Domain structure and magnetic anisotropy fluctuations in (Ga,Mn)As: Effect of annealing

We investigate the effect of post-growth annealing on the magnetic domain structure and magnetization reversal process of (Ga,Mn)As epilayers grown with tensile strain on a (Ga,In)As buffer. In the case of perpendicular magnetic easy-axis, annealing drastically changes the domain structure observed at magnetization reversal. In as-grown samples, strongly anisotropic domain growth is observed. Dendritic-like domain expansion with guided branching along the directions results in a grid-like pattern. This is tentatively attributed to spatial fluctuations of the uniaxial anisotropy constant, correlated with the cross-hatch pattern. In annealed samples, domain wall motion is much more isotropic, which likely results from a decrease of the relative amplitude of the uniaxial anisotropy fluctuations with increasing carrier density. However domain wall motion is impeded by linear or slightly curved defects, hundreds of micrometers long, and point-like pinning centers. The density of nucleation centers for magnetization reversal strongly decreases upon annealing.