  It is demonstrated that the nearest neighbor Mn pair on the GaAs (001)
surface has a lower energy for the [-110] direction comparing to the [110]
case. According to the group theory and the Luttinger's method of invariants,
this specific Mn distribution results in bulk uniaxial in-plane and
out-of-plane anisotropies. The sign and magnitude of the corresponding
anisotropy energies determined by a perturbation method and ab initio
computations are consistent with experimental results.
