Anisotropic AC Magnetic Susceptibility in (Ga,Mn)As Films

We report a systematic investigation of ac magnetic susceptibility in (Ga,Mn)As films as a function of temperature and magnetic field, carried out in parallel with dc magnetization measurements. The temperature dependence of ac susceptibility χ_ac shows anisotropic behavior: 1) a single peak in χ_ac is observed close to T_C for the [110] orientation of the driving ac field; 2) a single peak is also seen close to 22 K for the field along [110]; and 3) peaks at both these temperatures are observed for the field applied along [100]. A detailed analysis of the ac and dc data unambiguously indicates that the peak near T_C is related to the paramagnetic-to-ferromagnetic phase transition, with the ferromagnetic domains nucleating with their easy axes aligned with the [110] direction, providing a clear picture of uniaxial domain behavior near T_C. The peak near 22 K, on the other hand, is related to the onset of biaxial domain structure in (Ga,Mn)As induced by the competition between uniaxial and cubic anisotropy. More specifically, the ac susceptibility peak near T_C involves 180° magnetization flips along the [110] easy axis of the domains, while the peak near 22 K originates from magnetization wobbling between two biaxial easy axes separated by a small angle.