Crystal Structure, Magnetism, and Electronic Properties of New Rare-Earth-Free Ferromagnetic MnPt5As

The design and synthesis of targeted functional materials have been a long-term goal for material scientists. Although a universal design strategy is difficult to generate for all types of materials, however, it is still helpful for a typical family of materials to have such design rules. Herein, we incorporated several significant chemical and physical factors regarding magnetism, such as structure type, atom distance, spin-orbit coupling, and successfully synthesized a new rare-earth-free ferromagnet, MnPt5As, for the first time. MnPt5As can be prepared by using high-temperature pellet methods. According to X-ray diffraction results, MnPt5As crystallizes in a tetragonal unit cell with the space group P4/mmm (Pearson symbol tP7). Magnetic measurements on MnPt5As confirm ferromagnetism in this phase with a Curie temperature of ~301 K and a saturated moment of 3.5 uB per formula. Evaluation by applying the Stoner Criterion also indicates that MnPt5As is susceptible to ferromagnetism. Electronic structure calculations using the WIEN2k program with local spin density approximation imply that the spontaneous magnetization of this phase arises primarily from the hybridization of d orbitals on both Mn and Pt atoms. The theoretical assessments are consistent with the experimental results. Moreover, the spin-orbit coupling effects heavily influence on magnetic moments in MnPt5As. MnPt5As is the first high-performance magnetic material in this structure type. The discovery of MnPt5As offers a platform to study the interplay between magnetism and structure.


Introduction
How one can easily approach new ferromagnetic materials (e.g. a practical design strategy), especially those with high transition temperatures and saturated moments, is always a demanding necessity for materials synthetic scientists. Meanwhile, the exploration of ferromagnetic intermetallic materials without strategically vulnerable rare-earth elements is critical for information technology applications, such as magnetic and magnetoelastic sensors, 1 hard-disk drives, 2 spintronics, [3][4][5] and biomedical devices. 6 Most research focus on using magnetically active 3d transition metals to replace rare-earth elements, such as Fe, and Co. 7-10 Some of the 3d elements are ferromagnetic ordered above room temperature (e.g. Fe, Co, Ni). 11 When alloyed with other elements, the magnetic and mechanic properties can be tuned accordingly. 12,13 AlNiCo-type magnetic systems attracted attention for their temperature stability and mechanical properties.
As reported, Mn-Mn distances significantly affected how magnetic moments ordered in some systems, such as RMn2Ge2 and RMn6Ge6 (R = heavy-lanthanide elements or Y). [33][34][35] Moreover, the distribution pattern of Mn atoms in AMnX-based compounds (A = Li, Ni, Cu or La; X = As or Sb) are showing similar behaviors and also related to the saturated moment per Mn atom. 36 In such systems as mentioned, usually ferromagnetic ordering and high saturated magnetic moment are preferred by longer Mn-Mn distance while shorter Mn-Mn distance favors antiferromagnetic interactions. With this intriguing feature of Mn atoms, our candidate system is tetragonal MgPt5As structure, which can be considered anti-format of one of the most well-known heavy fermion superconductors, CeCoIn5. 37 Among all four atomic sites in MgPt5As-type structure, it is easily to be aware that if Mn atom can be placed onto Mg site in which Mg/As are distributed around 4.6 Å, the neighboring Mn-Mn distance can reach over 4 Å, due to the periodicity of unit cells and the change of atomic radii, which is comparable as the ferromagnetic cases in ref. 36.
Furthermore, with the electronic configuration of [Ar]3d 5 4s 2 , Mn is well-known to provide high magnetic moments in intermetallics. The spin-orbit coupling (SOC) effect is another crucial factor when designing new magnetic materials due to its ability to interplay with spin-polarization (magnetism) of the material. 38 For example, strong SOC effect can enhance magnetocrystalline anisotropy which is important for high coercivities and energy products in ferromagnets, such as FePt 39,40 . Heavy elements with 4d/5d or 5p/6p block are normally considered to be the source of strong SOC effects, for instance, platinum and bismuth.
With these design rules in mind, we successfully designed and synthesized a new rareearth-free ferromagnetic intermetallic compound MnPt5As. Magnetic properties measurements and theoretical calculations both show consistently that MnPt5As favors ferromagnetic ordering with a saturated moment ~ 3.5 B per formula. The magnetic moment in MnPt5As is theoretically proved to be strongly related to the atomic interaction between Mn and its neighboring Pt atoms.
Meanwhile, MnPt5As is the first high-Tc magnetic material in this structure type. The discovery of MnPt5As paves a way to more remarkable magnetic compounds with similar structures.

Experimental Section
Preparation of Polycrystalline MnPt5As: Polycrystalline MnPt5As was synthesized by the hightemperature solid-state method. Mn powder, Pt powder, and As powder were evenly mixed and pelletized in an argon-filled glovebox due to the toxicity of arsenic. 41 The obtained pellet was placed in an alumina crucible and sealed into an evacuated silica tube (<10 -5 torr). The sealed sample was heated up to 1050 o C at a rate of 30 o C per hour and stayed for 2 days followed by a cooling procedure to room temperature in 5 days. The polycrystalline product is air and moisture stable. frame. The scanning 2 width was set to 0.5. Direct methods and full-matrix least-squares on F 2 models within the SHELXTL package were applied to solve the structure. 43 Data acquisition was obtained via Bruker SMART software with the corrections on Lorentz and polarization effect done by SAINT program. Numerical absorption corrections were accomplished with XPREP. 44,45 Scanning Electron Microscope (SEM): Crystal picture was taken using a high vacuum scanning electron microscope (SEM) (JSM-6610 LV). Samples were placed on carbon tape prior to loading into the SEM chamber and were examined at 20 kV.

Physical Properties Measurements: The Quantum Design Dynacool Physical Property
Measurement System (PPMS) is used to measure the magnetic property and resistivity with the temperature range from 1.8 to 300 K with and without applied fields. The magnetic susceptibility is defined as χ = M/H. Here, M is the magnetization in units of emu, and H is the applied magnetic field. A standard relaxation calorimetry method was used to measure heat capacity and the data were collected in zero magnetic field between 220 K and 320 K using H-type grease. All the measurements were performed on polycrystalline samples manually selected from MnPt5As.

Electronic Structure Calculations:
The electronic and ferromagnetic structures were calculated using Tight-Binding, Linear Muffin-Tin Orbital-Atomic Spheres Approximation (TB-LMTO-ASA) with local (spin) density approximation (L(S)DA). [46][47][48] The empty spheres are required during the calculation with the overlap of Wigner-Seitz (WS) spheres limited to smaller than 16%.
A mesh of 9×9×6 k-points in the first Brillouin zone (BZ) was used to perform the detailed calculations and obtain the density of states (DOS) and Crystal Orbital Hamiltonian Population (COHP) curves. The band structure, Fermi surfaces, and density of states (DOS) of MnPt5As were also calculated using the WIEN2k program, which has the full-potential linearized augmented plane wave method (FP-LAPW) with local orbitals implemented. 49,50 The electron exchangecorrelation potential was used to treat the electron correlation within the generalized gradient approximation, which is parameterized by Perdew et. al. 51 The conjugate gradient algorithm was applied, and the energy cutoff was set at 500 eV. Reciprocal space integrations were completed over an 8×8×4 Monkhorst-Pack k-points mesh. 52 With these settings, the calculated total energy converged to less than 0.1 meV per atom. The spin-orbit coupling (SOC) effects were only applied for Pt atoms. The structural lattice parameters obtained from experiments are used for both calculations.

Results and Discussion
Crystal Structure and Phase Determination of New Phase MnPt5As: To obtain the structural feature of the new phase, the single crystal of MnPt5As was investigated. The refined results, crystallographic data and anisotropic displacement parameters are shown in Tables 1, 2 and 3. MnPt5As crystallize in the tetragonal structure with the space group P4/mmm (No. 123). Mn located at 1c (C4 symmetry), Pt atoms are on 1a (C4 symmetry) and 4i (C2 symmetry) sites, and As atom occupies 1b (C4 symmetry) sites. The partial occupancy refinements were tested, and no vacancy or mixture was discovered. A typical layered feature can be observed where the face-

Resistivity of MnPt5As:
To study the electronic properties of MnPt5As, the resistivity of a small single crystal of MnPt5As was picked up and measured using the 4-probe method. FIG. 4 illustrates the temperature-dependence of resistivity without an applied magnetic field. The resistivity measurement shows the metallic properties of MnPt5As. An anomaly can be observed around 300 K which reflects the ferromagnetic ordering transition, similar with other ferromagnetic intermetallics, for example, Ce2CoGe3, Ce5Co4Ge13 and Y(Fe1-xCox)2 55,56 . The reason for the kink observed could be that the major contribution of resistivity in MnPt5As changes from electronphonon scattering (at a higher temperature) to electron-electron scattering (below magnetic ordering temperature) while it is possible that additional magnetoresistance contribution develops below Tc. A high-quality single crystal may be necessary for further investigation. The resistivity curve was fitted by using power law (T) = 0 + AT n where 0 is residual resistivity due to defect scattering, A is a constant and n is an integer determined by the interaction pattern. At hightemperature (HT) region before the magnetic ordering temperature (310 K-350 K), as shown by the green line, we found that it fits well when n equals 1 which means the resistivity has mainly resulted from the collision between electron and phonon. As the temperature goes down, the phonon contribution decreases significantly while n is fitted to be 3 at the low-temperature (LT) region (4-25 K). This confirms that s-d electron scattering is a possible predominant mechanism at low temperature. 57 The residual resistivity 0 for HT and LT regions was fitted to be 18.

Conclusion
In this paper, we successfully synthesized the polycrystalline of a new rare-earth-free Pt-rich ferromagnet, MnPt5As. The new material holds a layered tetragonal crystal structure with a space group of P4/mmm. Magnetic properties measurements proved that MnPt5As is ferromagnetic ordered at room temperature. Resistivity and heat capacity measurements confirm the magnetic properties measurements. The theoretical electronic structure demonstrates that the Pt-Mn and Pt-Pt antibonding interactions are critical to the ferromagnetic properties in MnPt5As. Moreover, the spin-orbit coupling effect of Pt atoms is essential in decreasing the density of states of Mn atom at EF by splitting the van Hove singularity at the Fermi level, thus lowering the saturated moment.
Moreover, MnPt5As is the first high-Tc magnetic material holding this structure type. It provides a great platform to design and investigate new magnetic materials with targeted functional properties.

Supporting Information
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Author Information
Corresponding Author: weiweix@lsu.edu Notes: The authors declare no competing financial interest.