Magnetic anisotropy of Ga_1-x Mn_x As films with additional nonmagnetic donor doping

We have investigated the magnetic anisotropy properties of a series of ferromagnetic Ga_1-xMn_x As:Si films by transport measurements. The angular dependences of the planar Hall resistance (PHR) were analyzed in terms of the magnetic free energy to obtain the anisotropy fields. The cubic component of the magnetic anisotropy is much stronger than the uniaxial component at low temperature, although its dominance rapidly decreases with increasing temperature. The direction of the uniaxial anisotropy at 16 K lies along either the [1- 11] or the [110] direction depending on the Mn composition in the series. The uniaxial anisotropy direction, however, changed along the [110] direction regardless of the Mn concentration in the samples as the temperature increases.