Spin dynamics of the unsaturated magnetization state of a Co thin film

초록

A systematic study of the FMR spectra taken for unsaturated state of a Co thin film which exhibits hysteresis for the magnetization reversal region is presented; such studies are made possible by employing our new broadband FMR transmission technique(1) capable of tuning the resonance field close to that of the unsaturated state. Although there have been a few previous reports on the hysteretic behavior at a specific frequency (using the cavity-mode technique),(2,3) our results are unique in that they yield the frequency dependence of the hysteretic microwave absorption spectra and the evolution of the low-field responses, which are believed to provide possible origin of the low-field microwave absorption and valuable information about the spin dynamics of low-field magnetization reversal in ferromagnetic thin films. Our Co film has a g-factor of 2.25 and a Gilbert damping factor of 4.5 x 108 s-1, deduced from normal FMR spectra taken in the saturated state. In the unsaturated state, however, the evolution of the low-field hysteretic behavior were observed, which affects the FMR mode as the excitation frequency is lowered from 5.000 to 1.636 GHz. We found that the microwave absorption process persists in the unsaturated state for frequencies higher than 1.868 GHz such that the absorption minima occur at - 12 Oe on a down-field sweep and at +12 Oe on a up-field sweep, respectively. The implication and the possible origin of this phenomenon will be discussed.