Magnetic anisotropy in Ni–Mn–Ga martensites

Abstract

We study the temperature dependence of the magnetic anisotropy of three different martensites known to exist in the Ni–Mn–Ga alloys. The anisotropy constants were determined from magnetization curves measured at different temperatures. The anisotropy of five-layered modulated tetragonal martensite is uniaxial with easy magnetization direction along short crystallographic axis. At room temperature K1(rt)=1.65×105 J m−3 and K2 is negligible. Seven-layered modulated orthorhombic martensite exhibits easy magnetization direction along the shortest crystallographic axis. K1(rt)=1.7×105 J m−3 and K2(rt)=0.9×105 J m−3 referring to hard and mid-hard magnetization axes. Nonmodulated tetragonal martensite possesses a uniaxial anisotropy with easy plane and hard magnetization direction along the long crystallographic axis with K1(rt)=−2.3×105 J m−3 and K2(rt)=0.55×105 J m−3. The temperature dependence of K1(T) of five-layered martensite follows magnetization power law with exponent n=3 suggesting a single ion origin of the magnetic anisotropy in Ni–Mn–Ga martensite.