  Starting from microscopic and symmetry considerations, we derive the
Hamiltonian describing the exchange interaction between the localized Mn spins
and the valence band holes in $Ga_{1-x}Mn_x As$. We find that due to the strong
spin-orbit coupling in the valence band, this exchange interaction has a rather
complex structure and generates a highly anisotropic effective interaction
between the Mn spins. The corresponding ground state has a finite ferromagnetic
magnetization but is intrinsically spin-disordered even at zero temperature.
