Abstract : The subject of this thesis is the experimental study of the magnetic anisotropy in thin ferromagnetic semiconductor films and the magnetic coupling in bilayers formed with ferromagnetic metals. I have focussed my studies on two different systems: quaternary ferromagnetic GaMnAsP thin films and MnAs/GaMnAs bilayers. In these systems I have investigated how the magnetic anisotropies are influenced by the lattice mismatch induced biaxial strain and the hole concentration. The principal experimental techniques used are ferromagnetic resonance spectroscopy, SQUID magnetization measurements as well as transport and high resolution X-ray diffraction. Two series of GaMnAsP layers characterized by different Mn concentrations of 7% and 10% were investigated. For each series the P concentration has been varied over a wide range from 0 to 20%. Of particular interest is the case of highly P doped layers where the conductivity regime changes from metallic to impurity band conduction. This change induces a profound modification of all pertinent magnetic parameters. The variation of the four anisotropy constants as a function of temperature and P has been determined. For intermediate P concentrations layers with zero uniaxial strain can be obtained. The temperature induced magnetization reorientations have been evidenced for 6% doped P. High frequency FMR measurements have been used to study the magnetic anisotropy of MnAs epitaxial films on (111) and (100) GaAs. A ferromagnetic exchange coupling has been evidenced for MnAs/GaMnAs bilayers. I have further studied the magnetization relaxation via the Gilbert damping factor which has been deduced from FMR linewidth studies at two different microwave frequencies.