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Zintl compounds have been extensively studied for their outstanding thermoelectric properties, but their electronic structure remains largely unexplored. Here, we present a detailed investigation of the electronic structure of the isostructural thermopower materials YbMg$_2$Bi$_2$ and CaMg$_2$Bi$_2$ using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). The ARPES results show a significantly smaller Fermi surface and Fermi velocity in CaMg$_2$Bi$_2$ than in YbMg$_2$Bi$_2$. Our ARPES results also reveal that in the case of YbMg$_2$Bi$_2$, Yb-4$f$ states reside well below the Fermi level and likely have a negligible impact on transport properties. To properly model the position of 4$f$-states, as well as the overall electronic structure, a Hubbard $U$ at the Yb sites and spin-orbit coupling (SOC) have to be included in the DFT calculations. Interestingly, the theoretical results reveal that both materials belong to a $Z_2$ topological class and host robust topological surface states around $E_\mathrm {F}$. Due to the intrinsic hole doping, the topological states reside above the Fermi level, inaccessible by ARPES. Our results also suggest that in addition to SOC, vacancies and the resulting hole doping play an important role in the transport properties of these materials., 11 pages, 7 figures

We study IrCrMnZ (Z=Al, Ga, Si, Ge) systems using first-principles calculations from the perspective of their application as the electrode materials of MgO-based MTJs. These materials have highly spin-polarized conduction electrons with partially occupied $\Delta_1$ band, which is important for coherent tunneling in parallel magnetization configuration. The Curie temperatures of IrCrMnAl and IrCrMnGa are very high (above 1300 K) as predicted from mean-field-approximation. The stability of ordered phase against various antisite disorders has been investigated. We discuss here the effect of "spin-orbit-coupling" on the electronic structure around Fermi level. Further, we investigate the electronic structure of IrCrMnZ/MgO heterojunction along (001) direction. IrCrMnAl/MgO and IrCrMnGa/MgO maintain half-metallicity even at the MgO interface, with no interfacial states at/around Fermi level in the minority-spin channel. Large majority-spin conductance of IrCrMnAl/MgO/IrCrMnAl and IrCrMnGa/MgO/IrCrMnGa is reported from the calculation of ballistic spin-transport property for parallel magnetization configuration. We propose IrCrMnAl/MgO/IrCrMnAl and IrCrMnGa/MgO/IrCrMnGa as promising MTJs with a weaker temperature dependence of tunneling magnetoresistance ratio, owing to their very high Curie temperatures., Comment: 24 pages, 11 figures

Using first-principles calculations, we studied Mn$_2$RuZ (Z=Al, Ga, Si, Ge) and their heterojunctions with MgO along (001) direction. All these alloys possess Hg$_2$CuTi-type inverse Heusler alloy structure and ferrimagnetic ground state. Our study reveals the half-metallic electronic structure with highly spin-polarized $\Delta_1$ band, which is robust against atomic disorder. Next we studied the electronic structure of Mn$_2$RuAl/MgO and Mn$_2$RuGe/MgO heterojunctions. We found that the MnAl- or MnGe-terminated interface is energetically more favorable compared to the MnRu-terminated interface. Interfacial states appear at the Fermi level in the minority-spin gap for the Mn$_2$RuGe/MgO junction. We discuss the origin of these interfacial states in terms of local environment around each constituent atom. On the other hand, in the Mn$_2$RuAl/MgO junction, high spin polarization of bulk Mn$_2$RuAl is preserved independent of its termination., Comment: 15 pages, 7 figures

Using first-principles calculations based on density functional theory, we have studied the X-ray absorption spectra (XAS) and X-ray Magnetic Circular Dichroism (XMCD) spectra of Co 2 PtGa and Mn 2 PtGa alloys. Using relativistic calculations, we have estimated and analyzed the contribution of the orbital and spin moments to the total magnetic moment for the transition metal atoms, Pt, Co and Mn. It has been observed that while the ratio of orbital and spin magnetic moments of Pt atom in Co 2 PtGa in cubic phase is substantial, this is an order of magnitude lower in Mn 2 PtGa. On the other hand, this difference reduces drastically in case of the martensite phase. This signifies the crucial role of the neighborhood in determining the magnetic properties of these materials. Further, the XAS and XMCD spectra at the L 2 , 3 edges of different constituent atoms in various materials have been analyzed. The results (peaks and shoulders present in these XAS and XMCD spectra) have been corroborated with the atom-projected unoccupied density of states of the respective materials.

MgO-barrier magnetic tunnel junctions with half-metallic Heusler alloy electrodes have attracted considerable attention for spintronics applications. However, there remain a couple of issues related to materials that should be resolved before practical use. Recently, quarterly equiatomic Heusler alloys have attracted attention as advanced Heusler alloys. CoIrMnZ (Z = Al, Si, Ga, and Ge) half-metallic Heusler alloys were designed and predicted to have moderate Curie temperatures and to be lattice-matched with the MgO barrier, which is advantageous compared to traditional Co2 Heusler alloys T. T [T. Roy et al., J. Magn. Magn. Mater. 498 (2020) 166092]. Here, we experimentally investigated the structure and magnetic properties of thin films composed of one of these alloys, CoIrMnAl, fabricated by sputtering deposition. We successfully obtained films with the B2 chemical ordering, even without a post-annealing process. The lattice constants for the films annealed at 500–600 ∘C were approximately equal to the predicted values. The magnetization at 10 K was close to 500 kA/m, and the Curie temperature was approximately 400 K, which were approximately 70% of the values predicted for the fully ordered structure. The magnetic properties observed in the B2-ordered films were well explained by ferrimagnetism that appeared in the B2-ordered CoIrMnAl with full-swap disorders of Co-Ir and Mn-Al and almost full-swap disorder of Co-Mn, which was predicted from the first-principles calculations.

Using first-principles calculations based on density functional theory, we probe the electronic and magnetic properties of X$_{2}$PtGa (X being Cr, Mn, Fe, Co) Heusler alloys. Our calculations predict that all these systems possess inverse Heusler alloy structure in their respective ground states. Application of tetragonal distortion leads to lowering of energy with respect to the cubic phase for all the materials. The equilibrium volumes of both the phases are nearly the same. These results of our calculations indicate that all these materials are prone to undergo martensite transition, as has been recently shown theoretically for Mn$_{2}$PtGa in the literature. Ground state with a tetragonal symmetry of these materials is supported by the observation of soft tetragonal shear constants in their cubic phase. By comparing the energies of various types of magnetic configurations of these alloys we predict that Cr$_{2}$PtGa and Mn$_{2}$PtGa possess ferrimagnetic configuration whereas Fe$_{2}$PtGa and Co$_{2}$PtGa possess ferromagneic configuration in their respective ground states., 12 pages, 8 figures

The equiatomic quaternary Heusler alloy CoFeCrAl is a candidate material for spin-gapless semiconductors (SGSs). However, to date, there have been no experimental attempts at fabricating a junction device. This paper reports a fully epitaxial (001)-oriented MgO barrier magnetic tunnel junction (MTJ) with CoFeCrAl electrodes grown on a Cr buffer. X-ray and electron diffraction measurements show that the (001) CoFeCrAl electrode films with atomically flat surfaces have a $B2$-ordered phase. The saturation magnetization is $380\phantom{\rule{0.16em}{0ex}}{\mathrm{emu}/\mathrm{cm}}^{3}$, almost the same as the value given by the Slater--Pauling--like rule, and the maximum tunnel magnetoresistance ratios at 300 K and 10 K are 87% and 165%, respectively. Cross-sectional electron diffraction analysis shows that the MTJs have MgO interfaces with fewer dislocations. The temperature- and bias-voltage-dependence of the transport measurements indicates magnon-induced inelastic electron tunneling overlapping with the coherent electron tunneling. X-ray magnetic circular dichroism (XMCD) measurements show a ferromagnetic arrangement of the Co and Fe magnetic moments of $B2$-ordered CoFeCrAl, in contrast to the ferrimagnetic arrangement predicted for the $Y$-ordered state possessing SGS characteristics. Ab initio calculations taking account of the Cr-Fe swap disorder qualitatively explain the XMCD results. Finally, the effect of the Cr-Fe swap disorder on the ability for electronic states to allow coherent electron tunneling is discussed.

Using first principles calculations based on density functional theory we have studied the effects of Fe and Cr doping at the Mn site on mechanical, electronic, and magnetic properties of Pt2MnGa and Ni2MnGa. We predict on the basis of formation energy that all the substituted X2Mn1−xYxGa alloys (x=0.00, 0.25, 0.75, 1.00; X=Pt, Ni; Y=Fe and Cr) are stable materials. Further, all the substituted materials which we have studied here are likely to undergo martensite transition. In this work, we have reported how the stability of the austenite and martensite phase varies with the extent of substitution by Fe as well as Cr at the Mn site. Further, we study the bulk mechanical properties of the austenite and martensite phases of the stoichiometric systems only. We observe that Pt-based systems are inherently much less brittle in comparison to the Ni-based systems studied here. We also study the magnetic properties. Interestingly, contrary to the unsubstituted case as well as the case when Mn is substituted by Fe, the substitution by Cr at the Mn site leads to lowering of energy in case of an intra-sublattice anti-ferromagnetic configuration compared to the ferromagnetic configuration.

Nowadays advanced magnetic tunnel junction applications demand very high tunnel magnetoresistance at room temperature, thus it is quite important to explore high Curie temperature Tc half-metallic Heusler alloys. In this article rst-principles calculation unveiled that NiCrMnSi has Tc of 1200 K comparable to that of the traditional Co2MnSi Heusler alloys, even though it does not contain Co element. In addition, we examined whether NiCrMnSi Heulser phase lms can be obtained by a magnetron sputtering on MgO substrates. The results of the structural analysis and rst-principles calculations indicated that NiCrMnSi Heusler phase is metastable. A possible route to obtain metastable NiCrMnSi Heusler alloy is to utilize appropriate templates.

Using first principles calculations based on density functional theory, we study electronic and magnetic properties of CoIrMnZ (Z = Al, Si, Ga, Ge) Heusler systems. Based on the electronic structure calculations, CoIrMnSi and CoIrMnGe are predicted to be half-metallic. For the studied systems, the ground state magnetic configurations are found to be ferromagnetic with magnetic moment consistent with the Slater-Pauling rule. The calculated ferromagnetic transition temperatures are much higher than the room temperature for all the cases. The effect of spin orbit coupling has been discussed on the electronic structure for all the systems studied here. Furthermore, we construct junctions with MgO along (001) direction. The MnZ terminations are found to be energetically more favourable compared to the CoIr terminations. After discussing the tunnelling conductance, we find that CoIrMnSi/MgO/CoIrMnSi could be a promising magnetic tunnelling junction (MTJ) candidate.

Using first-principles calculations based on density functional theory, we have studied the mechanical, electronic, and magnetic properties of Heusler alloys, namely, Ni$_{2}BC$ and Co$_{2}BC$ ($B$ = Sc, Ti, V, Cr and Mn as well as Y, Zr, Nb, Mo and Tc; $C$ = Ga and Sn). On the basis of electronic structure (density of states) and mechanical properties (tetragonal shear constant), as well as magnetic interactions (Heisenberg exchange coupling parameters), we probe the properties of these materials in detail. We calculate the formation energy of these alloys in the (face-centered) cubic austenite structure to probe the stability of all these materials. From the energetic point of view, we have studied the possibility of the electronically stable alloys having a tetragonal phase lower in energy compared to the respective cubic phase. A large number of the magnetic alloys is found to have the cubic phase as their ground state. On the other hand, for another class of alloys, the tetragonal phase has been found to have lower energy compared to the cubic phase. Further, we find that the values of tetragonal shear constant show a consistent trend : a high positive value for materials not prone to tetragonal transition and low or negative for others. In the literature, materials, which have been seen to undergo the martensite transition, are found to be metallic in nature. We probe here if there is any Heusler alloy which has a tendency to undergo a tetragonal transition and at the same time possesses a high spin polarization at the Fermi level. From our study, it is found that out of the four materials, which exhibit a martensite phase as their ground state, three of these, namely, Ni$_{2}$MnGa, Ni$_{2}$MoGa and Co$_{2}$NbSn have a metallic nature; on the contrary, Co$_{2}$MoGa exhibits a high spin polarization., 31 pages, 17 figures

Using density functional theory based calculations, we have carried out in-depth studies of effect of Co substitution on the magnetic properties of Ni and Pt-based shape memory alloys. We show the systematic variation of the total magnetic moment, as a function of Co doping. A detailed analysis of evolution of Heisenberg exchange coupling parameters as a function of Co doping has been presented here. The strength of RKKY type of exchange interaction is found to decay with the increase of Co doping., Comment: 6 pages, 7 figures

In this work we have predicted the mechanical and magnetic properties of cubic austenite phase of the Cu substituted alloys of Ni2MnGa and Mn2NiGa at the Ga site i.e. Ni2MnCu and Mn2NiCu, respectively, as well as of Pt2MnCu and Pd2MnCu. We have observed that the conventional Heusler structure is lower in energy compared to the inverse Heusler structure for the Ni2MnGa, Pt2MnGa and Pd2MnGa and the Cu-substituted materials, whereas Mn2NiGa and its Cu-substituted alloy at Ga site assume the inverse structure. From the calculated elastic constants, we find that Pt2MnCu is expected to be the most ductile material among all the substituted materials studied here. We have calculated and compared the Curie temperatures derived from the Heisenberg exchange coupling parameters.

First-principles density functional theory based calculations have been used to predict the bulk mechanical properties of magnetic shape memory Heusler alloy Ni2MnGa substituted by copper (Cu), platinum (Pt), palladium (Pd) and manganese (Mn) at the Ni site. The elastic constants of Ni2MnGa alloy with and without substitution are calculated. We analyze and compare in detail the bulk mechanical properties for these alloys, in particular, the ratio between the calculated bulk and shear modulii, as well as the Poisson’s ratio and Young’s modulii. This analysis further based on an empirical relation, indicates that Pt2MnGa may inherently be the least brittle material, among the above-mentioned alloys. Interesting difference has been observed between the shear modulii calculated from Voigt’s and Reuss’s method. This has been explained in terms of the values of the tetragonal shear constant C′ of the materials. Study of Heisenberg exchange coupling parameters and Curie temperature as well as density of states of the materials shows the effect of substitution at the Ni site on the magnetic and electronic properties, respectively.

Using first-principles density functional theory based calculations, we study systematically the effect of medium to large Cu substitution at the Mn, Ga as well as Ni sites on the geometric, bulk mechanical, electronic, and magnetic properties of Ni${}_{2}$MnGa and Mn${}_{2}$NiGa. The calculations have been carried out for possible austenite and martensite phases using a supercell approach. Partial Cu substitutions at Mn and Ga sites show promises in terms of the electronic and magnetic properties for both Ni${}_{2}$MnGa and Mn${}_{2}$NiGa alloys from an application point of view. Our calculations predict that for certain partial substitutions, the austenite to martensite transition temperature is likely to increase and the system remains magnetic in nature. On the other hand, a significantly large amount of Cu substitution at the Ni site seems to stabilize the austenite phase in both of these alloy systems rendering a martensite transition unlikely. Interestingly, the overall trend in the changes in the structural, bulk mechanical, electronic, and magnetic properties of these two different types of alloy systems, Ni${}_{2}$MnGa and Mn${}_{2}$NiGa, as a result of substantial Cu substitution in all the three different sites, is found to be the same.

Influence of disorder, antisite defects, martensite transition and compositional variation on the magnetic properties and electronic structure of Mn$_2$NiGa and Mn$_{1+x}$Ni$_{2-x}$Ga magnetic shape memory alloys have been studied by using full potential spin-polarized scalar relativistic Korringa-Kohn-Rostocker (FP-SPRKKR) method. Mn$_2$NiGa is ferrimagnetic and its total spin moment increases when disorder in the occupancy of Mn$_{\rm Ni}$ (Mn atom in Ni position) is considered. The moment further increases when Mn-Ga antisite defect[1] is included in the calculation. A reasonable estimate of $T_C$ for Mn$_2$NiGa is obtained from the exchange parameters for the disordered structure. Disorder influences the electronic structure of Mn$_2$NiGa through overall broadening of the density of states and a decrease in the exchange splitting. Inclusion of antisite defects marginally broaden the minority spin partial DOS (PDOS), while the majority spin PDOS is hardly affected. For Mn$_{1+x}$Ni$_{2-x}$Ga where 1$\geq$$x$$\geq$0, as $x$ decreases, Mn$_{\rm Mn}$ moment increases while Mn$_{\rm Ni}$ moment decreases in both austenite and martensite phases. For $x$$\geq$ 0.25, the total moment of the martensite phase is smaller compared to the austenite phase, which indicates possible occurrence of inverse magnetocaloric effect. We find that the redistribution of Ni 3$d$- Mn$_{\rm Ni}$ 3$d$ minority spin electron states close to the Fermi level is primarily responsible for the stability of the martensite phase in Mn-Ni-Ga., Comment: 10 pages, 5 figures