Studies Of Ferromagnetic Resonance Linewidth Of Polycrystalline And Single Crystal (Ultra)Thin Films

In switching characteristic of magnetic storage materials and other devices of spintronics, the damping of magnetic moment plays a key role in spin dynamics. As far as applications are regarded, the magnetic relaxations in thin films, ultrathin films, nanostructure and ultrafine-particulate materials come to be a hot topic recently. Ferromagnetic resonance (FMR) is one of the fundamental methods for exploring properties in magnetic material, the half-power field width of spectrum of FMR ,viz. linewidth, relates directly to the damping of magnetic material . In this paper, with the aid of FMR, magnetic relaxations in magnetic thin and ultrathin films have been systematically studied. The following are the main results of our work:1. The general way for studying linewidth of FMR by phenomenological theory is given, with a comparison between Laudau-Lifshitz-Gilbert equation and Bloembergen-Bloch equation in describing dynamics of magnetic moment. In addition, we reviewed all of magnetic relaxation mechanisms employed to account for FMR linewidth and damping in magnetic materials.2. The data of FMR linewidth of a series of polycrystalline samples Ta / NiFe /Ta with NiFe thickness changing from 2.5 nm and 50 nm have been fitted with our deduced analytic expressions of various relaxation mechanisms. According to our analysis, the relaxation due to LS coupling contributes mainly to the damping in these polycrystalline films, and recently proposed spin pumping mechanism also has part contributions. But for some samples, the two-magnon scattering mechanism should not be ignored. For all samples, the apparent broadening of linewidth are more dependent on the specific conditions of preparation. For the first time we obtained one of parameters characterizing spin transport in spin pumping theory,g↑↓, from experiment at the interface of Ta/NiFe.3. The experimental data of FMR linewidth of a series of single crystal Fe ultrathin films epitaxially grown on GaAs with thickness of 5.2,8.4,12.7,25,33 ML have been fitted similarly to polycrystalline samples. During fitting, an experiential analytic expression for Gilbert damping factor has been found independently to account for its anisotropy. Besides, it is also shown in numerical calculation that the largest linewidth occurs when the magnetic moment has the largest deviation from external field.4. We have independently deduced a basic formulation for recently proposed magnetoelectronic circuit theory to show more clearly the physical meaning of parameter g↑↓. A fictitious experiment was designed and its related problem of spin transport has been solved based on the magnetoelectronic circuit theory.