Successive Trimming of a Permalloy Stripe to enhance the localized Edge Mode Spectrum probed by Ferromagnetic Resonance

Finite-size effects in ultrathin magnetic films are a well-known feature. They usually play a role, when the surface and interface layers dominate over the volume contribution of the sample and have different properties, due to roughness, texture, hybridization, modified magnetic moment, or dipolar fields. For micro- and especially nanostructures these effects might be there as well—but at the side walls. Regarding the magnetization dynamics these effects lead to additional spin wave modes, e.g. localized spin wave modes (edge modes). It has been shown that these edge modes are influenced by the quality of the side walls, namely by angled side walls, edge roughness, beveled edges, or even magnetic dilution [1]. As these structures have to be prepared by means of lithography involving masks a certain edge roughness or even side wall slope are inevitable. Nevertheless, when it comes to micromagnetic simulations to corroborate or explain measurements these contributions are usually excluded from the model. Here 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 ferromagnetic resonance (FMR). To achieve the sensitivity to detect the FMR of the weak edge modes of a single Permalloy stripe we use planar microresonator FMR [2,3]. The experimental results are corroborated by micromagnetic simulations. Including just edge roughness of ~4 nm (rms) in the simulations is enough to perfectly match the FMR experimental data. The residual edge roughness is in the order of the grain size of the polycrystalline permalloy. Although the focused ion beam and its motion are able to cut the side walls perfectly straight and vertical with sub-nm precision, the Ne ions penetrate the side wall up to 15 nm (called straggling). This is due to the collision cascade with the Ni and Fe atoms of the Permalloy causing possible lateral damage of the Permalloy lattice. 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 sputter yield for differently oriented Permalloy grains.

[1] R.D. McMichael, B.B. Maranville, Phys. Rev. B 74,024424 (2006).
[2] A. Banholzer et al., Nanotechnology 22, 295713 (2011).
[3] R. Narkowicz et al., Rev. Sci. Instrum. 79, 084702 (2008).