Broad-band FMR study of ferromagnetic thin films patterned with antidot lattices

Previous ferromagnetic resonance (FMR) studies of ferromagnetic (FM) thin films patterned with antidot (AD) arrays have generally avoided the low-field, hysteretic regime that is dominated by irreversible domain wall (DW) dynamics in unpatterned films. Moreover, FM vortices have not yet been identified and systematically studied in films patterned with AD lattices (ADLs). We have studied DC magnetization and broad-band FMR data for permalloy thin films of thickness t ≈ 25 nm, patterned with square lattices of square-shaped AD of width D and separation d = 1000 nm. We observe highly reproducible magnetic hysteresis curves and FMR spectra in the low-field reversal regime (i.e., applied magnetic fields H < HC, where HC is the coercive field), which indicates the ADL enforces a reproducible evolution of spin textures compared to the more random behavior of DW evolution in unpatterned films. The width of the reversal regime (2HC) and the field separation between observed FMR modes increases with D for a fixed separation d. Our micromagnetic simulations suggest these effects are consequences of both edge pinning of moments by individual AD, or DW pinning by the extended ADL, which involves two distinct length scales L ≈ d and L ≫ d, respectively. FM vortices are observed in our simulations, and their stability sensitively depends upon the AD size and applied magnetic field history.