Ferromagnetic Resonance for CoPt-Based Granular Films with Intergranular Magnetic Interaction

Abstract

In current perpendicular magnetic recording, granular media consist of ferromagnetic CoPtCr based magnetic grains and non $\square$ magnetic oxide boundaries are used. To control static and dynamic magnetization process of the granular media, evaluation of intrinsic magnetic constants such as saturation magnetization $(M_{\mathrm{s}})$ , magnetic anisotropy field $(H_{\mathrm{k}})$ , gyromagnetic ratio $(\gamma)$ , and damping constant $(\alpha)$ is quite essential. The ferromagnetic resonance (FMR), which is a resonance phenomenon of magnetic moments on effective fields, is one of the typical evaluation methods for $H_{\mathrm{k}}, \gamma$ , and $\alpha$ for a homogeneous material [1]. For the granular media, FMR signals thought to broaden due to distribution in magnetic properties and positions for each magnetic grain. However, its FMR signal was not significantly broader compared with that for alloy film with nearly the same composition for a Q $\square$ band (~34 GHz) magnetic cavity method [2] (fig. 1). In this study, the influences of intergranular magnetic coupling among columns, $H_{\mathrm{k}}$ distribution, and magnetic dipole interactions on FMR of the magnetic nano $\square$ column which assume the granular media is investigated by numerical calculations based on the Landau $\square$ Lifshitz $\square$ Gilbert equation. In the case of grains with $H_{\mathrm{k}}$ distribution and no $\square$ intergranular magnetic coupling, an FMR of assembly appears at an expected field based on the Kittel mode. This resonance had a long tail on the low magnetic field side (fig. 2). Such a resonance phenomenon is caused by a lot of coupling modes that originated from magnetic dipole interactions. In this case, $\alpha$ and $H_{\mathrm{k}}$ distribution can be evaluated from the full width at half maximum (FWHM) of the resonance and its long tail, respectively. On the other hand, the long tail reduced with increasing intergranular magnetic coupling. In this case, FWHM related to only $\alpha$ . This indicates that the resonance changed from the incoherent mode to a state near the Kittel mode, because of the increase in intergranular magnetic coupling even though there was distribution of $H_{\mathrm{k}}$ in the magnetic grains.