Effect of spin diffusion on Gilbert damping for a very thin permalloy layer in Cu/permalloy/Cu/Pt films

Ferromagnetic resonance (FMR) was measured for Cu/permalloy (Py) (20, 30, 40 Angstrom)/Cu (d(Cu))/Pt (0, 50 Angstrom) films with various d(Cu) to clarify the effect of spin diffusion driven by the precession of magnetization on Gilbert damping. The peak-to-peak linewidth DeltaH(pp) of the FMR spectra for Cu/Py/Cu/Pt films was very large at d(Cu)=0 Angstrom, and decreased remarkably at d(Cu)=30 Angstrom. Above d(Cu)=30 Angstrom, it decreased gradually with increasing d(Cu) in the anomalously wide range of d(Cu). The out-of-plane angular dependence of the FMR of Cu/Py(30 Angstrom)/Cu (d(Cu))/Pt (0, 50 Angstrom) films was measured and analyzed using a Landau-Lifshitz-Gilbert equation that took into account the local variation of the effective demagnetizing field. The Gilbert damping coefficient G obtained from the analysis for Cu/Py/Cu/Pt films was about twice as large as that for Cu/Py/Cu films even at d(Cu)=100 Angstrom and decreased gradually as d(Cu) increased. At d(Cu)=2000-3000 Angstrom, G for Cu/Py/Cu/Pt and Cu/Py/Cu films has the same value. We discussed the influence of spin diffusion driven by the precession of magnetization in FMR on G using a previously proposed model. The calculated G vs d(Cu) fitted well to the experimental one, and the other features of the experimental results are well explained by the model.