Magnetic anisotropy in Ni-Mn-Ga martensites

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 K-1(rt)=1.65x10(5) J m(-3) and K-2 is negligible. Seven-layered modulated orthorhombic martensite exhibits easy magnetization direction along the shortest crystallographic axis. K-1(rt)=1.7x10(5) J m(-3) and K-2(rt)=0.9x10(5) 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 K-1(rt)=-2.3x10(5) J m(-3) and K-2(rt)=0.55x10(5) J m(-3). The temperature dependence of K-1(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. (C) 2003 American Institute of Physics.