Pair distribution function study of Ni2MnGa magnetic shape memory alloy: Evidence for the precursor state of the premartensite phase

Abstract

Precursor phenomena preceding the martensite phase transition play a critical role in understanding the important technological properties of shape memory and magnetic shape memory alloys (MSMAs). Since the premartensite phase of Ni2MnGa MSMA, earlier considered as the precursor state of the martensite phase, has in recent years been shown to be a thermodynamically stable phase, Singh et al. [Nat. Commun. 8, 1006 (2017)10.1038/s41467-017-00883-z], there is a need to revisit the precursor effects in these materials. We present here evidence for the existence of a precursor state of the premartensite phase in Ni2MnGa MSMA by atomic pair distribution function analysis of high-energy, high-flux, and high-Q synchrotron x-ray powder diffraction data. It is shown that the local structure of the cubic austenite phase corresponds to the short-range ordered (SRO) precursor state of the 3M premartensite phase at temperatures well above the actual premartensite phase transition temperature TPM and even above the ferromagnetic (FM) transition temperature TC. The presence of such a SRO precursor state of the premartensite phase is shown to lead to significant volume strain, which scales quadratically with spontaneous magnetization. The experimentally observed first-order character of the paramagnetic-to-FM phase transition and the anomalous reduction in the value of the magnetization in the temperature range TPM<T<TC are explained in terms of the coupling of the magnetoelastic strain with the FM order parameter and the higher magnetocrystalline anisotropy of the precursor state of the premartensite phase, respectively. © 2021 American Physical Society.