A study of eddy current field interactions in ferromagnetic / normal metal thin film structures and its impact on ferromagnetic resonance

Abstract

This study investigates the eddy current field interaction in ferromagnetic (FM) / normal-metal (NM) thin film structures and its impact on ferromagnetic resonance (FMR). Eddy currents correspond to circulating currents induced in conductors by time varying magnetic fields. When induced in the NM layer of a FM/NM thin film structure these currents give rise to secondary inhomogeneous magnetic fields that may perturb the magnetization-dynamics in the FM layer. To probe this interaction an FMR setup was employed to both excite eddy currents in fabricated FM/NM structures and to study how the associated magnetic fields perturb the dynamics of the system. A series of thin film structures composed of patterned 10\si{\nano\meter} Permalloy / 10\si{\nano\meter} gold thin films with lateral dimensions in the micron-millimeter range were prepared using a lift off process combining optical lithography, DC magnetron sputtering and Electron beam evaporation. The FMR response of produced thin film samples was measured using a commercial electron paramagnetic resonance (EPR) setup and variations in extracted FMR lineshape parameters used to map the eddy current field interaction. To interpret obtained FMR results a theoretical model describing the impact of eddy current fields on FMR lineshapes was developed. The derived model was found to be in qualitative agreement with FMR results obtained for some, but not all analyzed thin film structures.

FMR results obtained from uncapped circular Permalloy thin film structures indicated significant eddy current field interactions in Permalloy alone. While complicating the analysis of results obtained from FM/NM thin film structures, this observation may be of interest due to the widespread use of Permalloy in magnonics related research.

In the study of FM/NM ring structures a tunable suppression of the FMR signal amplitude was observed when varying the width of the NM loop enveloping the inner FM sheet, this indicating that the FMR response can be tuned by manipulating the geometry of NM thin film structures in close proximity to FM thin films.

Results obtained from the FMR analysis of rectangular FM/NM bilayer structures indicate that the dynamics of the system is locally dominated by in-plane eddy current field components, this implying a local amplification of the applied microwave magnetic field.