Threshold for reorientation of the magnetization in F/AF bilayers

We study reorientation of magnetization of a ferromagnetic film grown on a compensated antiferromagnetic substrate and propose a method to determine a lower bound of the interface exchange coupling strength. For uniaxial ferromagnetic films, we show that the frustration induced by the interface exchange leads to a reorientation of the magnetization only if the strength of the interface field is beyond a threshold value. For ferromagnetic films with fourfold crystalline anisotropy the reorientation of the magnetization occurs for any value of the interface exchange field strength. We use a self-consistent two spin mean field model and apply the theoretical model to Fe/FeF2 and Fe/MnF2 bilayers. For these systems the reorientation of the magnetization produces minor changes in the antiferromagnetic arrangement of the spins near the interface, allowing a simple analytical formula which relates the threshold value of the interface exchange coupling to the magnetic parameters of the antiferromagnetic substrate and to the anisotropy and thickness of the ferromagnetic film.