Effect of Fe and Co substitution on martensitic stability, elastic, electronic and magnetic properties of Mn$_2$NiGa: insights from \textit{ab initio} calculations

Abstract

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.