Trimming of permalloy stripes to enhance the localized edge mode spectrum probed by ferromagnetic resonance

Finite-size effects in ultrathin magnetic films are a well-known feature, i.e., when the surface or interfaces dominate the volume of the sample due to different roughness, texture, hybridization, modified magnetic moment, or dipolar fields. For nanostructures these effects could arise at the side walls as well. This leads to localized spin wave modes (edge modes).
It has been shown that the quality of the side walls (angled side walls or roughness) influence these modes [1]. During preparation of samples by lithography a certain edge roughness and side wall slope are sometimes inevitable. Nevertheless, in micromagnetic simulations these contributions are usually excluded from the model. We show, how successive trimming the sides of a 5 μm x 1 μm Permalloy stripe by a focused Ne ion beam improves the spin wave spectrum and enhances the edge mode spectrum as probed by planar microresonator ferromagnetic resonance (FMR) [2,3] as depicted in Figure 1. Including an rms edge roughness of ~2 nm (within the order of the permalloy grain size) in the simulations is enough to match the FMR data. Hence, we attribute the residual roughness to the ion induced damage by the lateral penetration during trimming of the side walls, and a small remaining edge roughness due to changes in the sputter yield for differently oriented Permalloy grains.