Ferromagnetic resonance in as-deposited and annealed Fe-SiO2 heterogeneous thin films

The magnetic properties of sputtered Fe-SiO2 composite thin films, containing an Fe volume fraction ranging from 0.42 to 0.90, have been characterized by means of ferromagnetic resonance. Experiments at X-band (v=9.4 GHz) and Q-band frequencies (v=35.4 GHz) were done on as-deposited and annealed (2 h, 435 degrees C) samples as a function of the Fe concentration. In general only one line corresponding to the uniform precession mode is observed. However, when the external field is applied perpendicular to the film plane one or two extra lines are observed at lower fields. More interestingly, when the concentration is very close to the percolation threshold for the Fe granules (f(p)similar to 0.62), more than 15 equally spaced lines are found. The origin of these additional resonances does not seem to be related to the film thickness, as usually happens with standing spin waves in continuous films, but to the divergence of the correlation length at the percolation concentration. It was found that the effective anisotropy held H-eff, which takes into account demagnetization and anisotropy fields, decreases almost linearly as the Fe fraction is reduced up to f(p), where a sudden decrease is observed. The resonance linewidth and the calculated g value also have distinctive features near the percolation threshold. Sample annealing produced a broader distribution of particle sizes and shapes that reflects in a resonance linewidth 3-6 times larger than for similar as-deposited samples. All these effects are closely related to the intricate geometry of the Fe granules near the percolation concentration. [S0163-1829(99)01338-7].