Vortex magnetodynamics: Ferromagnetic resonance in permalloy dot arrays

Permalloy dot arrays (square lattices of 100, 200, 500, and 1000 nm dots, 40 nm thick) have been fabricated using e-beam lithography. Hysteresis loops show vortex states at remanence in all samples. Variable frequency ferromagnetic resonance was used to probe the dynamics of both vortex and single domain states. The uniform resonance mode is present at fields above the vortex annihilation when field sweeping up and vortex nucleation field when field sweeping down. In the vortex state multiple resonances are observed in the 500 and 1000 nm samples, but interestingly, not in the 100 and 200 nm. In the frequency/field dispersion curve, these vortex resonances exhibit optical mode characteristics, whereas the uniform mode exhibits the expected acoustic mode behavior. Resonance modes corresponding to the annihilation and creation field (transition from vortex to single domain or verse versa) are also observed and discussed.