  We describe a systematic study of the exchange coupling between a
magnetically hard metallic ferromagnet (MnAs) and a magnetically soft
ferromagnetic semiconductor (GaMnAs) in bilayer and trilayer heterostructures.
An exchange spring model of MnAs/GaMnAs bilayers accounts for the variation of
the exchange bias field with layer thickness and composition. We also present
evidence for hole-mediated interlayer exchange coupling in MnAs/p-GaAs/\GaMnAs
trilayers and study the dependence of the exchange bias field on the thickness
of the spacer layer.
