Time-domain ferromagnetic resonance in epitaxial thin films

Time-resolved ferromagnetic resonance is used to study magnetic relaxation in epitaxial Fe _1-xCo _x films grown on vicinal GaAs(100) surfaces. The magnetic free energy of these films is determined by a combination of a four-fold volume anisotropy and a two-fold anisotropy due to surface bonding and morphology. The response following a high bandwidth (∼10 GHz) magnetic field pulse is measured using the polar Kerr effect. Samples with transverse dimensions much larger than the spatial extent of the field pulse show simple free induction decays. The measured precession frequencies are in good agreement with a coherent rotation model in the case of films grown on (100) surfaces with little or no (<2°) miscut. The magnetic response of 20 μm diameter disks is more complicated, particularly for static fields along the hard direction. Long-lived features appear in the response, and single precession frequencies are no longer observed. Micromagnetic simulations indicate the critical role played by the combination of the anisotropy and demagnetizing fields in the patterned structures.