Localized Edge Modes in Permalloy Square Antidot Arrays

We have carried comprehensive experiments on and simulations of the ferromagnetic resonance spectrum of various thin films of permalloy patterned with a periodic array of holes on a square lattice. These so-called antidot lattices show a rich multi-line spectrum covering a wide range of frequencies and magnetic fields; they also exhibit striking angular dependences. One mode that is predicted in our simulations is highly localized at the edges of the holes. Our simulations further show that for a fixed field or excitation frequency the angular dependence of this mode strongly depends on the shape of the holes in the antidot array. It has been suggested\footnote{S. Neusser, B. Botters and D. Grundler, Phys. Rev. B, 78, 054406 (2008).} that, in comparison to other more extended modes, this localized mode would be difficult to find experimentally due to: i) the effects of varying shape and roughness present in an actual array, and ii) the fact that the mode is concentrated within a small fraction of the unit cell of the sample. Our experiments, performed with a broadband meanderline-based ferromagnetic resonance (FMR) spectrometer,\footnote{C. C. Tsai, J. Choi, S. Cho, B. K. Sarma, C. Thompson, O. Chernyashevskyy, I. Nevirkovets, and J. B. Ketterson, Rev. of Sci. Instr. \textbf{80}, 023904 (2009).} show a very weak resonance (requiring extensive signal averaging) that is in rough agreement with the simulations, which we propose as a candidate for this elusive edge mode.