Chaos in magnetic garnet thin films

Abstract

Ferromagnetic resonance in thin magnetic garnet films has been studied at excitation frequencies of 10 GHz and from room temperature to 4.2 K. Powers sufficient to drive the magnetization to large precession angles have been studied. Second-order Suhl instabilities of the main resonance have been observed as an onset of auto-oscillations of the magnetization. These oscillations include regular and irregular oscillations, very noisy collective oscillations, as well as spiking. They have been observed in real time and by spectral analysis. In pure yttrium iron garnet (YIG) films the oscillations have been observed in the frequency range from 0.5 to 2 MHz at 300 and 4.2 K. In going from 300 to 4.2 K the threshold power level pth in YIG films is lowered by three orders of magnitude, while the frequency of the regular oscillations are nearly independent of temperature. If pth is connected to the intrinsic damping parameter γ, then γ is temperature dependent, which is in fair agreement with the large number of narrow resonance features observed in the films. Preliminary simulations using a model extending the S theory of Zakharov and Nakamura for large precession angles give time evolutions that are qualitatively similar to the experimental observations. Regular and weekly irregular oscillations are found for one spin-wave mode coupled to a linear resonance circuit, i.e., a phonon mode. More irregularity is found for two coupled spin-wave modes excited to large precession angles.