Premartensite to martensite transition and its implications for the origin of modulation inNi2MnGaferromagnetic shape-memory alloy

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 ! 1 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.

Magnetostriction of Ni2Mn1−xCrxGa Heusler Alloys

“…Identical TP values were obtained for the cooling and heating processes; the difference was at most 0.2 K. Singh et al [24] suggested from their high-resolution X-ray spectroscopy of single-crystal Ni2MnGa that a 3M-like structure develops in the premartensite phase. Considering this X-ray spectroscopy result, the linear thermal expansion results of polycrystalline sample in this study indicate that lattice deformation occurred below TP.…”

“…In the premartensite phase, the absolute magnetostriction value increased drastically, reaching a magnetostriction value of −180 ppm at 250 K. The sudden decrease between 260 K and 240 K in Figure 6a corresponds to the temperature range in which the dip in permeability is seen ( Figure 2a). Singh et al [24] suggested that a 3M premartensite phase appears below TP; therefore, the magnetostriction is correlated with lattice deformation and magnetic anisotropy. Ultrasonic measurements indicate that the shear elastic C' coefficient, C' = (C11 − C12)/2, decreases around TP [17], indicating that lattice softening occurred around TP.…”

“…The permeability demonstrates a dip at T P [23]. Singh et al [24] have suggested that a 3M-like incommensurate premartensite phase exists in Ni 2 MnGa.…”

Premartensitic transition and relevant magnetic effects in Ni50Mn34In15.5Al0.5 alloy

“…The premartensitic phase shows approximately cubic symmetry with a micromodulated domain structure of parent phase, indicating the weak first-order nature of the premartensitic transition2345. Anomaly of the elastic26789, thermal29101112, resistivity810111314151617, and magnetic properties3789101112151617181920 can be observed across this phase transition in Ni-Mn-Ga alloys. From the view of developing the application of Ni-Mn-Ga alloys, much effort has been devoted to research the origin of the premartensitic transition from parent phase to the intermediate state by theoretical and experimental methods.…”

Temperature Dependences of the Electrical Resistivity on the Heusler Alloy System Ni2MnGa1−xFex

“…On cooling, the premartensitic phase appears below T p = 260 K. With further decrease of temperature, Ni 2 MnGa undergoes a first-order martensitic transition at T M = 200 K [7]. Recently, Singh et al performed a high-resolution synchrotron X-ray powder diffraction study for Ni 2 MnGa and discussed the incommensurate nature of the modulate structures of the premartensitic (intermediate) and martensitic phases [8,9]. The ferromagnetic state remains below T M .…”