Effect of Fe and Co substitution on martensitic stability, elastic, electronic and magnetic properties of Mn2_2NiGa: insights from \textit{ab initio} calculations

We investigate the effects of Fe and Co substitutions on the phase stability of the martensitic phase, mechanical, electronic and magnetic properties of magnetic shape memory system Mn$_{2}$NiGa by first-principles Density functional theory(DFT) calculations. The evolution of these aspects upon substitution of Fe and Co at different crystallographic sites are investigated by computing the electronic structure, mechanical properties and magnetic exchange parameters. We find that the martensitic phase of Mn$_{2}$NiGa gradually de-stabilises with increase in concentration of Fe/Co due to the weakening of the minority spin hybridisation of Ni and Mn atoms occupying crystallographically equivalent sites. The interplay between relative structural stability and the compositional changes are understood from the variations in the elastic modulii and electronic structures. We find that the elastic shear modulus C$^{\prime}$ can be considered as a predictor of composition dependence of martensitic transformation temperature in substituted Mn$_{2}$NiGa. The magnetic properties of Mn$_{2}$NiGa are found to be greatly improved by the substitutions due to stronger ferromagnetic interactions in the compounds. The gradually weaker(stronger) Jahn-Teller distortion (covalent bonding) in the minority spin densities of states due to substitutions lead to a half-metallic like gap in these compounds resulting in materials with high spin-polarisation when the substitutions are complete. The substitutions at the Ga site result in two new compounds Mn$_{2}$NiFe and Mn$_{2}$NiCo with very high magnetic moments and Curie temperatures. Thus, our work indicates that although the substitutions de-stabilise the martensitic phase in Mn$_{2}$NiGa, new magnetic materials with very good magnetic parameters and potentially useful for novel magnetic applications can be obtained.Comment: Supplementary material can be obtained from the author upon reques

First-principles investigations into the thermodynamics of cation disorder and it's impact on electronic structure and magnetic properties of spinel Co(Cr1−xMnx)2O4Co\left(Cr_{1-x}Mn_{x} \right)_{2}O_{4}

Recent experiments on Mn doped multiferroic $CoCr_{2}O_{4}$ indicate that a possible distribution of Mn atoms among tetrahedrally and octahedrally coordinated sites in the spinel lattice give rise to different variations in the structural parameters and saturation magnetisations in different concentration regimes of Mn atoms substituting the Cr. A composition dependent magnetic compensation behaviour points to the role conversions of the magnetic constituents. In this work, we have investigated the thermodynamics of cation disorder in $Co\left(Cr_{1-x}Mn_{x}\right)_{2}O_{4}$ system and it's consequences on the structural, electronic and magnetic properties, using results from first-principles electronic structure calculations. We have computed the variations in the cation-disorder as a function of Mn concentration and the temperature and found that at the annealing temperature of the experiment many of the systems exhibit cation disorder. Our results support the interpretations of the experimental results regarding the qualitative variations in the sub-lattice occupancies and the associated magnetisation behaviour, with composition. We have analysed the variations in structural, magnetic and electronic properties of this system with variations in the compositions and the degree of cation disorder from the variations in their electronic structures and by using the ideas from crystal field theory. Our study provides a complete microscopic picture of the effects that are responsible for composition dependent behavioural differences of the properties of this system. This work lays down a general framework, based upon results from first-principles calculations, to understand and analyse the substitutional magnetic spinel oxides $A\left(B_{1-x}C_{x} \right)_{2}O_{4}$ in presence of cation disorder.Comment: 13 pages, 5 figures, submitted to Physical review

First principles study of the structural phase stability and magnetic order in various structural phases of Mn2_2FeGa

We investigate the structural and magnetic properties of Mn$_{2}$FeGa for different phases(cubic, hexagonal and tetragonal) reported experimentally using density functional theory. The relative structural stabilities, and the possible phase transformation mechanisms are discussed using results for total energy, electronic structure and elastic constants. We find that the phase transformation form hexagonal to ground state tetragonal structure would take place through a Heusler-like phase which has a pronounced electronic instability. The electronic structures, the elastic constants and the supplementary phonon dispersions indicate that the transition from the Heusler-like to the tetragonal phase is of pure Jahn-Teller origin. We also describe the ground state magentic structures in each phase by computations of the exchange interactions. For Heusler-like and tetragonal phases, the ferromagnetic exchange interactions associated with the Fe atoms balance the dominating antiferromagnetic interactions between the Mn atoms leading to collinear magnetic structures. In the hexagonal phase, the direction of atomic moment are completely in the planes with a collinear like structure, in stark contrast to the well known non-collinear magnetic structure in the hexagonal phase of Mn$_{3}$Ga, another material with similar structural properties. The overwhelmingly large exchange interactions of Fe with other magnetic atoms destroy the possibility of magnetic frustration in the hexagonal phase of Mn$_{2}$FeGa. This comprehensive study provides significant insights into the microscopic physics associated with the structural and magnetic orders in this compound.Comment: Supplementary material can be obtained on reques

Site occupancies and their effects on the physical properties of spinel Co(Cr1−xFex)2O4Co\left(Cr_{1-x}Fe_{x} \right)_{2}O_{4}: an {\it ab initio} study

Recent experimental studies on Fe substituted spinel CoCr$_{2}$O$_{4}$ have discovered multiple functional properties in the system such as temperature and composition dependent magnetic compensation, tunable exchange bias and magnetostriction. These properties are attributed to the renormalisation of the inter-atomic magnetic exchange interactions arising due to the non-regular site occupancies of the magnetic cations in the system. In this work, we perform {\it ab initio} electronic structure calculations by DFT+U method and combine with a generalised thermodynamic model to compute the site occupancy patterns of the magnetic cations, the structural properties and the magnetic exchange interactions of Co$\left(Cr_{1-x}Fe_{x} \right)_{2}$O$_{4}$ for the entire composition range $0<x<1$. We find that the substituting Fe atoms prefer to occupy the tetrahedral sites of the spinel structure for the entire range of $x$, in agreement with the experimental inferences. Our results on the variations of the structural parameters with compositions agree very well with the experiments. By computing the variations of the various inter-atomic magnetic exchange interactions, we provide a microscopic picture of the evolution of a collinear structure from a non-collinear one due to substitution of Fe in CoCr$_{2}$O$_{4}$. The computed results are analysed in terms of the elements of the crystal field theory, and the features in the atoms and orbital-projected densities of states. The results and analysis presented in this work is the first comprehensive study on this system which would help understanding the complexities associated with the site occupancies, the electronic structures and the magnetic interactions in this multi-functional material.Comment: Submitted in Journal of Physics

Site occupancy, composition and magnetic structure dependencies of martensitic transformation in Mn2_{2}Ni1+x_{1+x}Sn1−x_{1-x}

A delicate balance between various factors such as site occupancy, composition and magnetic ordering seems to affect the stability of the martensitic phase in Mn$_{2}$Ni$_{1+x}$Sn$_{1-x}$. Using first-principles DFT calculations, we explore the impacts of each one of these factors on the martensitic stability of this system. Our results on total energies, magnetic moments and electronic structures upon changes in the composition, the magnetic configurations and the site occupancies show that the occupancies at the 4d sites in the Inverse Heusler crystal structure play the most crucial role. The presence of Mn at the 4d sites originally occupied by Sn and it's interaction with the Mn atoms at other sites decide the stability of the martensitic phases. This explains the discrepancy between the experiments and earlier DFT calculations regarding phase stability in Mn$_{2}$NiSn. Our results qualitatively explain the trends observed experimentally with regard to martensitic phase stability and the magnetisations in Ni-excess, Sn-deficient Mn$_{2}$NiSn system

Half-metallicity in quaternary Heusler alloys with 3dd and 4dd elements: observations and insights from DFT calculations

In this work, we provide important insights into the evolution of half-metallicity in quaternary Heusler alloys. Employing {\it ab initio} electronic structure methods we study 18 quaternary Heusler compounds having the chemical formula CoX$^\prime$Y$^\prime$Al, where Y$^\prime$ = Mn, Fe; and X$^\prime$ a 4$d$ element. Along with the search for new materials for spintronics applications, the trends in structural, electronic, magnetic properties and Curie temperature were investigated. We have made comparative studies with the compounds in the quaternary series CoX$^{\prime}$Y$^{\prime}$Si with X$^{\prime}$ materials from 3$d$ and 4$d$ transition metal series in the periodic table. We observe that the half-metallic behaviour depends primarily on the crystal structure type based on atomic arrangements and the number of valence electrons. As long as these two are identical, the electronic structures and the magnetic exchange interactions bear close resemblances. Consequently, the materials exhibit identical electronic properties, by and large. We analysed the roles of different transition metal atoms in affecting hybridisations and correlated them with the above observations. This work, therefore, provides important perspectives regarding the underlying physics of half-metallic behaviour in quaternary Heusler compounds which goes beyond specifics of a given material. This, thus, paves way for smart prediction of new half-metals. This work also figures out an open problem of understanding how different ternary Heuslers with different electronic behaviour may lead to half-metallic behaviour in quaternary Heuslers with 4$d$ transition metal elements.Comment: Supplementary material available on reques

Giant magnetocaloric effect driven by first-order magneto-structural transition in cosubstituted Ni-Mn-Sb Heusler compounds: predictions from \textit{Ab initio} and Monte Carlo calculations

Using Density Functional Theory and a thermodynamic model [Physical Review B 86, 134418 (2012)], in this paper, we provide an approach to systematically screen compounds of a given Heusler family to predict ones that can yield giant magnetocaloric effect driven by a first-order magneto-structural transition. We apply this approach to two Heusler series Ni$_{2-x}$Fe$_{x}$Mn$_{1+z-y}$Cu$_{y}$Sb$_{1-z}$ and Ni$_{2-x}$Co$_{x}$Mn$_{1+z-y}$Cu$_{y}$Sb$_{1-z}$, obtained by cosubstitution at Ni and Mn sites. We predict four new compounds with potentials to achieve the target properties. Our computations of the thermodynamic parameters, relevant for magnetocaloric applications, show that the improvement in the parameters in the predicted cosubstituted compounds can be as large as four times in comparison to the off-stoichiometric Ni-Mn-Sb and a compound derived by single substitution at the Ni site, where magnetocaloric effects have been observed experimentally. This work establishes a protocol to select new compounds that can exhibit large magnetocaloric effects and demonstrate cosubstitution as a route for more flexible tuneability to achieve outcomes, better than the existing ones.Comment: Supplementary Materials can be obtained on request by sending e-mail to [email protected]

Systematic analysis of structural and magnetic properties of spinel CoB2O4CoB_2O_4(B=Cr,Mn and Fe)compounds from their electronic structures

The structural and magnetic properties of spinel compounds $CoB_2O_4$ (B=Cr,Mn and Fe) are studied using the DFT+U method and generalized gradient approximation (GGA). We concentrate on understanding the trends in the properties of these materials as the B cation changes, in terms of relative strengths of crystal fields and exchange fields through an analysis of their electronic densities of states. We find that the electron-electron correlation plays a significant role in obtaining the correct structural and electronic ground states. Significant structural distortion in CoMn$_{2}$O$_{4}$ and "inverted" sublattice occupancy in CoFe$_{2}$O$_{4}$ affects the magnetic exchange interactions substantially. The trends in the magnetic exchange interactions are analysed in terms of the structural parameters and the features in their electronic structures. We find that the Fe states in CoFe$_{2}$O$_{4}$ are extremely localised, irrespective of the symmetry of the site, which makes it very different from the features of the states of the B cations in other two compounds. These results provide useful insights into the trends in the properties of CoB$_{2}$O$_{4}$ compounds with variation of B cation which would help in understanding the results of recent experiments on doping of Mn and Cr in multiferroic CoCr$_{2}$O$_{4}$.Comment: 10 pGES, 7 FIGURES, To appear in Journal of Physics Condensed Matte

Anti-site disorder and improved functionality of Mn2_{2}Ni{\it X} ({\it X}= Al, Ga, In, Sn) inverse Heusler alloys

Recent first-principles calculations have predicted Mn$_{2}$Ni{\it X} ({\it X}=Al, Ga, In, Sn) alloys to be magnetic shape memory alloys. Moreover, experiments on Mn$_{2}$NiGa and Mn$_{2}$NiSn suggest that the alloys deviate from the perfect inverse Heusler arrangement and that there is chemical disorder at the sublattices with tetrahedral symmetry. In this work, we investigate the effects of such chemical disorder on phase stabilities and magnetic properties using first-principles electronic structure methods. We find that except Mn$_{2}$NiAl, all other alloys show signatures of martensitic transformations in presence of anti-site disorder at the sublattices with tetrahedral symmetry. This improves the possibilities of realizing martensitic transformations at relatively low fields and the possibilities of obtaining significantly large inverse magneto-caloric effects, in comparison to perfect inverse Heusler arrangement of atoms. We analyze the origin of such improvements in functional properties by investigating electronic structures and magnetic exchange interactions

A new first principles approach to calculate phonon spectra of disordered alloys

The lattice dynamics in substitutional disordered alloys with constituents having large size differences is driven by strong disorder in masses, inter-atomic force constants and local environments. In this letter, a new first-principles approach based on special quasi random structures and itinerant coherent potential approximation to compute the phonon spectra of such alloys is proposed and applied to Ni$_{0.5}$Pt$_{0.5}$ alloy. The agreement between our results with the experiments is found to be much better than for previous models of disorder due to an accurate treatment of the interplay of inter-atomic forces among various pairs of chemical species. This new formalism serves as a potential solution to the longstanding problem of a proper microscopic understanding of lattice dynamical behavior of disordered alloys.Comment: 10 pages, 2 figure