Mesoscopic structure of Ni_{2 + x} Mn_{1 − x} Ga alloys in the concentration range corresponding to a coupled magnetic-structural phase stransition

Abstract

The Ni_{2 + x} Mn_{1 − x} Ga magnetic shape memory alloys at concentrations x = 0.18–0.36 have been studied using small-angle polarized neutron scattering in magnetic fields 0 < H ≤ 5.7 kOe. In this concentration range, the alloy undergoes a coupled magnetic-structural phase transition. At x < 0.18, the martensitic phase transition occurs, as usual, in the ferromagnetic state, i.e., T _m < T _C (T _m and T _C are the temperatures of the martensitic and magnetic phase transitions, respectively). However, at x > 0.27, the relationship between the characteristic temperatures is changed, i.e., T _m > T _C. The small-angle polarized neutron scattering data indicate that, in the concentration range under investigation, there occurs a complex transformation of the magnetic and atomic structures. All phase transitions exhibit a temperature hysteresis of scattering and polarization, which is characteristic of first-order phase transitions. A comparative analysis of the mesoscopic structures of the Ni_{2 + x} Mn_{1 − x} Ga and Ni_{2 + x + y} Mn_{1 − x} Ga_{1 − y} alloys studied using the small-angle polarized neutron scattering technique has been carried out.