Temperature dependence of magnetic anisotropy in ferromagnetic (Ga,Mn)As films: Investigation by the planar Hall effect

We carried out systematic planar Hall effect (PHE) measurements of GaMnAs ferromagnetic semiconductor film as a function of temperature. The two-step switching of the PHE occurring in the magnetization-reversal process was observed to change significantly as the temperature was increased. To investigate the mechanism responsible for such behavior, the temperature dependence of the PHE was continuously measured (with and without an external magnetic field) after the sample was first magnetized along one of the easy axes to produce an initial single-domain state at 3 K. A detailed temperature dependence of the magnetization direction was then obtained by taking the ratio of the planar Hall resistance measured with and without a magnetic field. As the temperature was increased, the direction of the easy axis of magnetization was observed to change from the [010] crystallographic direction to [110]. This reorientation of the easy axis direction can be understood in terms of the temperature dependence of the relative strengths of the magnetic anisotropy constants (i.e., of the ratio of uniaxial-to-cubic anisotropy) of the GaMnAs film.