Standing spin waves in granular Fe–SiO2 thin films

Abstract

Magnetically heterogeneous Fe–SiO2 composite thin films (20 nm thick) prepared using standard rf sputtering techniques have been investigated by means of ferromagnetic resonance (FMR). Besides a single FMR absorption line, corresponding to the uniform precession mode, several nearly equally spaced absorption lines could be observed close to the percolation threshold of the Fe granules fp if the applied field H is parallel to the film normal. These extra absorptions have been identified as standing spin waves. A constant separation between lines suggests that volume inhomogeneities are important (as expected in a granular film). We have determined the following characteristic lengths of the standing spin waves for three Fe volume concentrations: L=370 nm, 680 nm, and 270 nm for f=0.63, 0.66, and 0.69, respectively. In continuous thin films L is normally coincident with the film thickness but in this case L is more than an order of magnitude larger than the film thickness. This discrepancy and the absence of additional lines for f≠fp suggests that L is not related to the film thickness but to the correlation length of the percolating network, eventually modulated by the sample magnetization M. The temperature dependence of the average separation between lines supports this picture.