Abstract

In the search for new magnetic functional materials, non-stoichiometric compounds remain a relatively unexplored territory. While experimentalists create new compositions looking for improved functional properties, their work is not guided by systematic theoretical predictions. Being designed for perfect periodic crystals, the majority of first-principles approaches struggle with the concept of a non-stoichiometric system. In this work, we attempt a systematic computational study of magnetic and structural properties of Ni–Mn–Ga, mapped onto ternary composition diagrams. Compositional stability was examined using the convex hull analysis. We show that the cubic austenite has its stability region close to the stoichiometric Ni2MnGa, in agreement with experimental data, while the tetragonal martensite spreads its stability over a wider range of Mn and Ni contents. The unstable compositions in both austenite and martensite states are located in the Ga-rich corner of the ternary diagram. We note that simultaneous stability of the austenite and martensite should be considered for potentially stable compounds suitable for synthesis. The majority of compounds are predicted to be ferrimagnetically ordered in both austenitic and martensitic states. The methodology used in this work is computationally tractable, yet it delivers some predictive power. For experimentalists who plan to synthesize stable Ni–Mn–Ga compounds with ferromagnetic order, we narrow the target compositional range substantially.

Highlights

  • During the last decades, shape memory Ni–Mn-based Heusler alloys have received notable attention as promising materials for a wide variety of engineering applications such as a magnetic actuator, controller, sensor, and damping technologies

  • Its distinguishing feature is the martensitic transformation in the ferromagnetic (FM) state between the high-temperature cubic austenite and the low-temperature modulated martensite with twin boundary structure occurring at about

  • We carried out a systematic investigation of Ni–Mn–Ga austenite and martensite with respect to their magnetic and structural properties

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Summary

Introduction

Shape memory Ni–Mn-based Heusler alloys have received notable attention as promising materials for a wide variety of engineering applications such as a magnetic actuator, controller, sensor, and damping technologies. That success was followed by extensive research of non-stoichiometric Ni–Mn–Ga compositions, covering various aspects such as the crystal structure of the austenite and martensite [3,4,5,6,7], magnetic and magnetoresistance properties [8,9], thermally and magnetically induced deformation [2,10,11], magnetocaloric properties [12,13], heat treatment processes [3,14], and phase diagrams [15,16,17,18,19,20,21,22]

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