Surface anisotropy and resonance modes in Co-SiO2 heterogeneous films -: art. no. 054428

We have studied the magnetic properties of Co-SiO2 films as a function of the Co concentration. Films were prepared using rf sputtering and Co volume concentrations in the range 0.18less than or equal toxless than or equal to0.62 were obtained. The samples were measured using ferromagnetic resonance at X-band (nu=9.5 GHz) and Q-band (nu=35 GHz). A main absorption associated to the uniform precession of the magnetic moments was observed in all films. From this resonance field we have evaluated an effective anisotropy field (H-eff) as a function of x. For low Co concentrations the shape anisotropy is the major contribution to H-eff. However, above a critical concentration x(p)similar to0.37 (coincident with the percolation of the Co granules) it is necessary to assume the presence of an additional anisotropy to explain the experimental data. At this same concentration the resonance spectra change with the appearance of an extra absorption at fields higher than that of the uniform resonance mode. For larger Co concentrations another absorption is observed at intermediate fields. We have analyzed these extra lines using the surface inhomogeneity model that takes into account the different environment of the surface moments. The surface anisotropy was determined as a function of x with a behavior quite similar to the additional anisotropy obtained from H-eff. In situ annealing studies support the assumption that the observed behavior originates in the effects of the surface and suggest the nonequivalence of both film surfaces.