Anisotropy of ultrathin ferromagnetic films and the spin reorientation transition

The influence of uniaxial anisotropy and the dipole interaction on the direction of the magnetization of ultrathin ferromagnetic films in the ground state is studied. The ground-state energy can be expressed in terms of anisotropy constants which are calculated in detail as a function of the system parameters and the film thickness. In particular noncollinear spin arrangements are taken into account. Conditions for the appearance of a spin reorientation transition are given and analytic results for the width of the canted phase and its shift in applied magnetic fields associated with this transition are derived.