Effect of Fe and Co substitution on the martensitic stability and the elastic, electronic, and magnetic properties ofMn2NiGa: Insights fromab initiocalculations

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 Mn2NiGa 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 (tetragonal shear constant, Pugh ratio and Cauchy pressure) and magnetic exchange parameters. We find that the martensitic phase of Mn2NiGa 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 like in the Ni2MnGa-based systems, the elastic shear modulus C can be considered as a predictor of composition dependence of martensitic transformation temperature Tm 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 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 Mn2NiFe and Mn2NiCo with very high magnetic moments and Curie temperatures. Thus, our work indicates that although the substitutions de-stabilise the martensitic phase in Mn2NiGa, new magnetic materials with very good magnetic parameters and potentially useful for novel magnetic applications can be obtained. This can trigger interests in the experimental community for further research on substituted Mn2NiGa.

Understanding the origin of the magnetocaloric effects in substitutional Ni-Mn-Sb-Z(Z=Fe,Co,Cu) compounds: Insights from first-principles calculations

“…The configurational ordering of the constituent elements in the crystal lattice affects both the martensitic transformation characteristics and the magnetic properties of Ni-Mn-based alloys 42,49,59,60 . First-principles calculations [61][62][63] demonstrated that the substituent 3dtransition metals do not always prefer to occupy the sites of substitutions. Therefore, we first focus on determination of the site preferences and the associated magnetic ground states of the substituted Ni 2 Mn 1.5 Sb 0.5 compounds in their high temperature austenite phases, by comparing total energies of various possible site ordered and magnetic configurations at fixed compositions.…”

“…Quaternary off-stoichiometric Ni-Mn based Heusler compounds are found to exhibit better mechanical properties such as ductility, corrosion resistance, machinability, all of which ease manufacturing and increase the product longevity. For example, Fe-addition improves the toughness of Ni-Mn-Ga allloys without sacrificing its magnetic and thermoelastic properties 25,62 . In this section, we, therefore, explore the changes in the mechanical properties of Ni 2 Mn 1.5 Sb 0.5 compounds upon substitution by the fourth element.…”

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

“…Since, site occupancies in the compounds do not always follow a regular pattern (the excess atoms occupying the sites of deficient atoms in the reference system) 52,[59][60][61][62][63] affecting the MCE properties as a consequence, we first found out the minimum energy configuration for each composition by comparing the total energies of configurations with different site occupancy patterns and magnetic configurations. We found that the substituting Fe/Co atoms prefer to occupy the Ni sites, whereas the Cu atoms prefer to occupy the Sb sites, the same as found out in Ref 29.…”

“…These exchange parameters were then used for the calculation of T c . The Curie temperatures were estimated with two different approaches: the mean-field approximation (MFA) 48,49 and the Monte Carlo simulation (MCS) method [50][51][52] . Details of the calculations using these methods are given in the supplementary material.…”

“…The ∆E values are also listed in Table I. In literature, ∆E is routinely used to predict the martensitic transformation temperature (T M ) 52,58,61,63,64 . Here we have used it first to understand the trends in the T M so that compositions with higher T M can be screened.…”