Oblique pumping, resonance saturation, and spin wave instability processes in thin Permalloy films
Date
2008
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Abstract
The study of nonlinear dynamics in metal films is of increasing importance as advancements are made in magnetic recording. In this dissertation, these interactions are examined by the study of first order spin wave instability (SWI) processes that occur for external static magnetic fields well below ferromagnetic resonance (FMR), and second order SWI processes that occur for static fields over the full FMR field range. This work is concerned specifically with the study of the high power resonance saturation and oblique pumping responses in thin Permalloy films, the microwave threshold amplitudes at which the instabilities occur, and the theoretical analysis of the relevant SWI processes. To greatly increase measurement accuracy and reduce measurement time, the high power FMR system has been modified and new calibration techniques implemented. The modifications to the system allow for fully automated and calibrated microwave threshold amplitude vs. static field measurements, termed butterfly curves. Resonance saturation butterfly curves have been measured for an in-plane field configuration for 35 - 123 nm thin Permalloy films. The butterfly curves show a jump on the low field side associated with a low field shift of the FMR profile and a foldover like asymmetry development. Apart from the jump, the second order Suhl SWI theory, suitably modified for thin films, provides good fits to the butterfly curve data through the use of constant spin wave relaxation rates that are on the same order as expected for intrinsic magnon-electron scattering processes. The FMR in-plane precession cone angles at threshold are small. Oblique pumping butterfly curves have been measured at different in-plane field configurations for 104 and 123 nm thin Permalloy films. The butterfly curves show thickness dependent high field cutoffs that agree with the field points at which the bottom of the spin wave band moves above one half the pump frequency. A combination of parallel and perpendicular first order SWI theory, suitably modified for thin films, shows good fits to the data except at low fields where the thin film approximation is not applicable. The damping trial functions used for the fits correspond to magnon-electron and three-magnon scattering processes.
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Subject
ferromagnetic resonance
oblique pumping
permalloy
resonance saturation
spin wave instability
Suhl
electromagnetics
condensed matter physics
materials science