curie temp. Tc = 332 K, and studied its magnetic properties by vibrating sample magnetometry (VSM) and broadband ferromagnetic resonance (FMR) spectroscopy. The expts. were performed with external magnetic fields applied along the c-axis (H‖c) and the ab-plane (H‖ab), with temps. ranging from 300 to 10 K. We have found a sizable Lande g-factor difference between the H‖c and H‖ab cases. In both cases, the Lande g-factor values deviated from g = 2. This indicates contribution of orbital angular momentum to the magnetic moment. The FMR measurements reveal that Fe5GeTe2 has a damping const. comparable to Permalloy. With reducing temp., the linewidth was broadened. Together with the VSM data, our measurements indicate that Fe5GeTe2 transitions from ferromagnetic to ferrimagnetic at lower temps. Our expts. highlight key information regarding the magnetic state and spin scattering processes in Fe5GeTe2, which promote the understanding of magnetism in Fe5GeTe2, leading to implementations of Fe5GeTe2 based room-temp. spintronic devices.
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  49. 49
    Chacon, A.; Heinen, L.; Halder, M.; Bauer, A.; Simeth, W.; Mühlbauer, S.; Berger, H.; Garst, M.; Rosch, A.; Pfleiderer, C. Observation of Two Independent Skyrmion Phases in a Chiral Magnetic Material. Nat. Phys. 2018, 14, 936– 941,  DOI: 10.1038/s41567-018-0184-y
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    Observation of two independent skyrmion phases in a chiral magnetic material
    Chacon, A.; Heinen, L.; Halder, M.; Bauer, A.; Simeth, W.; Muehlbauer, S.; Berger, H.; Garst, M.; Rosch, A.; Pfleiderer, C.
    Nature Physics (2018), 14 (9), 936-941CODEN: NPAHAX; ISSN:1745-2473. (Nature Research)
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  • Abstract

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    Figure 1

    Figure 1. Structure and characterization of the slow-cooled and quenched Fe5GeTe2 samples. (a) Crystal structure of Fe5GeTe2, with the positions of Fe1, Fe2, Fe3, Ge, and Te indicated. The half-colored Fe1 and Ge atoms indicate split vacancy sites. (b–e) Magnetization M of the bulk slow-cooled (b,c) and the quenched (d,e) Fe5GeTe2 crystals. Measurements were performed as a function of temperature T following both field-cooling (FC) and field-warming (FW) processes, under a field of 30 mT applied either perpendicular (B⊥c) or parallel (B∥c) to the c crystal axis. (f,g) Optical images of the two investigated slow-cooled flakes, stamped on Si3N4 membranes. Flake 1 was capped with an hBN flake and investigated in the STXM measurements. Flake 2 was investigated in the LTEM measurements. The labeled regions of interest (ROI, red dashed lines) indicate the areas investigated in the imaging experiments. The ROI of flake 1 has a thickness of ∼120 nm, while flake 2 has a thickness of ∼150 nm.

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    Figure 2

    Figure 2. X-ray and LTEM imaging of slow-cooled Fe5GeTe2 under an out-of-plane applied magnetic field. (a–d) X-ray micrographs of the ROI of the exfoliated slow-cooled Fe5GeTe2 flake 1, acquired at a range of temperatures and applied out-of-plane magnetic fields. The images were taken following the field-sweep protocol: at each temperature, the out-of-plane applied magnetic field was increased stepwise after being initialized at −250 mT. The color map scales with the out-of-plane magnetization of the flake, mz (light is down, dark is up). (e) Selected LTEM images of the slow-cooled Fe5GeTe2 flake 2 acquired at 98 K. The upper two panels show the raw contrast achieved when defocusing the electron beam, revealing magnetic contrast of stripe domains and, at higher applied fields, magnetic bubble states. The lower three panels show the in-plane magnetic induction reconstructed using the transport-of-intensity equation (TIE) of the different observed bubble objects: type-II bubbles and type-I bubbles of left- and right-handed chirality. The color wheel indicates the direction of the in-plane magnetic induction.

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    Figure 3

    Figure 3. X-ray imaging of slow-cooled Fe5GeTe2 under an in-plane applied magnetic field. (a–c) X-ray micrographs of the ROI of the exfoliated Fe5GeTe2 flake 1, acquired at a range of temperatures and applied in-plane magnetic fields. The images were taken following the field-sweep protocol: at each temperature, the in-plane applied magnetic field was increased stepwise after being initialized at −250 mT. The color map is scaled with the out-of-plane magnetization of the flake, mz (light is down, dark is up). Dashed green lines indicate example locations of the linescans used to evaluate the stripe domain spacing. (d) Average stripe domain size d at 0 mT as a function of the sample temperature. Error bars indicate the standard error acquired when averaging the results of multiple line scans across the domains in each image. TK indicates the temperature at which the domain size suddenly changes, which we argue is due to a sudden alteration of the uniaxial anisotropy by the Fe1 sublattice ordering.

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    Figure 4

    Figure 4. Magnetic phase diagrams of slow-cooled Fe5GeTe2. (a,b) Magnetic phase diagrams of the slow-cooled Fe5GeTe2 flake 1, acquired by the field-sweep (FS) process for out-of-plane and in-plane applied magnetic fields, respectively. The presence of the uniformly magnetized (UM, purple crosses), stripe domain (SD, orange circles), dense bubble array (B, green complete stars), isolated bubble (iB, green empty stars), and combined stripe and bubble (B + SD, green squares) states is indicated. Each point corresponds to an acquired X-ray micrograph. The direction of the field sweep, initialized at −250 mT, is indicated by the red arrow. (c) Measured saturation magnetization, MS, of the bulk slow-cooled F5GT crystal, plotted as a function of temperature. (d) Measured uniaxial anisotropy of the bulk slow-cooled F5GT crystal, Keff (orange circles) plotted as a function of temperature, determined from integrating the difference between magnetization loops of the bulk crystal measured with the field parallel and perpendicular to the c axis. Also plotted is the estimated shape anisotropy contribution, Ksh (purple crosses), and the resulting estimated uniaxial anisotropy, KU (red stars), obtained from Keff – Ksh. In all plots, the value of TC and the characteristic transition temperature TK is indicated by vertical gray lines.

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    Figure 5

    Figure 5. Bubble density in the slow-cooled Fe5GeTe2 flake 1 sample. (a–i) Bubble density ρ measured as a function of the applied magnetic field using STXM (a–c) and LTEM (d–i) at selected temperatures for the slow-cooled Fe5GeTe2 flakes. LTEM measurements were performed at both the sample edge (d–f) and center (g–i), as labeled.

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    Figure 6

    Figure 6. X-ray microscopy of the quenched Fe5GeTe2 flake. (a) Overview X-ray micrograph of the quenched F5GT flake. The region of interest (ROI) of subsequent images is indicated. (b) X-ray micrograph of the ROI at 300 K before any cooling, revealing significant contrast indicating structural inhomogeneities within the flake sample. (c,d) Two representative X-ray micrographs of a region of the quenched Fe5GeTe2 flake at 75 K, measured following the field sweep procedure for both out-of-plane and in-plane field, respectively. The images are for a single X-ray polarization, and thus, the contrast is both structural and magnetic in origin, allowing the correlation between magnetic and structural features to be seen.

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      Thin van der Waals (vdW) layered magnetic materials hold the possibility of realizing vdW heterostructures with new functionalities. Here, the authors report on the realization and investigation of tunneling spin valves based on van der Waals heterostructures consisting of an atomically thin hBN layer acting as tunnel barrier and two exfoliated Fe3GeTe2 crystals acting as ferromagnetic electrodes. Low-temp. anomalous Hall effect measurements show that thin Fe3GeTe2 crystals are metallic ferromagnets with an easy axis perpendicular to the layers and a very sharp magnetization switching at magnetic field values that depends slightly on their geometry. In Fe3GeTe2/hBN/Fe3GeTe2 heterostructures, the authors observe textbook behavior of the tunneling resistance, which is min. (max.) when the magnetization in the two electrodes is parallel (antiparallel) to each other. The magnetoresistance is 160% at low temp., from which we det. the spin polarization of Fe3GeTe2 to be 0.66, corresponding to 83% and 17% of the majority and minority carriers, resp. The measurements also show that, with increasing temp., the evolution of the spin polarization extd. from the tunneling magnetoresistance is proportional to the temp. dependence of the magnetization extd. from the anal. of the anomalous Hall cond. This suggests that the magnetic properties of the surface are representative of those of the bulk, as may be expected for vdW materials.
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    7. 7
      Li, X.; Lü, J. T.; Zhang, J.; You, L.; Su, Y.; Tsymbal, E. Y. Spin-Dependent Transport in van der Waals Magnetic Tunnel Junctions with Fe3GeTe2 Electrodes. Nano Lett. 2019, 19, 5133– 5139,  DOI: 10.1021/acs.nanolett.9b01506
      7
      Spin-Dependent Transport in van der Waals Magnetic Tunnel Junctions with Fe3GeTe2 Electrodes
      Li, Xinlu; Lu, Jing-Tao; Zhang, Jia; You, Long; Su, Yurong; Tsymbal, Evgeny Y.
      Nano Letters (2019), 19 (8), 5133-5139CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Van der Waals (vdW) heterostructures, stacking different two-dimensional materials, have opened up unprecedented opportunities to explore new physics and device concepts. Esp. interesting are recently discovered two-dimensional magnetic vdW materials, providing new paradigms for spintronic applications. Here, using d. functional theory (DFT) calcns., we investigate the spin-dependent electronic transport across vdW magnetic tunnel junctions (MTJs) composed of Fe3GeTe2 ferromagnetic electrodes and a graphene or hexagonal boron nitride (h-BN) spacer layer. For both types of junctions, we find that the junction resistance changes by thousands of percent when the magnetization of the electrodes is switched from parallel to antiparallel. Such a giant tunneling magnetoresistance (TMR) effect is driven by dissimilar electronic structure of the two spin-conducting channels in Fe3GeTe2, resulting in a mismatch between the incoming and outgoing Bloch states in the electrodes and thus suppressed transmission for an antiparallel-aligned MTJ. The vdW bonding between electrodes and a spacer layer makes this result virtually independent of the type of the spacer layer, making the predicted giant TMR effect robust with respect to strain, interface distance, and other parameters, which may vary in the expt. We hope that our results will further stimulate exptl. studies of vdW MTJs and pave the way for their applications in spintronics.
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    8. 8
      Deiseroth, H.-J.; Aleksandrov, K.; Reiner, C.; Kienle, L.; Kremer, R. K. Fe3GeTe2 and Ni3GeTe2 – Two New Layered Transition-Metal Compounds: Crystal Structures, HRTEM Investigations, and Magnetic and Electrical Properties. Eur. J. Inorg. Chem. 2006, 2006, 1561– 1567,  DOI: 10.1002/ejic.200501020
      There is no corresponding record for this reference.
    9. 9
      May, A. F.; Bridges, C. A.; McGuire, M. A. Physical Properties and Thermal Stability of Fe5GeTe2 single Crystals. Phys. Rev. Mater. 2019, 3, 104401,  DOI: 10.1103/PhysRevMaterials.3.104401
      There is no corresponding record for this reference.
    10. 10
      Jiang, H.; Zang, Z.; Wang, X.; Que, H.; Wang, L.; Si, K.; Zhang, P.; Ye, Y.; Gong, Y. Thickness-Tunable Growth of Composition-Controllable Two-Dimensional FexGeTe2. Nano Lett. 2022, 22, 9477– 9484,  DOI: 10.1021/acs.nanolett.2c03562
      There is no corresponding record for this reference.
    11. 11
      Liu, Q.; Xing, J.; Jiang, Z.; Guo, Y.; Jiang, X.; Qi, Y.; Zhao, J. Layer-Dependent Magnetic Phase Diagram in FenGeTe2 (3 ≤ n ≤ 7) Ultrathin Films. Commun. Phys. 2022, 5, 140,  DOI: 10.1038/s42005-022-00921-3
      11
      Layer-dependent magnetic phase diagram in FenGeTe2 (3 = n = 7) ultrathin films
      Liu, Qinxi; Xing, Jianpei; Jiang, Zhou; Guo, Yu; Jiang, Xue; Qi, Yan; Zhao, Jijun
      Communications Physics (2022), 5 (1), 140CODEN: CPOHDJ; ISSN:2399-3650. (Nature Portfolio)
      Two-dimensional (2D) ferromagnets with high Curie temp. TC are desirable for spintronics applications. However, they are rarely obtained in expts. mainly due to the challenge of synthesizing high-quality 2D crystals, and their TC values are below room temp. Using first-principles calcns., we design a family of stable 2D FenGeTe2 (4 = n = 7) ultrathin films with coexisting itinerant and localized magnetism. Among them, 2D Fe3GeTe2 and Fe4GeTe2 are ferromagnetic metals with TC = 138 and 68 K; 2D Fe5GeTe2, Fe6GeTe2, and Fe7GeTe2 are Ne´el's P-, R-, and R-type ferrimagnetic metals with TC = 320, 450, and 570 K. A thickness-induced magnetic phase transition originates from competition between itinerant and localized states, and also correlates with Fe3+ and Fe2+ content. A valence/orbital-dependent magnetic exchange model is proposed for these effects. Our results reveal a universal mechanism for magnetic coupling in complex magnetic systems.
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    12. 12
      May, A. F.; Calder, S.; Cantoni, C.; Cao, H.; McGuire, M. A. Magnetic Structure and Phase Stability of the van der Waals Bonded Ferromagnet Fe3GeTe2. Phys. Rev. B 2016, 93, 014411,  DOI: 10.1103/PhysRevB.93.014411
      There is no corresponding record for this reference.
    13. 13
      Park, S. Y.; Kim, D. S.; Liu, Y.; Hwang, J.; Kim, Y.; Kim, W.; Kim, J. Y.; Petrovic, C.; Hwang, C.; Mo, S. K. Controlling the Magnetic Anisotropy of the van der Waals Ferromagnet Fe3GeTe2 through Hole Doping. Nano Lett. 2020, 20, 95– 100,  DOI: 10.1021/acs.nanolett.9b03316
      13
      Controlling the Magnetic Anisotropy of the van der Waals Ferromagnet Fe3GeTe2 through Hole Doping
      Park, Se Young; Kim, Dong Seob; Liu, Yu; Hwang, Jinwoong; Kim, Younghak; Kim, Wondong; Kim, Jae-Young; Petrovic, Cedomir; Hwang, Choongyu; Mo, Sung-Kwan; Kim, Hyung-jun; Min, Byoung-Chul; Koo, Hyun Cheol; Chang, Joonyeon; Jang, Chaun; Choi, Jun Woo; Ryu, Hyejin
      Nano Letters (2020), 20 (1), 95-100CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Identifying material parameters affecting properties of ferromagnets is key to optimize materials better suited for spintronics. Magnetic anisotropy is of particular importance in van der Waals magnets, since it not only influences magnetic and spin transport properties, but also is essential to stabilizing magnetic order in the two dimensional limit. Here, we report that hole doping effectively modulates the magnetic anisotropy of a van der Waals ferromagnet, and explore the phys. origin of this effect. Fe3-xGeTe2 nanoflakes show a significant suppression of the magnetic anisotropy with hole doping. Electronic structure measurements and calcns. reveal that the chem. potential shift assocd. with hole doping is responsible for the reduced magnetic anisotropy by decreasing the energy gain from the spin-orbit induced band splitting. Our findings provide an understanding of the intricate connection between electronic structures and magnetic properties in two-dimensional magnets and propose a method to engineer magnetic properties through doping.
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    14. 14
      Birch, M. T.; Powalla, L.; Litzius, K.; Nehruji, V.; Hovorka, O.; Wintz, S.; Schulz, F.; Mayoh, D. A.; Balakrishnan, G.; Weigand, M.; Burghard, M.; Schütz, G. Control of Stripe, Skyrmion and Skyrmionium Formation in the 2D Magnet Fe3–xGeTe2 by Varying Composition. 2D Mater. 2024, 11, 025008,  DOI: 10.1088/2053-1583/ad1a6b
      There is no corresponding record for this reference.
    15. 15
      Seo, J.; Kim, D. Y.; An, E. S.; Kim, K.; Kim, G. Y.; Hwang, S. Y.; Kim, D. W.; Jang, B. G.; Kim, H.; Eom, G. Nearly Room Temperature Ferromagnetism in a Magnetic Metal-Rich van der Waals Metal. Sci. Adv. 2020, 6, eaay8912  DOI: 10.1126/sciadv.aay8912
      There is no corresponding record for this reference.
    16. 16
      May, A. F.; Ovchinnikov, D.; Zheng, Q.; Hermann, R.; Calder, S.; Huang, B.; Fei, Z.; Liu, Y.; Xu, X.; McGuire, M. A. Ferromagnetism Near Room Temperature in the Cleavable van der Waals Crystal Fe5GeTe2. ACS Nano 2019, 13, 4436– 4442,  DOI: 10.1021/acsnano.8b09660
      16
      Ferromagnetism Near Room Temperature in the Cleavable van der Waals Crystal Fe5GeTe2
      May, Andrew F.; Ovchinnikov, Dmitry; Zheng, Qiang; Hermann, Raphael; Calder, Stuart; Huang, Bevin; Fei, Zaiyao; Liu, Yaohua; Xu, Xiaodong; McGuire, Michael A.
      ACS Nano (2019), 13 (4), 4436-4442CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Two-dimensional materials with intrinsic functionality are becoming increasingly important in exploring fundamental condensed matter science and for developing advanced technologies. Bulk crystals that can be exfoliated are particularly relevant to these pursuits as they provide the opportunity to study the role of phys. dimensionality and explore device physics in highly cryst. samples and designer heterostructures in a routine manner. Magnetism is a key element in these endeavors; however, relatively few cleavable materials are magnetic and none possess magnetic order at ambient conditions. Here, we introduce Fe5-xGeTe2 as a cleavable material with ferromagnetic behavior at room temp. We established intrinsic magnetic order at room temp. in bulk crystals (TC = 310 K) through magnetization measurements and in exfoliated, thin flakes (TC ≈ 280 K) using the anomalous Hall effect. Our work reveals Fe5GeTe2 as a prime candidate for incorporating intrinsic magnetism into functional van der Waals heterostructures and devices near room temp.
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    17. 17
      May, A. F.; Du, M.-H.; Cooper, V. R.; McGuire, M. A. Tuning Magnetic Order in the van der Waals Metal Fe5GeTe2 by Cobalt Substitution. Phys. Rev. Mater. 2020, 4, 074008,  DOI: 10.1103/PhysRevMaterials.4.074008
      17
      Tuning magnetic order in the van der Waals metal Fe5GeTe2 by cobalt substitution
      May, Andrew F.; Du, Mao-Hua; Cooper, Valentino R.; McGuire, Michael A.
      Physical Review Materials (2020), 4 (7), 074008CODEN: PRMHBS; ISSN:2475-9953. (American Physical Society)
      This paper is a contribution to the joint Phys. Review Applied and Phys. Review Materials collection titledTwo-Dimensional Materials and Devices. Fe5-xGeTe2 is a van der Waals material with one of the highest reported bulk Curie temps., TC'310K. In this study, theor. calcns. and expts. are utilized to demonstrate that the magnetic ground state is highly sensitive to local at. arrangements and the interlayer stacking. Cobalt substitution is found to be an effective way to manipulate the magnetic properties while also increasing the ordering temp. In particular, cobalt substitution up to '30% enhances TC and changes the magnetic anisotropy, while '50% cobalt substitution yields an antiferromagnetic state. Single crystal x-ray diffraction evidences a structural change upon increasing the cobalt concn., with a rhombohedral cell obsd. in the parent material and a primitive cell obsd. for '46% cobalt content relative to iron. First-principles calcns. demonstrate that it is a combination of high cobalt content and the concomitant change to primitive layer stacking that produces antiferromagnetic order. These results illustrate the sensitivity of magnetism in Fe5-xGeTe2 to compn. and structure, and emphasize the important role of local structural order-disorder and layer stacking in cleavable magnetic materials.
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    18. 18
      Mayoh, D. A.; Wood, G. D. A.; Holt, S. J. R.; Beckett, G.; Dekker, E. J. L.; Lees, M. R.; Balakrishnan, G. Effects of Fe Deficiency and Co Substitution in Polycrystalline and Single Crystals of Fe3GeTe2. Cryst. Growth Des. 2021, 21, 6786– 6792,  DOI: 10.1021/acs.cgd.1c00684
      There is no corresponding record for this reference.
    19. 19
      May, A. F.; Yan, J.; Hermann, R.; Du, M.-H.; McGuire, M. A. Tuning the Room Temperature Ferromagnetism in Fe5GeTe2 by Arsenic Substitution. 2D Mater. 2022, 9, 015013,  DOI: 10.1088/2053-1583/ac34d9
      19
      Tuning the room temperature ferromagnetism in Fe5GeTe2 by arsenic substitution
      May, Andrew F.; Yan, Jiaqiang; Hermann, Raphael; Du, Mao-Hua; McGuire, Michael A.
      2D Materials (2022), 9 (1), 015013CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
      In order to tune the magnetic properties of the cleavable high-Curie temp. ferromagnet Fe5-xGeTe2, the effect of increasing the electron count through arsenic substitution has been investigated. Small addns. of arsenic (2.5% and 5%) seemingly enhance ferromagnetic order in polycryst. samples by quenching fluctuations on one of the three magnetic sublattices, whereas larger As concns. decrease the ferromagnetic Curie temp. (TC) and satn. magnetization. This work also describes the growth and characterization of Fe4.8AsTe2 single crystals that are structurally analogous to Fe5-xGeTe2 but with some phase stability complications. Magnetization measurements reveal dominant antiferromagnetic behavior in Fe4.8AsTe2 with a N´eel temp. of TN ≈ 42 K. A field-induced spin-flop below TN results in a switch from neg. to pos. magnetoresistance, with significant hysteresis causing butterfly-shaped resistance loops. In addn. to reporting the properties of Fe4.8AsTe2, this work shows the importance of manipulating the individual magnetic sublattices in Fe5-xGeTe2 and motivates further efforts to control the magnetic properties in related materials by fine tuning of the Fermi energy or crystal chem.
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    20. 20
      Deng, Y.; Yu, Y.; Song, Y.; Zhang, J.; Wang, N. Z.; Sun, Z.; Yi, Y.; Wu, Y. Z.; Wu, S.; Zhu, J.; Wang, J.; Chen, X. H.; Zhang, Y. Gate-Tunable Room-Temperature Ferromagnetism in Two-Dimensional Fe3GeTe2. Nature 2018, 563, 94– 99,  DOI: 10.1038/s41586-018-0626-9
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      Gate-tunable room-temperature ferromagnetism in two-dimensional Fe3GeTe2
      Deng, Yujun; Yu, Yijun; Song, Yichen; Zhang, Jingzhao; Wang, Nai Zhou; Sun, Zeyuan; Yi, Yangfan; Wu, Yi Zheng; Wu, Shiwei; Zhu, Junyi; Wang, Jing; Chen, Xian Hui; Zhang, Yuanbo
      Nature (London, United Kingdom) (2018), 563 (7729), 94-99CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Materials research has driven the development of modern nano-electronic devices. In particular, research in magnetic thin films has revolutionized the development of spintronic devices1,2 because identifying new magnetic materials is key to better device performance and design. Van der Waals crystals retain their chem. stability and structural integrity down to the monolayer and, being atomically thin, are readily tuned by various kinds of gate modulation3,4. Recent expts. have demonstrated that it is possible to obtain two-dimensional ferromagnetic order in insulating Cr2Ge2Te6 (ref. 5) and CrI3 (ref. 6) at low temps. Here we develop a device fabrication technique and isolate monolayers from the layered metallic magnet Fe3GeTe2 to study magnetotransport. We find that the itinerant ferromagnetism persists in Fe3GeTe2 down to the monolayer with an out-of-plane magnetocryst. anisotropy. The ferromagnetic transition temp., Tc, is suppressed relative to the bulk Tc of 205 K in pristine Fe3GeTe2 thin flakes. An ionic gate, however, raises Tc to room temp., much higher than the bulk Tc. The gate-tunable room-temp. ferromagnetism in two-dimensional Fe3GeTe2 opens up opportunities for potential voltage-controlled magnetoelectronics7-11 based on atomically thin van der Waals crystals.
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    21. 21
      Tan, C.; Xie, W.-Q.; Zheng, G.; Aloufi, N.; Albarakati, S.; Algarni, M.; Li, J.; Partridge, J.; Culcer, D.; Wang, X.; Yi, J. B.; Tian, M.; Xiong, Y.; Zhao, Y.-J.; Wang, L. Gate-Controlled Magnetic Phase Transition in a van der Waals Magnet Fe5GeTe2. Nano Lett. 2021, 21, 5599– 5605,  DOI: 10.1021/acs.nanolett.1c01108
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      Gate-Controlled Magnetic Phase Transition in a van der Waals Magnet Fe5GeTe2
      Tan, Cheng; Xie, Wen-Qiang; Zheng, Guolin; Aloufi, Nuriyah; Albarakati, Sultan; Algarni, Meri; Li, Junbo; Partridge, James; Culcer, Dimitrie; Wang, Xiaolin; Yi, Jia Bao; Tian, Mingliang; Xiong, Yimin; Zhao, Yu-Jun; Wang, Lan
      Nano Letters (2021), 21 (13), 5599-5605CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Magnetic van der Waals (vdW) materials are poised to enable all-elec. control of magnetism in the two-dimensional limit. However, tuning the magnetic ground state in vdW itinerant ferromagnets by voltage-induced charge doping remains a significant challenge, due to the extremely large carrier densities in these materials. Here, by cleaving the vdW itinerant ferromagnet Fe5GeTe2 (F5GT) into 5.4 nm (around two unit cells), we find that the ferromagnetism (FM) in F5GT can be substantially tuned by the thickness. Moreover, by utilizing a solid protonic gate, an electron doping concn. of above 1021 cm-3 has been exhibited in F5GT nanosheets. Such a high carrier accumulation exceeds that possible in widely used elec. double-layer transistors (EDLTs) and surpasses the intrinsic carrier d. of F5GT. Importantly, it is accompanied by a magnetic phase transition from FM to antiferromagnetism (AFM). The realization of an antiferromagnetic phase in nanosheet F5GT suggests the promise of applications in high-temp. antiferromagnetic vdW devices and heterostructures.
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    22. 22
      Tomasello, R.; Martinez, E.; Zivieri, R.; Torres, L.; Carpentieri, M.; Finocchio, G. A Strategy for the Design of Skyrmion Racetrack Memories. Sci. Rep. 2014, 4, 6784,  DOI: 10.1038/srep06784
      22
      A strategy for the design of skyrmion racetrack memories
      Tomasello, R.; Martinez, E.; Zivieri, R.; Torres, L.; Carpentieri, M.; Finocchio, G.
      Scientific Reports (2014), 4 (), 6784CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
      Magnetic storage based on racetrack memory is very promising for the design of ultra-dense, low-cost and low-power storage technol. Information can be coded in a magnetic region between two domain walls or, as predicted recently, in topol. magnetic objects known as skyrmions. Here, we show the technol. advantages and limitations of using Bloch and Ne´el skyrmions manipulated by spin current generated within the ferromagnet or via the spin-Hall effect arising from a non-magnetic heavy metal underlayer. We found that the Ne´el skyrmion moved by the spin-Hall effect is a very promising strategy for technol. implementation of the next generation of skyrmion racetrack memories (zero field, high thermal stability, and ultra-dense storage). We employed micromagnetics reinforced with an anal. formulation of skyrmion dynamics that we developed from the Thiele equation. We identified that the excitation, at high currents, of a breathing mode of the skyrmion limits the maximal velocity of the memory.
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    23. 23
      Grollier, J.; Querlioz, D.; Camsari, K.; Everschor-Sitte, K.; Fukami, S.; Stiles, M. D. Neuromorphic Spintronics. Nat. Electron. 2020, 3, 360– 370,  DOI: 10.1038/s41928-019-0360-9
      There is no corresponding record for this reference.
    24. 24
      Mühlbauer, S.; Binz, B.; Jonietz, F.; Pfleiderer, C.; Rosch, A.; Neubauer, A.; Georgii, R.; Böni, P. Skyrmion Lattice in a Chiral Magnet. Science 2009, 323, 915– 919,  DOI: 10.1126/science.1166767
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      Skyrmion Lattice in a Chiral Magnet
      Muehlbauer, S.; Binz, B.; Jonietz, F.; Pfleiderer, C.; Rosch, A.; Neubauer, A.; Georgii, R.; Boeni, P.
      Science (Washington, DC, United States) (2009), 323 (5916), 915-919CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Skyrmions represent topol. stable field configurations with particle-like properties. We used neutron scattering to observe the spontaneous formation of a two-dimensional lattice of skyrmion lines, a type of magnetic vortex, in the chiral itinerant-electron magnet MnSi. The skyrmion lattice stabilizes at the border between paramagnetism and long-range helimagnetic order perpendicular to a small applied magnetic field regardless of the direction of the magnetic field relative to the at. lattice. Our study exptl. establishes magnetic materials lacking inversion symmetry as an arena for new forms of cryst. order composed of topol. stable spin states.
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    25. 25
      Yu, X. Z.; Onose, Y.; Kanazawa, N.; Park, J. H.; Han, J. H.; Matsui, Y.; Nagaosa, N.; Tokura, Y. Real-Space Observation of a Two-Dimensional Skyrmion Crystal. Nature 2010, 465, 901– 904,  DOI: 10.1038/nature09124
      25
      Real-space observation of a two-dimensional skyrmion crystal
      Yu, X. Z.; Onose, Y.; Kanazawa, N.; Park, J. H.; Han, J. H.; Matsui, Y.; Nagaosa, N.; Tokura, Y.
      Nature (London, United Kingdom) (2010), 465 (7300), 901-904CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      Crystal order is not restricted to the periodic at. array, but can also be found in electronic systems such as the Wigner crystal or as orbital order, stripe order and magnetic order. In the case of magnetic order, spins align parallel to each other in ferromagnets and antiparallel in antiferromagnets. In other, less conventional, cases, spins can sometimes form highly nontrivial structures called spin textures. Among them is the unusual, topol. stable skyrmion spin texture, in which the spins point in all the directions wrapping a sphere. The skyrmion configuration in a magnetic solid is anticipated to produce unconventional spin-electronic phenomena such as the topol. Hall effect. The crystn. of skyrmions as driven by thermal fluctuations has recently been confirmed in a narrow region of the temp./magnetic field (T-B) phase diagram in neutron scattering studies of the three-dimensional helical magnets MnSi (ref. 17) and Fe1-xCoxSi (ref. 22). Here the authors report real-space imaging of a two-dimensional skyrmion lattice in a thin film of Fe0.5Co0.5Si using Lorentz TEM. With a magnetic field of 50-70 mT applied normal to the film, the authors observe skyrmions as a hexagonal arrangement of swirling spin textures, with a lattice spacing of 90 nm. The related T-B phase diagram is in good agreement with Monte Carlo simulations. In this two-dimensional case, the skyrmion crystal seems very stable and appears over a wide range of the phase diagram, including near zero temp. Such a controlled nanometer-scale spin topol. in a thin film may be useful in observing unconventional magneto-transport effects.
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    26. 26
      Back, C.; Cros, V.; Ebert, H.; Everschor-Sitte, K.; Fert, A.; Garst, M.; Ma, T.; Mankovsky, S.; Monchesky, T. L.; Mostovoy, M. The 2020 Skyrmionics Roadmap. J. Phys. D: Appl. Phys. 2020, 53, 363001,  DOI: 10.1088/1361-6463/ab8418
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      The 2020 skyrmionics roadmap
      Back, C.; Cros, V.; Ebert, H.; Everschor-Sitte, K.; Fert, A.; Garst, M.; Ma, Tianping; Mankovsky, S.; Monchesky, T. L.; Mostovoy, M.; Nagaosa, N.; Parkin, S. S. P.; Pfleiderer, C.; Reyren, N.; Rosch, A.; Taguchi, Y.; Tokura, Y.; von Bergmann, K.; Zang, Jiadong
      Journal of Physics D: Applied Physics (2020), 53 (36), 363001CODEN: JPAPBE; ISSN:0022-3727. (IOP Publishing Ltd.)
      The notion of non-trivial topol. winding in condensed matter systems represents a major area of present-day theor. and exptl. research. Magnetic materials offer a versatile platform that is particularly amenable for the exploration of topol. spin solitons in real space such as skyrmions. First identified in non-centrosym. bulk materials, the rapidly growing zool. of materials systems hosting skyrmions and related topol. spin solitons includes bulk compds., surfaces, thin films, heterostructures, nano-wires and nano-dots. This underscores an exceptional potential for major breakthroughs ranging from fundamental questions to applications as driven by an interdisciplinary exchange of ideas between areas in magnetism which traditionally have been pursued rather independently. The skyrmionics Roadmap provides a review of the present state of the art and the wide range of research directions and strategies currently under way. These are, for instance, motivated by the identification of the fundamental structural properties of skyrmions and related textures, processes of nucleation and annihilation in the presence of non-trivial topol. winding, an exceptionally efficient coupling to spin currents generating spin transfer torques at tiny current densities, as well as the capability to purpose-design broad-band spin dynamic and logic devices.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXisVykurjE&md5=0ed854c9d764effc997c4816539bf144
    27. 27
      Büttner, F.; Lemesh, I.; Beach, G. S. D. Theory of Isolated Magnetic Skyrmions: From Fundamentals to Room Temperature Applications. Sci. Rep. 2018, 8, 4464,  DOI: 10.1038/s41598-018-22242-8
      27
      Theory of isolated magnetic skyrmions: From fundamentals to room temperature applications
      Buttner Felix; Lemesh Ivan; Beach Geoffrey S D
      Scientific reports (2018), 8 (1), 4464 ISSN:.
      Magnetic skyrmions are topological quasiparticles of great interest for data storage applications because of their small size, high stability, and ease of manipulation via electric current. However, although models exist for some limiting cases, there is no universal theory capable of accurately describing the structure and energetics of all skyrmions. The main barrier is the complexity of non-local stray field interactions, which are usually included through crude approximations. Here we present an accurate analytical framework to treat isolated skyrmions in any material, assuming only a circularly-symmetric 360° domain wall profile and a homogeneous magnetization profile in the out-of-plane direction. We establish the first rigorous criteria to distinguish stray field from DMI skyrmions, resolving a major dispute in the community. We discover new phases, such as bi-stability, a phenomenon unknown in magnetism so far. We predict materials for sub-10 nm zero field room temperature stable skyrmions suitable for applications. Finally, we derive analytical equations to describe current-driven dynamics, find a topological damping, and show how to engineer materials in which compact skyrmions can be driven at velocities >1000 m/s.
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    28. 28
      Han, M.-G.; Garlow, J. A.; Liu, Y.; Zhang, H.; Li, J.; DiMarzio, D.; Knight, M. W.; Petrovic, C.; Jariwala, D.; Zhu, Y. Topological Magnetic-Spin Textures in Two-Dimensional van der Waals Cr2Ge2Te6. Nano Lett. 2019, 19, 7859– 7865,  DOI: 10.1021/acs.nanolett.9b02849
      28
      Topological Magnetic-Spin Textures in Two-Dimensional van der Waals Cr2Ge2Te6
      Han, Myung-Geun; Garlow, Joseph A.; Liu, Yu; Zhang, Huiqin; Li, Jun; DiMarzio, Donald; Knight, Mark W.; Petrovic, Cedomir; Jariwala, Deep; Zhu, Yimei
      Nano Letters (2019), 19 (11), 7859-7865CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Two-dimensional (2D) van der Waals (vdW) materials show a range of profound phys. properties that can be tailored through their incorporation in heterostructures and manipulated with external forces. The recent discovery of long-range ferromagnetic order down to at. layers provides an addnl. degree of freedom in engineering 2-dimensional materials and their heterostructure devices for spintronics, valleytronics, and magnetic tunnel junction switches. Here, using direct imaging by cryo-Lorentz TEM topol. nontrivial magnetic-spin states, skyrmionic bubbles, can be realized in exfoliated insulating 2-dimensional vdW Cr2Ge2Te6. Due to the competition between dipolar interactions and uniaxial magnetic anisotropy, hexagonally packed nanoscale bubble lattices emerge by field cooling with magnetic field applied along the out-of-plane direction. Despite a range of topol. spin textures in stripe domains arising due to pair formation and annihilation of Bloch lines, bubble lattices with single chirality are prevalent. Observation of topol. nontrivial homochiral skyrmionic bubbles in exfoliated vdW materials provides a new avenue for novel quantum states in atomically thin insulators for magneto-electronic and quantum devices.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXitVGmsLbE&md5=d01e64e4224e6d4ebfe73e8ee6577fae
    29. 29
      Wu, Y.; Francisco, B.; Chen, Z.; Wang, W.; Zhang, Y.; Wan, C.; Han, X.; Chi, H.; Hou, Y.; Lodesani, A.; Yin, G.; Liu, K.; Cui, Y.-t.; Wang, K. L.; Moodera, J. S. A Van der Waals Interface Hosting Two Groups of Magnetic Skyrmions. Adv. Mater. 2022, 34, 2110583,  DOI: 10.1002/adma.202110583
      29
      A Van der Waals Interface Hosting Two Groups of Magnetic Skyrmions
      Wu, Yingying; Francisco, Brian; Chen, Zhijie; Wang, Wei; Zhang, Yu; Wan, Caihua; Han, Xiufeng; Chi, Hang; Hou, Yasen; Lodesani, Alessandro; Yin, Gen; Liu, Kai; Cui, Yong-tao; Wang, Kang L.; Moodera, Jagadeesh S.
      Advanced Materials (Weinheim, Germany) (2022), 34 (16), 2110583CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Multiple magnetic skyrmion phases add an addnl. degree of freedom for skyrmion-based ultrahigh-d. spin memory devices. Extending the field to 2D van der Waals magnets is a rewarding challenge, where the realizable degree of freedoms (e.g., thickness, twist angle, and elec. gating) and high skyrmion d. result in intriguing new properties and enhanced functionality. In this work, a van der Waals interface, formed by two 2D ferromagnets Cr2Ge2Te6 and Fe3GeTe2 with a Curie temp. of ≈65 and ≈205 K, resp., hosting two groups of magnetic skyrmions, is reported. Two sets of topol. Hall effect signals are obsd. below 6s0 K when Cr2Ge2Te6 is magnetically ordered. These two groups of skyrmions are directly imaged using magnetic force microscopy, and supported by micromagnetic simulations. Interestingly, the magnetic skyrmions persist in the heterostructure with zero applied magnetic field. The results are promising for the realization of skyrmionic devices based on van der Waals heterostructures hosting multiple skyrmion phases.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38Xms1yhu7g%253D&md5=42b91dd86082c238ea70de70ccbf1328
    30. 30
      Ding, B.; Li, Z.; Xu, G.; Li, H.; Hou, Z.; Liu, E.; Xi, X.; Xu, F.; Yao, Y.; Wang, W. Observation of Magnetic Skyrmion Bubbles in a van der Waals Ferromagnet Fe3GeTe2. Nano Lett. 2020, 20, 868– 873,  DOI: 10.1021/acs.nanolett.9b03453
      30
      Observation of Magnetic Skyrmion Bubbles in a van der Waals Ferromagnet Fe3GeTe2
      Ding, Bei; Li, Zefang; Xu, Guizhou; Li, Hang; Hou, Zhipeng; Liu, Enke; Xi, Xuekui; Xu, Feng; Yao, Yuan; Wang, Wenhong
      Nano Letters (2020), 20 (2), 868-873CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Two-dimensional (2D) van der Waals (vdW) magnetic materials have recently been introduced as a new horizon in materials science, and they enable potential applications for next-generation spintronic devices. Here, the observations of stable Bloch-type magnetic skyrmions in single crystals of 2-dimensional vdW Fe3GeTe2 (FGT) are reported by using in situ Lorentz TEM. The ground-state magnetic stripe domains in FGT transform into skyrmion bubbles when an external magnetic field is applied perpendicularly to the (001) thin plate with temps. below the Curie temp. TC are found. Most interestingly, a hexagonal lattice of skyrmion bubbles is obtained via field-cooling manipulation with magnetic field applied along the [001] direction. Owing to their topol. stability, the skyrmion bubble lattices are stable to large field-cooling tilted angles and further reproduced by using the micromagnetic simulations. These observations directly demonstrate that the 2-dimensional vdW FGT possesses a rich variety of topol. spin textures, being of great promise for future applications in the field of spintronics.
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    31. 31
      Park, T.-E.; Peng, L.; Liang, J.; Hallal, A.; Yasin, F. S.; Zhang, X.; Song, K. M.; Kim, S. J.; Kim, K.; Weigand, M. Néel-Type Skyrmions and Their Current-Induced Motion in van der Waals Ferromagnet-Based Heterostructures. Phys. Rev. B 2021, 103, 104410,  DOI: 10.1103/physrevb.103.104410
      There is no corresponding record for this reference.
    32. 32
      Peng, L.; Yasin, F. S.; Park, T.-E.; Kim, S. J.; Zhang, X.; Nagai, T.; Kimoto, K.; Woo, S.; Yu, X. Tunable Néel–Bloch Magnetic Twists in Fe3GeTe2 with van der Waals Structure. Adv. Funct. Mater. 2021, 31, 2103583,  DOI: 10.1002/adfm.202103583
      32
      Tunable Neel-Bloch Magnetic Twists in Fe3GeTe2 with van der Waals Structure
      Peng, Licong; Yasin, Fehmi S.; Park, Tae-Eon; Kim, Sung Jong; Zhang, Xichao; Nagai, Takuro; Kimoto, Koji; Woo, Seonghoon; Yu, Xiuzhen
      Advanced Functional Materials (2021), 31 (37), 2103583CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The advent of ferromagnetism in 2D van der Waals (vdW) magnets has stimulated high interest in exploring topol. magnetic textures, such as skyrmions for use in future skyrmion-based spintronic devices. To engineer skyrmions in vdW magnets by transforming Bloch-type magnetic bubbles into Neel-type skyrmions, a heavy metal/vdW magnetic thin film heterostructure has been made to induce interfacial Dzyaloshinskii-Moriya interaction (DMI). However, the unambiguous identification of the magnetic textures inherent to vdW magnets, for example, whether the magnetic twists (skyrmions/domain walls) are Neel- or Bloch-type, is unclear. Here we demonstrate that the magnetic twists can be tuned between Neel and Bloch-type in the vdW magnet Fe3GeTe2 (FGT) with/without interfacial DMI. We use an in-plane magnetic field to align the modulation wavevector q of the magnetizations in order to distinguish the Neel- or Bloch-type magnetic twists. We observe that q is perpendicular to the in-plane field in the heterostructure (Pt/oxidized-FGT/FGT/oxidized-FGT), while q aligns at a rotated angle with respect to the field direction in the FGT thin plate thinned from bulk. We find that the aligned domain wall twists hold fan-like modulations, coinciding qual. with our computational results.
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    33. 33
      Wu, Y.; Zhang, S.; Zhang, J.; Wang, W.; Zhu, Y. L.; Hu, J.; Yin, G.; Wong, K.; Fang, C.; Wan, C. Néel-Type Skyrmion in WTe2/Fe3GeTe2 van der Waals Heterostructure. Nat. Commun. 2020, 11, 3860,  DOI: 10.1038/s41467-020-17566-x
      33
      Neel-type skyrmion in WTe2/Fe3GeTe2 van der Waals heterostructure
      Wu Yingying; Yin Gen; Wong Kin; Shao Qiming; Wang Kang L; Zhang Senfu; Zhang Junwei; Zhang Xixiang; Wang Wei; Zhu Yang Lin; Mao Zhiqiang; Hu Jin; Fang Chi; Wan Caihua; Han Xiufeng; Taniguchi Takashi; Watanabe Kenji; Zang Jiadong
      Nature communications (2020), 11 (1), 3860 ISSN:.
      The promise of high-density and low-energy-consumption devices motivates the search for layered structures that stabilize chiral spin textures such as topologically protected skyrmions. At the same time, recently discovered long-range intrinsic magnetic orders in the two-dimensional van der Waals materials provide a new platform for the discovery of novel physics and effects. Here we demonstrate the Dzyaloshinskii-Moriya interaction and Neel-type skyrmions are induced at the WTe2/Fe3GeTe2 interface. Transport measurements show the topological Hall effect in this heterostructure for temperatures below 100 K. Furthermore, Lorentz transmission electron microscopy is used to directly image Neel-type skyrmion lattice and the stripe-like magnetic domain structures as well. The interfacial coupling induced Dzyaloshinskii-Moriya interaction is estimated to have a large energy of 1.0 mJ m(-2). This work paves a path towards the skyrmionic devices based on van der Waals layered heterostructures.
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    34. 34
      Yang, M. Creation of Skyrmions in van der Waals Ferromagnet Fe3GeTe2 on (Co/Pd)n Superlattice. Sci. Adv. 2020, 6, eabb5157  DOI: 10.1126/sciadv.abb5157
      There is no corresponding record for this reference.
    35. 35
      Chakraborty, A.; Srivastava, A. K.; Sharma, A. K.; Gopi, A. K.; Mohseni, K.; Ernst, A.; Deniz, H.; Hazra, B. K.; Das, S.; Sessi, P.; Kostanovskiy, I.; Ma, T.; Meyerheim, H. L.; Parkin, S. S. P. Magnetic Skyrmions in a Thickness Tunable 2D Ferromagnet from a Defect Driven Dzyaloshinskii–Moriya Interaction. Adv. Mater. 2022, 34, 2108637,  DOI: 10.1002/adma.202108637
      There is no corresponding record for this reference.
    36. 36
      Birch, M. T.; Powalla, L.; Wintz, S.; Hovorka, O.; Litzius, K.; Loudon, J.; Turnbull, L.; Nehruji, V.; Son, K.; Bubeck, C. History-Dependent Domain and Skyrmion Formation in 2D van der Waals Magnet Fe3GeTe2. Nat. Commun. 2022, 13, 3035,  DOI: 10.1038/s41467-022-30740-7
      36
      History-dependent domain and skyrmion formation in 2D van der Waals magnet Fe3GeTe2
      Birch, M. T.; Powalla, L.; Wintz, S.; Hovorka, O.; Litzius, K.; Loudon, J. C.; Turnbull, L. A.; Nehruji, V.; Son, K.; Bubeck, C.; Rauch, T. G.; Weigand, M.; Goering, E.; Burghard, M.; Schuetz, G.
      Nature Communications (2022), 13 (1), 3035CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
      The discovery of two-dimensional magnets has initiated a new field of research, exploring both fundamental low-dimensional magnetism, and prospective spintronic applications. Recently, observations of magnetic skyrmions in the 2D ferromagnet Fe3GeTe2 (FGT) have been reported, introducing further application possibilities. However, controlling the exhibited magnetic state requires systematic knowledge of the history-dependence of the spin textures, which remains largely unexplored in 2D magnets. In this work, we utilize real-space imaging, and complementary simulations, to det. and explain the thickness-dependent magnetic phase diagrams of an exfoliated FGT flake, revealing a complex, history-dependent emergence of the uniformly magnetized, stripe domain and skyrmion states. The results show that the interplay of the dominant dipolar interaction and strongly temp. dependent out-of-plane anisotropy energy terms enables the selective stabilization of all three states at zero field, and at a single temp., while the Dzyaloshinksii-Moriya interaction must be present to realize the obsd. Neel-type domain walls. The findings open perspectives for 2D devices incorporating topol. spin textures.
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    37. 37
      Zhang, H.; Chen, R.; Zhai, K.; Chen, X.; Caretta, L.; Huang, X.; Chopdekar, R. V.; Cao, J.; Sun, J.; Yao, J.; Birgeneau, R.; Ramesh, R. Itinerant Ferromagnetism in van der Waals Fe5GeTe2 Crystals Above Room Temperature. Phys. Rev. B 2020, 102, 064417,  DOI: 10.1103/PhysRevB.102.064417
      There is no corresponding record for this reference.
    38. 38
      Lv, X.; Pei, K.; Yang, C.; Qin, G.; Liu, M.; Zhang, J.; Che, R. Controllable Topological Magnetic Transformations in the Thickness-Tunable van der Waals Ferromagnet Fe5GeTe2. ACS Nano 2022, 16, 19319– 19327,  DOI: 10.1021/acsnano.2c08844
      There is no corresponding record for this reference.
    39. 39
      Ly, T. T.; Park, J.; Kim, K.; Ahn, H.; Lee, N. J.; Kim, K.; Park, T.; Duvjir, G.; Lam, N. H.; Jang, K. Direct Observation of Fe-Ge Ordering in Fe5–xGeTe2 Crystals and Resultant Helimagnetism. Adv. Funct. Mater. 2021, 31, 2009758,  DOI: 10.1002/adfm.202009758
      39
      Direct Observation of Fe-Ge Ordering in Fe5-xGeTe2 Crystals and Resultant Helimagnetism
      Ly, Trinh Thi; Park, Jungmin; Kim, Kyoo; Ahn, Hyo-Bin; Lee, Nyun Jong; Kim, Kwangsu; Park, Tae-Eon; Duvjir, Ganbat; Lam, Nguyen Huu; Jang, Kyuha; You, Chun-Yeol; Jo, Younghun; Kim, Se Kwon; Lee, Changgu; Kim, Sanghoon; Kim, Jungdae
      Advanced Functional Materials (2021), 31 (17), 2009758CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Microscopic structures and magnetic properties are investigated for Fe5-xGeTe2 single crystal, recently discovered as a promising van der Waals (vdW) ferromagnet. An Fe atom (Fe(1)) located in the outermost Fe5Ge sublayer has two possible split-sites which are either above or below the Ge atom. Scanning tunneling microscopy shows √3 × √3 superstructures which are attributed to the ordering of Fe(1) layer. The √3 × √3 superstructures have two different phases due to the symmetry of Fe(1) ordering. Intriguingly, the obsd. √3 × √3 ordering breaks the inversion symmetry of crystal, resulting in substantial antisym. exchange interaction. The temp. dependence of magnetization reveals a sharp magnetic anomaly suggesting helical magnetism of the Fe5-xGeTe2 due to its non-centrosymmetricity. Anal. study also supports that the obsd. ordering can give rise to the helimagnetism. The work will provide essential information to understand the complex magnetic properties and the origin of the new vdW ferromagnet, Fe5-xGeTe2 for future topol.-based spin devices.
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    40. 40
      Zhang, C.; Liu, C.; Zhang, S.; Zhou, B.; Guan, C.; Ma, Y.; Algaidi, H.; Zheng, D.; Li, Y.; He, X. Magnetic Skyrmions with Unconventional Helicity Polarization in a Van Der Waals Ferromagnet. Adv. Mater. 2022, 34, 2204163,  DOI: 10.1002/adma.202204163
      40
      Magnetic Skyrmions with Unconventional Helicity Polarization in a Van Der Waals Ferromagnet
      Zhang, Chenhui; Liu, Chen; Zhang, Senfu; Zhou, Bojian; Guan, Chaoshuai; Ma, Yinchang; Algaidi, Hanin; Zheng, Dongxing; Li, Yan; He, Xin; Zhang, Junwei; Li, Peng; Hou, Zhipeng; Yin, Gen; Liu, Kai; Peng, Yong; Zhang, Xi-Xiang
      Advanced Materials (Weinheim, Germany) (2022), 34 (42), 2204163CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Skyrmion helicity, which defines the spin swirling direction, is a fundamental parameter that may be utilized to encode data bits in future memory devices. Generally, in centrosym. ferromagnets, dipole skyrmions with helicity of -π/2 and π/2 are degenerate in energy, leading to equal populations of both helicities. On the other hand, in chiral materials where the Dzyaloshinskii-Moriya interaction (DMI) prevails and the dipolar interaction is negligible, only a preferred helicity is selected by the type of DMI. However, whether there is a rigid boundary between these two regimes remains an open question. Herein, the observation of dipole skyrmions with unconventional helicity polarization in a van der Waals ferromagnet, Fe5-δGeTe2, is reported. Combining magnetometry, Lorentz transmission electron microscopy, elec. transport measurements, and micromagnetic simulations, the short-range superstructures in Fe5-δGeTe2 resulting in a localized DMI contribution, which breaks the degeneracy of the opposite helicities and leads to the helicity polarization, is demonstrated. Therefore, the helicity feature in Fe5-δGeTe2 is controlled by both the dipolar interaction and DMI that the former leads to Bloch-type skyrmions with helicity of ±π/2 whereas the latter breaks the helicity degeneracy. This work provides new insights into the skyrmion topol. in van der Waals materials.
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    41. 41
      Schmitt, M.; Denneulin, T.; Kovács, A.; Saunderson, T. G.; Rüßmann, P.; Shahee, A.; Scholz, T.; Tavabi, A. H.; Gradhand, M.; Mavropoulos, P.; Lotsch, B. V.; Dunin-Borkowski, R. E.; Mokrousov, Y.; Blügel, S.; Kläui, M. Skyrmionic Spin structures in Layered Fe5GeTe2 up to Room Temperature. Commun. Phys. 2022, 5, 254,  DOI: 10.1038/s42005-022-01031-w
      41
      Skyrmionic spin structures in layered Fe5GeTe2 up to room temperature
      Schmitt, Maurice; Denneulin, Thibaud; Kovacs, Andras; Saunderson, Tom G.; Ruessmann, Philipp; Shahee, Aga; Scholz, Tanja; Tavabi, Amir H.; Gradhand, Martin; Mavropoulos, Phivos; Lotsch, Bettina V.; Dunin-Borkowski, Rafal E.; Mokrousov, Yuriy; Bluegel, Stefan; Klaeui, Mathias
      Communications Physics (2022), 5 (1), 254CODEN: CPOHDJ; ISSN:2399-3650. (Nature Portfolio)
      Abstr.: The role of the crystal lattice, temp. and magnetic field for the spin structure formation in the 2D van der Waals magnet Fe5GeTe2 with magnetic ordering up to room temp. is a key open question. Using Lorentz transmission electron microscopy, we exptl. observe topol. spin structures up to room temp. in the metastable pre-cooling and stable post-cooling phase of Fe5GeTe2. Over wide temp. and field ranges, skyrmionic magnetic bubbles form without preferred chirality, which is indicative of centrosymmetry. These skyrmions can be obsd. even in the absence of external fields. To understand the complex magnetic order in Fe5GeTe2, we compare macroscopic magnetometry characterization results with microscopic d. functional theory and spin-model calcns. Our results show that even up to room temp., topol. spin structures can be stabilized in centrosym. van der Waals magnets.
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    42. 42
      Fujita, R.; Bassirian, P.; Li, Z.; Guo, Y.; Mawass, M. A.; Kronast, F.; van der Laan, G.; Hesjedal, T. Layer-Dependent Magnetic Domains in Atomically Thin Fe5GeTe2. ACS Nano 2022, 16, 10545– 10553,  DOI: 10.1021/acsnano.2c01948
      42
      Layer-Dependent Magnetic Domains in Atomically Thin Fe5GeTe2
      Fujita, Ryuji; Bassirian, Pedram; Li, Zhengxian; Guo, Yanfeng; Mawass, Mohamad A.; Kronast, Florian; van der Laan, Gerrit; Hesjedal, Thorsten
      ACS Nano (2022), 16 (7), 10545-10553CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Magnetic domain formation in two-dimensional (2D) materials gives perspectives into the fundamental origins of 2D magnetism and also motivates the development of advanced spintronics devices. However, the characterization of magnetic domains in atomically thin van der Waals (vdW) flakes remains challenging. Here, we employ X-ray photoemission electron microscopy (XPEEM) to perform layer-resolved imaging of the domain structures in the itinerant vdW ferromagnet Fe5GeTe2 which shows near room temp. bulk ferromagnetism and a weak perpendicular magnetic anisotropy (PMA). In the bulk limit, we observe the well-known labyrinth-type domains. Thinner flakes, on the other hand, are characterized by increasingly fragmented domains. While PMA is a characteristic property of Fe5GeTe2, we observe a spin-reorientation transition with the spins canting in-plane for flakes thinner than six layers. Notably, a bubble phase emerges in four-layer flakes. This thickness dependence, which clearly deviates from the single-domain behavior obsd. in other 2D magnetic materials, demonstrates the exciting prospect of stabilizing complex spin textures in 2D vdW magnets at relatively high temps.
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    43. 43
      Högen, M.; Fujita, R.; Tan, A. K. C.; Geim, A.; Pitts, M.; Li, Z.; Guo, Y.; Stefan, L.; Hesjedal, T.; Atatüre, M. Imaging Nucleation and Propagation of Pinned Domains in Few-Layer Fe5GeTe2. ACS Nano 2023, 17, 16879– 16885,  DOI: 10.1021/acsnano.3c03825
      There is no corresponding record for this reference.
    44. 44
      Zhang, H.; Raftrey, D.; Chan, Y. T.; Shao, Y. T.; Chen, R.; Chen, X.; Huang, X.; Reichanadter, J. T.; Dong, K.; Susarla, S. Room-Temperature Skyrmion Lattice in a Layered Magnet (Fe0.5Co0.5)5GeTe2. Sci. Adv. 2022, 8, eabm7103  DOI: 10.1126/sciadv.abm7103
      There is no corresponding record for this reference.
    45. 45
      Zhang, H.; Shao, Y. T.; Chen, R.; Chen, X.; Susarla, S.; Raftrey, D.; Reichanadter, J. T.; Caretta, L.; Huang, X.; Settineri, N. S. A Room Temperature Polar Magnetic Metal. Phys. Rev. Mater. 2022, 6, 044403,  DOI: 10.1103/PhysRevMaterials.6.044403
      45
      A room temperature polar magnetic metal
      Zhang, Hongrui; Shao, Yu-Tsun; Chen, Rui; Chen, Xiang; Susarla, Sandhya; Raftrey, David; Reichanadter, Jonathan T.; Caretta, Lucas; Huang, Xiaoxi; Settineri, Nicholas S.; Chen, Zhen; Zhou, Jingcheng; Bourret-Courchesne, Edith; Ercius, Peter; Yao, Jie; Fischer, Peter; Neaton, Jeffrey B.; Muller, David A.; Birgeneau, Robert J.; Ramesh, Ramamoorthy
      Physical Review Materials (2022), 6 (4), 044403CODEN: PRMHBS; ISSN:2475-9953. (American Physical Society)
      The emergence of long-range magnetic order in noncentrosym. compds. has stimulated interest in the possibility of exotic spin transport phenomena and topol. protected spin textures for applications in next-generation spintronics. Polar magnets, with broken symmetries of spatial inversion and time reversal, usually host chiral spin textures. This work reports on a wurtzite-structure polar magnetic metal, identified as AA'-stacked (Fe0.5Co0.5)5GeTe2, which exhibits a Neel-type skyrmion lattice as well as a Rashba-Edelstein effect at room temp. Atomic resoln. imaging of the structure reveals a structural transition as a function of Co-substitution, leading to the emergence of the polar phase at 50% Co. This discovery reveals an unprecedented layered polar magnetic system for investigating intriguing spin topologies, and it ushers in a promising new framework for spintronics.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38XhtlamurfF&md5=10063a2b48adc59971e7fbd1e3400196
    46. 46
      Meisenheimer, P.; Zhang, H.; Raftrey, D.; Chen, X.; Shao, Y.-T.; Chan, Y.-T.; Yalisove, R.; Chen, R.; Yao, J.; Scott, M. C.; Wu, W.; Muller, D. A.; Fischer, P.; Birgeneau, R. J.; Ramesh, R. Ordering of Room-Temperature Magnetic Skyrmions in a Polar van der Waals Magnet. Nat. Commun. 2023, 14, 3744,  DOI: 10.1038/s41467-023-39442-0
      There is no corresponding record for this reference.
    47. 47
      Stahl, J.; Shlaen, E.; Johrendt, D. The van der Waals Ferromagnets Fe5−δGeTe2 and Fe5−δ−xNixGeTe2 – Crystal Structure, Stacking Faults, and Magnetic Properties. Z. Anorg. Allg. Chem. 2018, 644, 1923– 1929,  DOI: 10.1002/zaac.201800456
      47
      The van der Waals Ferromagnets Fe5-δGeTe2 and Fe5-δ-xNixGeTe2 - Crystal Structure, Stacking Faults, and Magnetic Properties
      Stahl, Juliane; Shlaen, Evgeniya; Johrendt, Dirk
      Zeitschrift fuer Anorganische und Allgemeine Chemie (2018), 644 (24), 1923-1929CODEN: ZAACAB; ISSN:1521-3749. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Fe5-δGeTe2 was synthesized by heating the elements at 1050 K and characterized by single crystal and powder X-ray anal. The structure [R3m, a = 4.0376(4) Å, c = 29.194(6) Å] consists of Fe5-δGe layers sepd. by tellurium double layers forming a van der Waals gap. The pronounced two-dimensional character of Fe5-δGeTe2 causes stacking faults along the c direction. Simulations of different stacking variants using the DIFFaX software reveal disorder occurring in domains. Magnetic measurements of Fe5-δGeTe2 show ferromagnetism below 279 K with a satn. moment of 1.80 μB at 1.8 K. Nickel substitution of the iron sites has little influence on the structure but changes the satn. moment, which passes through a max. of 2.11 μB in Fe4.11Ni0.50GeTe2. This indicates that structural influences as well as the diln. of the magnetic iron atoms play a decisive role.
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    48. 48
      Alahmed, L.; Nepal, B.; Macy, J.; Zheng, W.; Casas, B.; Sapkota, A.; Jones, N.; Mazza, A. R.; Brahlek, M.; Jin, W. Magnetism and Spin Dynamics in Room-Temperature van der Waals Magnet Fe5GeTe2. 2D Mater. 2021, 8, 045030,  DOI: 10.1088/2053-1583/ac2028
      48
      Magnetism and spin dynamics in room-temperature van der Waals magnet Fe5GeTe2
      Alahmed, Laith; Nepal, Bhuwan; Macy, Juan; Zheng, Wenkai; Casas, Brian; Sapkota, Arjun; Jones, Nicholas; Mazza, Alessandro R.; Brahlek, Matthew; Jin, Wencan; Mahjouri-Samani, Masoud; Zhang, Steven S.-L.; Mewes, Claudia; Balicas, Luis; Mewes, Tim; Li, Peng
      2D Materials (2021), 8 (4), 045030CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
      Two-dimensional van der Waals (vdWs) materials have gathered a lot of attention recently. However, the majority of these materials have Curie temps. that are well below room temp., making it challenging to incorporate them into device applications. In this work, we synthesized a room-temp. vdW magnetic crystal Fe5GeTe2 with a Curie temp. Tc = 332 K, and studied its magnetic properties by vibrating sample magnetometry (VSM) and broadband ferromagnetic resonance (FMR) spectroscopy. The expts. were performed with external magnetic fields applied along the c-axis (H‖c) and the ab-plane (H‖ab), with temps. ranging from 300 to 10 K. We have found a sizable Lande g-factor difference between the H‖c and H‖ab cases. In both cases, the Lande g-factor values deviated from g = 2. This indicates contribution of orbital angular momentum to the magnetic moment. The FMR measurements reveal that Fe5GeTe2 has a damping const. comparable to Permalloy. With reducing temp., the linewidth was broadened. Together with the VSM data, our measurements indicate that Fe5GeTe2 transitions from ferromagnetic to ferrimagnetic at lower temps. Our expts. highlight key information regarding the magnetic state and spin scattering processes in Fe5GeTe2, which promote the understanding of magnetism in Fe5GeTe2, leading to implementations of Fe5GeTe2 based room-temp. spintronic devices.
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    49. 49
      Chacon, A.; Heinen, L.; Halder, M.; Bauer, A.; Simeth, W.; Mühlbauer, S.; Berger, H.; Garst, M.; Rosch, A.; Pfleiderer, C. Observation of Two Independent Skyrmion Phases in a Chiral Magnetic Material. Nat. Phys. 2018, 14, 936– 941,  DOI: 10.1038/s41567-018-0184-y
      49
      Observation of two independent skyrmion phases in a chiral magnetic material
      Chacon, A.; Heinen, L.; Halder, M.; Bauer, A.; Simeth, W.; Muehlbauer, S.; Berger, H.; Garst, M.; Rosch, A.; Pfleiderer, C.
      Nature Physics (2018), 14 (9), 936-941CODEN: NPAHAX; ISSN:1745-2473. (Nature Research)
      Magnetic materials can host skyrmions, which are topol. non-trivial spin textures. In chiral magnets with cubic lattice symmetry, all previously obsd. skyrmion phases require thermal fluctuations to become thermodynamically stable in bulk materials, and therefore exist only at relatively high temp., close to the helimagnetic transition temp. Other stabilization mechanisms require a lowering of the cubic crystal symmetry. Here, we report the identification of a second skyrmion phase in Cu2OSeO3 at low temp. and in the presence of an applied magnetic field. The new skyrmion phase is thermodynamically disconnected from the well-known, nearly isotropic, high-temp. phase, and exists, in contrast, when the external magnetic field is oriented along the 〈100〉 crystal axis only. Theor. modeling provides evidence that the stabilization mechanism is given by well-known cubic anisotropy terms, and accounts for an addnl. observation of metastable helixes tilted away from the applied field. The identification of two distinct skyrmion phases in the same material and the generic character of the underlying mechanism suggest a new avenue for the discovery, design and manipulation of topol. spin textures.
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    50. 50
      Halder, M.; Chacon, A.; Bauer, A.; Simeth, W.; Mühlbauer, S.; Berger, H.; Heinen, L.; Garst, M.; Rosch, A.; Pfleiderer, C. Thermodynamic Evidence of a Second Skyrmion Lattice Phase and Tilted Conical Phase in Cu2OSeO3. Phys. Rev. B 2018, 98, 144429,  DOI: 10.1103/PhysRevB.98.144429
      There is no corresponding record for this reference.
    51. 51
      Callen, E. R.; Callen, H. B. Anisotropic Magnetization. J. Phys. Chem. Solids 1960, 16, 310– 328,  DOI: 10.1016/0022-3697(60)90161-X
      There is no corresponding record for this reference.
    52. 52
      Teodorescu, C. M. Kittel’s Model for Ferromagnetic Domains, Revised and Completed, Including the Derivation of the Magnetic Hysteresis. Results Phys. 2023, 46, 106287,  DOI: 10.1016/j.rinp.2023.106287
      There is no corresponding record for this reference.
    53. 53
      Kotani, A.; Nakajima, H.; Harada, K.; Ishii, Y.; Mori, S. Field-Temperature Phase Diagram of Magnetic Bubbles Spanning Charge/Orbital Ordered and Metallic Phases in La1–xSrxMnO3. Phys. Rev. B 2017, 95, 144403,  DOI: 10.1103/PhysRevB.95.144403
      There is no corresponding record for this reference.
    54. 54
      Peng, L.; Iakoubovskii, K. V.; Karube, K.; Taguchi, Y.; Tokura, Y.; Yu, X. Formation and Control of Zero-Field Antiskyrmions in Confining Geometries. Advanced Science 2022, 9, 2202950,  DOI: 10.1002/advs.202202950
      There is no corresponding record for this reference.
    55. 55
      Casas, B. W.; Li, Y.; Moon, A.; Xin, Y.; McKeever, C.; Macy, J.; Petford-Long, A. K.; Phatak, C. M.; Santos, E. J. G.; Choi, E. S.; Balicas, L. Coexistence of Merons with Skyrmions in the Centrosymmetric Van Der Waals Ferromagnet Fe5–xGeTe2. Adv. Mater. 2023, 35, 2212087,  DOI: 10.1002/adma.202212087
      55
      Coexistence of Merons with Skyrmions in the Centrosymmetric Van Der Waals Ferromagnet Fe5-xGeTe2
      Casas, Brian W.; Li, Yue; Moon, Alex; Xin, Yan; McKeever, Conor; Macy, Juan; Petford-Long, Amanda K.; Phatak, Charudatta M.; Santos, Elton J. G.; Choi, Eun Sang; Balicas, Luis
      Advanced Materials (Weinheim, Germany) (2023), 35 (17), 2212087CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Fe5-xGeTe2 is a centrosym., layered van der Waals (vdW) ferromagnet that displays Curie temps. Tc (270-330 K) that are within the useful range for spintronic applications. However, little is known about the interplay between its topol. spin textures (e.g., merons, skyrmions) with technol. relevant transport properties such as the topol. Hall effect (THE) or topol. thermal transport. Here, via high-resoln. Lorentz transmission electron microscopy, it is shown that merons and anti-meron pairs coexist with Neel skyrmions in Fe5-xGeTe2 over a wide range of temps. and probe their effects on thermal and elec. transport. A THE is detected, even at room T, that senses merons at higher T's, as well as their coexistence with skyrmions as T is lowered, indicating an on-demand thermally driven formation of either type of spin texture. Remarkably, an unconventional THE is also obsd. in absence of Lorentz force, and it is attributed to the interaction between charge carriers and magnetic field-induced chiral spin textures. These results expose Fe5-xGeTe2 as a promising candidate for the development of applications in skyrmionics/meronics due to the interplay between distinct but coexisting topol. magnetic textures and unconventional transport of charge/heat carriers.
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    56. 56
      Nagaosa, N.; Tokura, Y. Topological Properties and Dynamics of Magnetic Skyrmions. Nat. Nanotechnol. 2013, 8, 899– 911,  DOI: 10.1038/nnano.2013.243
      56
      Topological properties and dynamics of magnetic skyrmions
      Nagaosa, Naoto; Tokura, Yoshinori
      Nature Nanotechnology (2013), 8 (12), 899-911CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      A review. Magnetic skyrmions are particle-like nanometer-sized spin textures of topol. origin found in several magnetic materials, and are characterized by a long lifetime. Skyrmions have been obsd. both by means of neutron scattering in momentum space and microscopy techniques in real space, and their properties include novel Hall effects, current-driven motion with ultralow c.d. and multiferroic behavior. These properties can be understood from a unified viewpoint, namely the emergent electromagnetism assocd. with the non-coplanar spin structure of skyrmions. From this description, potential applications of skyrmions as information carriers in magnetic information storage and processing devices are envisaged.
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    57. 57
      Litzius, K.; Leliaert, J.; Bassirian, P.; Rodrigues, D.; Kromin, S.; Lemesh, I.; Zazvorka, J.; Lee, K. J.; Mulkers, J.; Kerber, N. The Role of Temperature and Drive Current in Skyrmion Dynamics. Nat. Electron. 2020, 3, 30– 36,  DOI: 10.1038/s41928-019-0359-2
      57
      The role of temperature and drive current in skyrmion dynamics
      Litzius, Kai; Leliaert, Jonathan; Bassirian, Pedram; Rodrigues, Davi; Kromin, Sascha; Lemesh, Ivan; Zazvorka, Jakub; Lee, Kyu-Joon; Mulkers, Jeroen; Kerber, Nico; Heinze, Daniel; Keil, Niklas; Reeve, Robert M.; Weigand, Markus; Van Waeyenberge, Bartel; Schuetz, Gisela; Everschor-Sitte, Karin; Beach, Geoffrey S. D.; Klaeui, Mathias
      Nature Electronics (2020), 3 (1), 30-36CODEN: NEALB3; ISSN:2520-1131. (Nature Research)
      Magnetic skyrmions are topol. stabilized nanoscale spin structures that could be of use in the development of future spintronic devices. When a skyrmion is driven by an elec. current it propagates at an angle relative to the flow of current-known as the skyrmion Hall angle (SkHA)-that is a function of the drive current. This drive dependence, as well as thermal effects due to Joule heating, could be used to tailor skyrmion trajectories, but are not well understood. Here we report a study of skyrmion dynamics as a function of temp. and drive amplitude. We find that the skyrmion velocity depends strongly on temp., while the SkHA does not and instead evolves differently in the low- and high-drive regimes. In particular, the max. skyrmion velocity in ferromagnetic devices is limited by a mechanism based on skyrmion surface tension and deformation (where the skyrmion transitions into a stripe). Our mechanism provides a complete description of the SkHA in ferromagnetic multilayers across the full range of drive strengths, illustrating that skyrmion trajectories can be engineered for device applications.
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  • Supporting Information

    Supporting Information

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    • X-ray absorption spectra of the F5GT flakes; additional LTEM images and accompanying phase diagrams; magnetometry data of the bulk slow-cooled F5GT crystal; additional STXM images of two further quenched F5GT flakes; EDX data; and analysis of the LTEM data concerning the helical state at zero applied field (PDF)


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