Standing spin waves in granular Fe-SiO2 thin films

Magnetically heterogeneous Fe-SiO2 composite thin films (20 nm thick) prepa red using standard rf sputtering techniques have been investigated by means of ferromagnetic resonance (FMR). Besides a single FMR absorption line, co rresponding to the uniform precession mode, several nearly equally spaced a bsorption lines could be observed close to the percolation threshold of the Fe granules f(p) if the applied field H is parallel to the film normal. Th ese extra absorptions have been identified as standing spin waves. A consta nt separation between lines suggests that volume inhomogeneities are import ant (as expected in a granular film). We have determined the following char acteristic lengths of the standing spin waves for three Fe volume concentra tions: L = 370 nm, 680 nm, and 270 nm for f = 0.63, 0.66, and 0.69, respect ively. In continuous thin films L is normally coincident with the film thic kness 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 not equal f(p) 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 sepa ration between lines supports this picture. (C) 2000 American Institute of Physics. [S0021-8979(00)37908-7].