MAGNETIC INSTABILITIES IN ULTRATHIN FERROMAGNETS

We examine aspects of the dispersion relation of spin waves in ultrathin ferromagnetic films. When the easy axis is normal to the surfaces, and an external field is applied parallel to the surface, we find the state of uniform magnetization to be unstable, for magnetic fields in the near vicinity of that for which the magnetization just becomes parallel to the surface. We present results of a study of the temperature variation of the spin-wave dispersion relation, as a consequence of interactions between spin waves. From the structure of the result, we comment on the nature of the spin reorientation transition observed by Pappas, Kamper, and Hopster. It is noted that an earlier study of temperature-dependent renormalization of thin film parameters reported by Pescia and Pokrovsky overestimates the magnitude of the temperature corrections.