Influence of Magnetic Fields on the Spin Reorientation Transition in Ultra-Thin Films

The dependence of the spin reorientation transition in ultra-thin ferromagnetic films on external magnetic fields is studied. For different orientations of the applied field with respect to the film, phase diagrams are calculated within a mean field theory for the classical Heisenberg model. In particular we find that the spin reorientation transition present in this model is not suppressed completely by an applied field, as the magnetization component perpendicular to the field may show spontaneous order in a certain temperature interval. 1 Introduction Experimentally it became possible in recent years to grow epitaxial thin films of ferromagnetic materials on non-magnetic substrates with a very high quality. This offers the possibility to stabilize crystallographic structures which are not present in nature, and which may exhibit new properties of high technological impact. To understand the magnetic structure of these systems is a challenging problem both experimentally and theoret..