Premartensite to martensite transition and its implications on the origin of modulation in Ni2MnGa ferromagnetic shape memory alloy

TL;DR

This study uses high-resolution synchrotron x-ray diffraction to investigate phase transitions in Ni2MnGa, revealing that incommensurate premartensite transforms directly into martensite, challenging previous models and suggesting the incommensurate 7M modulation as the ground state.

Contribution

It provides direct experimental evidence that the incommensurate premartensite phase leads to martensite, refuting the adaptive phase model and supporting the soft phonon model for Ni2MnGa.

Findings

Premartensite transforms to martensite via a first-order transition.

Incommensurate 7M modulation persists down to 5K, indicating it as the ground state.

No evidence of lock-in phases or intermediate commensurate states.

Abstract

We present here results of temperature dependent high resolution synchrotron x-ray powder diffraction study of sequence of phase transitions in Ni2MnGa. Our results show that the incommensurate martensite phase results from the incommensurate premartensite phase, and not from the austenite phase assumed in the adaptive phase model. The premartensite phase transforms to the martensite phase through a first order phase transition with coexistence of the two phases in a broad temperature interval (~40K), discontinuous change in the unit cell volume as also in the modulation wave vector across the transition temperature and considerable thermal hysteresis in the characteristic transition temperatures. The temperature variation of the modulation wave vector q shows smooth analytic behaviour with no evidence for any devilish plateau corresponding to an intermediate or ground state commensurate lock-in phases. The existence of the incommensurate 7M like modulated structure down to 5K suggests that the incommensurate 7M like modulation is the ground state of Ni2MnGa and not the Bain distorted tetragonal L10 phase or any other lock-in phase with a commensurate modulation. These findings can be explained within the framework of the soft phonon model.