Surface anisotropy and resonance modes in Co-SiO2 heterogeneous films

J. GÓMEZ; A. BUTERA; J. A. BARNARD

PHYSICAL REVIEW B - CONDENSED MATTER AND MATERIALS PHYSICS

Año: 2004 vol. 70 p. 54428 - 54428

0163-1829

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 0.18 < x <  0.62 were  obtained. The samples were measured using ferromagnetic resonance at X-band (v  =9.5 GHz) and Q-band  (v=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 (Heff) as a function of x. For low Co concentrations the shape  anisotropy is the major contribution to Heff.  However, above a critical concentration xp~0.37  (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 Heff. 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.