Anisotropy effects on the magnetic excitations of epitaxial ultrathin films below and above the Curie temperature

Brillouin light scattering is exploited to study magnetic excitations in magnetic films consisting of a few atomic layers, characterized by a strong in-plane uniaxial anisotropy, namely NiFe/Cu(110) and Fe/Cu(110). Well below the Curie temperature, a nonmonotonic field dependence of the magnon frequency is measured for magnetic field H applied along the hard in plane direction, while for field along the easy direction the frequency is found to increase almost linearly with increasing H. When temperature is increased, the measured magnon gap at zero field decreases until, for all the investigated systems, it vanishes at room temperature. The anisotropy of the magnetic excitations is found to persist even in the paramagnetic phase: in fact, the magnon frequency is found to increase almost linearly with increasing H, but with different slopes depending on the field direction. The smaller slope is found for the field along the hard axis. A finite-temperature Green's function theory, recently developed for the study of the field-driven reorientation transition of an anisotropic monolayer, is able to account for the experimental data of the investigated ultrathin films in the whole temperature range.