Tuning the magnetic phase diagram of Ni-Mn-Ga by Cr and Co substitution

M Schröter,A Grünebohm,H C Herper

doi:10.1088/1361-6463/ac2a66

Abstract

Ni-Mn-based Heusler alloys have a high technical potential related to a large change of magnetization at the structural phase transition. These alloys show a subtle dependence of magnetic properties and structural phase stability on composition and substitution by 3d elements and although they have been extensively investigated, there are still ambiguities in the published results and their interpretation. To shed light on the large spread of reported properties, we perform a comprehensive study by means of density functional theory calculations. We focus on Cr and Co co-substitution whose benefit has been predicted previously for the expensive Ni-Mn-In-based alloy and study the more abundant iso-electronic counterpart Ni-Mn-Ga. We observe that substituting Ni partially by Co and/or Cr enhances the magnetization of the Heusler alloy and at the same time reduces the structural transition temperature. Thereby, Cr turns out to be more efficient to stabilize the ferromagnetic alignment of the Mn spins by strong antiferromagnetic interactions between Mn and Cr atoms. In a second step, we study Cr on the other sublattices and observe that an increase in the structural transition temperature is possible, but depends critically on the short-range order of Mn and Cr atoms. Based on our results, we are able to estimate composition dependent magnetic phase diagrams. In particular, we demonstrate that neither the atomic configuration with the lowest energy nor the results based on the coherent potential approximation are representative for materials with a homogeneous distribution of atoms and we also predict a simple method for fast screening of different concentrations which can be viewed as a blueprint for the study of high entropy alloys. Our results help to explain the large variation of experimentally found materials properties.

Highlights

Magnetic Ni-Mn-(In, Ga, Sn) based Heusler alloys show a variety of complex magnetic [1,2,3,4] and structural phases [1, 5, 6], which partly transform into each other in first-order magneto-structural phase transitions
In order to close this gap, we discuss the influence of atomic structure and ordering on the relative energies of different magnetic phases in Ga-based Heusler alloys and depict concentration ranges allowing for large changes of magnetization during a structural phase transition
For high Co and Cr concentrations the potential jump of magnetization is reduced from 2.5 μB f.u.−1 to 1.8 μB f.u.−1 for uu to udt transition and in the lower right corner of the diagram we find a dd to dut transition with ∆M = 1.5 μB f.u.−1 Interestingly in the latter two cases, the magnetization of the tetragonal phase is potentially larger than the magnetization in the cubic phase

Summary

Introduction

Magnetic Ni-Mn-(In, Ga, Sn) based Heusler alloys show a variety of complex magnetic [1,2,3,4] and structural phases [1, 5, 6], which partly transform into each other in first-order magneto-structural phase transitions. Some investigations for the Ga-based system have been made by Zagrebin et al They have shown that if averaging over possible isomers is taken into account, Cr in Ni8Mn2Cr2Ga4 [52] leads to ferrimagnetic and FM cubic and tetragonal phases, respectively They have predicted that the structural transition temperature in Ni2Mn1−xCrxGa increases with x [53]. In order to close this gap, we discuss the influence of atomic structure and ordering on the relative energies of different magnetic phases in Ga-based Heusler alloys and depict concentration ranges allowing for large changes of magnetization during a structural phase transition. Detailed information on lattice constants and energy differences are summarized in the appendix

Computational details

Discussion and results

Findings

Phase diagrams

Conclusions and outlook