Pair distribution function study ofNi2MnGamagnetic 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 Ni 2 MnGa 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], 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 Ni 2 MnGa 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 T PM and even above the ferromagnetic (FM) transition temperature T C . 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 T PM < T < T C 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.

Intrinsic anomalous Hall conductivity and real space Berry curvature induced topological Hall effect in Ni2MnGa magnetic shape memory alloy

“…Finally, a sharp drop attributes to the premartensite to martensite phase transition (MPT) with martensite start temperature M C S = T M ∼ 175 K, where T M is the MPT temperature. The presence of thermal hysteresis across MPT confirms the firstorder character of MPT in Ni 2 MnGa MSMA [60,70,71]. The magnetic behavior of each phase is verified by the magnetic field dependence of magnetization (M(H)) measurements in the temperature range of 385-2 K, shown in figure 2(b).…”

“…This manifests the dominance of the band structure dictated by intrinsic Berry mechanism in the premartensite phase as well. Further, existence of local premartensite phase has been observed even in the austenite phase [71], which anticipates the presence of the intrinsic mechanism to the AHE even in the austenite phase of Ni 2 MnGa.…”

“…In contrast, a merely temperature dependent variation of ρ Tmax xy can be noted in the austenite phase (figure 6(a)). This attributes to the fact that THE may be induced by non-coplanar spin structure with finite spin chirality [15], which possibly arises due to the local premartensite phase present in the austenite phase of Ni 2 MnGa as reported [71]. Further, this temperature dependent THE behavior may arise because of change in nucleation probability of non-coplanar spin structure with finite chirality near FM transition temperature T C [21].…”

“…Further, the premartensite phase, which is a precursor phase of the martensite phase [59], has been proposed to be closely linked with zero-field skyrmion formation in the Ni 50 Mn 35.2 In 14.8 MSMA [98], which explains the appearance of THE even in the premartensite phase. Moreover, the local precursor state of the premartensite phase present in the austenite phase of Ni 2 MnGa [71], emerges the possibility of THE even in the cubic austenite phase of Ni 2 MnGa. Therefore, it is important to investigate the origin of present THE in Ni 2 MnGa i.e.…”

Robust evidence for the stabilization of the premartensite phase in Ni-Mn-In magnetic shape memory alloys by chemical pressure

“…The observed, calculated, and difference profiles so obtained, shown in Fig. 4 The magnetoelastic coupling has been suggested as one of the factors for the stabilization of the PM phase in MSMAs [18,23,50,81]. The effect of magnetoelastic coupling is manifested through the variation of T PM with magnetic field [18,19,23,50].…”