Effect of Fe and Co substitution on the martensitic stability and the elastic, electronic, and magnetic properties of Mn2NiGa: Insights from ab initio calculations

We investigate the effects of Fe and Co substitutions on the phase stability of the martensitic phase and mechanical, electronic, and magnetic properties of the magnetic shape memory system Mn2NiGa by first-principles density functional theory calculations. The evolution of these aspects upon substitution of Fe and Co at different crystallographic sites is investigated by computing the electronic structure, mechanical properties (tetragonal shear constant, Pugh ratio, and Cauchy pressure), and magnetic exchange parameters. We find that the austenite phase of Mn2NiGa gradually stabilizes with increase in concentration of Fe/Co due to the weakening of the minority spin hybridization of Ni and Mn atoms occupying crystallographically equivalent sites. The interplay between relative structural stability and the compositional changes is understood from the variations in the elastic moduli and electronic structures. We find that like in the Ni2MnGa-based systems, the elastic shear modulus C' can be considered as a predictor of composition dependence of martensitic transformation temperature T-m in substituted Mn2NiGa, thus singling it out as the universally acceptable predictor for martensitic transformation in Ni-Mn-Ga compounds over a wide composition range. The magnetic properties of Mn2NiGa 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 leads to a half-metallic-like gap in these compounds resulting in materials with high spin polarization when the substitutions are complete. The substitutions at the Ga site result in the two compounds Mn2NiFe and Mn2NiCo with very high magnetic moments and Curie temperatures. Thus, our work indicates that although the substitutions destroy the martensitic transformation and thus the possibility of realization of shape memory properties in Mn2NiGa, magnetic materials with very good magnetic parameters that are potentially useful for novel magnetic applications can be obtained. This can trigger interest in the experimental community in further research on substituted Mn2NiGa.