  We carefully investigated the ferromagnetic coupling in the as-grown and
annealed ferromagnetic semiconductor GaMnAs/AlGaMnAs bilayer devices. We
observed that the magnetic interaction between the two layers strongly affects
the magnetoresistance of the GaMnAs layer with applying out of plane magnetic
field. After low temperature annealing, the magnetic easy axis of the AlGaMnAs
layer switches from out of plane into in-plane and the interlayer coupling
efficiency is reduced from up to 0.6 to less than 0.4. However, the magnetic
coupling penetration depth for the annealed device is twice that of the
as-grown bilayer device.
