Magnetic properties of various martensitic phases in Ni-Mn-Ga alloy

Single crystalline Ni_48.7Mn_29.1Ga_22.2 sample was studied in the temperature range 110 K-425 K. Magnetic properties of four different structure phases were measured at different temperatures. The magnetic anisotropies of the different phases were determined from magnetization curves. At room temperature, the structure is five-layered modulated tetragonal martensite with c/a<1 and uniaxial anisotropy K_u1=1.6×10⁵ J.m^-3. Below 251 K, the structure is orthorhombic c5 J.m^-3 between c and a. At 218 K, the orthorhombic phase transforms to the tetragonal phase with c/a>1. A single variant of low-temperature martensite was obtained by cooling in magnetic field 1.4 T. At 173 K, the sum of magnetic anisotropy constants is K₁+K₂=1.5×10⁵ J.m^-3 assuming an easy-plane magnetic anisotropy.