Ferromagnetic resonance in granular soft magnetic films

Ferromagnetic resonance (FMR) is a powerful tool in probing the local magnetic properties in composite magnetic systems. Reactive sputter deposited Fe-R-O (R = Hf, Nd, and Dy) films were systematically studied using in-plane FMR. The as-deposited films render broad FMR spectra, with multiple peaks becoming obvious on annealing. The overall local saturation magnetization M_s at the atomic sites, extracted from in-plane FMR data, was compared with grain size, as measured by X-ray diffraction. M_s values decreased with decreasing grain size, being as low as 0.6 T for 5-nm grains. These values are lower than the bulk value calculated based on the Fe concentration determined by compositional analysis. The deviation increases with decreasing grain size. The linear dependence on the inverse of the grain size indicates a strong surface effect. The results were independent of the R element and, therefore, of the nature of the oxide.