Reorientation transitions of first and second order in thin ferromagnetic films

The order of the reorientation transition of the magnetization vector of thin ferromagnetic films is studied. It is shown within a quantum-mechanical molecular-field approach that both first- and second-order transitions are possible and that it depends in a very sensitive way on system parameters of which type of transition actually occurs. Systems in which the uniaxial anisotropy energy is the same for all sites tend to have first-order transitions while films with only a surface anisotropy have reorientation transitions of second order.