Intermetallic compounds of Mn and Group III elements constitute an interesting class of magnetic material, suitable for epitaxy on semiconductor substrates. By exploiting the epitaxial relationship to GaAs, e.g. metastable magnetic tau MnAl can be fabricated with the c-axis of the tetragonal crystal parallel to the substrate normal. Consequently, perpendicular magnetization due to the large magnetocrystalline anisotropy in tau MnAl is found. The resulting magneto-optic Kerr and extraordinary Hall effect has generated interest in applications such as optical storage and non-volatile magnetic solid state memories. Tayloring the magnetic properties of the epitaxial heterostructures can be done by post-growth annealing or micro-patterning. More flexibility is achieved with multilayers (e.g. tau MnAl/NiAl) and superlattices (e.g. tau MnAl/Co). In the latter all-magnetic superlattice on GaAs, a forced bce Co structure and strong coupling effects are under study. Further, we recently introduced heterogeneous magnet-semiconductor materials based on GaAs with a high density of nanometer-scale spherical magnetic MnAs clusters grown by MBE. Some of the interesting physics to be explored in magnetic material and semiconductor combinations will be discussed.