Film thickness dependence of the magnetic resonance in Fe–SiO2 nanocomposites

The magnetic properties of Fe–SiO₂ nanogranular composite thin films were studied as a function of film thickness and Fe concentration, f, using ferromagnetic resonance at X-band (9.4 GHz) and Q-band (35 GHz). Films with an Fe volume percent ranging from 17% to 70% were fabricated from a mosaic target using RF sputtering techniques. Film thickness was varied between 10 and 200 nm. From measurements made at room temperature with the external field applied parallel and perpendicular to the film plane, it was possible to determine an almost linear dependence of the effective anisotropy field with Fe concentration. Small differences observed between X- and Q-band, specially at low f, were attributed to the effects that the different fields applied during the experiment cause on the magnetic state of the sample. No systematic change of the effective field or the g value was observed in films of different thickness. The absorption line width, on the other hand, was found to depend on film thickness indicating a larger distribution of particle shape and size with increasing film thickness. A maximum in the line width was observed around f30–35% and is probably caused by the transition from single domain ferromagnetic clusters to superparamagnetic particles.