SPIN-REORIENTATION TRANSITIONS IN ULTRATHIN FERROMAGNETIC-FILMS UNDERAPPLIED-FIELD

The influence of external field on thin ferromagnetic films with spin- reorientation transitions (SRTs) is explored in the presence of differ ent orders of thickness-dependent uniaxial anisotropy. The two princip al configurations are addressed and the first two anisotropy contribut ions are examined. A simple and natural representation of the field-in duced SRTs has been found. It preserves the linearity of the thickness -driven trajectories even with field. The crosspoints of these traject ories with the phase borderlines correspond to crossover thicknesses o f a linear type for positive second-order anisotropy contributions and of both linear and nonlinear types for negative second-order anisotro pies. In the latter case, the nonlinear boundaries correlate with the coexistence of phases which brings about first-order-like reorientatio ns and concommittant hysteresis effects. Practical schemes are propose d for the determination of the complete set of anisotropy parameters f rom measured field dependences of the crossover thicknesses.