Simultaneous existence of two spin-wave modes in ultrathin Fe/GaAs(001) films studied by Brillouin light scattering: experiment and theory

A double-peaked structure was observed in the in situ Brillouin Light Scattering (BLS) spectra of a 6 Åthick epitaxial Fe/GaAss001d film for values of an external magnetic field H, applied along the hard in planedirection, lower than a critical value Hc.0.9 kOe. This experimental finding is theoretically interpreted interms of a model which assumes a nonhomogeneous magnetic ground state characterized by the presence ofperperpendicular up/down stripe domains. For such a ground state, two spin-wave modes, namely an acousticand an optic mode, can exist. Upon increasing the field the magnetization tilts in the film plane, and for HùHc the ground state is homogeneous, thus allowing the existence of just a single spin-wave mode. Thefrequencies of the two spin-wave modes were calculated and successfully compared with the experimentaldata. The field dependence of the intensities of the corresponding two peaks that are present in the BLS spectrawas also estimated, providing further support to the above-mentioned interpretation.