Structural and magnetic study of heterogeneous CoxAg1-x films by resonanceand magnetometric techniques

The combined study of structural and magnetic properties of CoxAg1-x hetero geneous films in a wide concentration range 0.18<x<0.82 was performed using nuclear magnetic resonance (NMR), X-band ferromagnetic resonance (FMR), an d superconductive quantum interferometer (SQUID) magnetometry. The structur al investigation by NMR shows that at low Co concentrations rather large an d pure cobalt clusters (similar to 3-nm diameter) coexist with much smaller ones of Co embedded in Ag rich, disordered (alloyed) regions. At x approxi mate to 0.4 large Co clusters coalesce (topological percolation). Above x = 0.5 there are Ag clusters in the Co matrix, which are larger and/or more i rregular than the Co clusters for an equivalent, complementary composition. Both SQUID and FMR measurements indicate that the magnetic percolation, ma nifested by collective magnetic behavior of the granule ensemble, onsets al ready at x approximate to 0.3. Below this concentration the average size of the granules, obtained using both resonance techniques and SQUID, is 2.7-3 .6 nm. At low Co concentration, the analysis of the SQUID magnetization dat a yields an atomic fraction of small clusters (similar to 0.5 nm) which is significantly larger than that observed in the NMR spectra, showing that mo st of the small clusters are still superparamagnetic at liquid-helium tempe rature. The observed FMR and magnetization behavior of CoxAg1-x films agree s well with the theoretical model developed in the framework of spherical i sotropic interacting particles at low Co concentrations. At higher Co conce ntrations, a complex dynamics of uniform FMR precession is found, with two split modes due to the coupling between magnetic moments of individual gran ules and infinite ferromagnetic cluster, in agreement with a recent theoret ical prediction. S0163-1829(99)05437-5].