Ferromagnetic resonance in granular thin films

A theoretical analysis has been done of the effects of granule size, shape, orientation, and concentration and the temperature effects on the ferromagnetic resonance (FMR) field H-r in granular thin films. The granular CoxAg1-x thin films with 0.2<x<0.85 (the volumetric filling factor 0.17<f<0.83) were prepared by electron-beam oblique coevaporation. Our room temperature FMR data show an abrupt change in the H-r(f) dependence at f = f(p) approximate to 0.3, the percolation point for this system (as evidenced from the magnetization and magnetoresistance measurements). This change is associated to the development of great clusters [ferromagnetic (FM) phase] beside single granules [superparamagnetic (SPM) phase] at f>f(p). For the Co-Ag system, persistence of a considerable SPM fraction is revealed by the superconducting quantum interference device data up to the highest f, and the effect at f = f(p) consists in a discontinuous jump of the H-r(f) slope. Otherwise, the FMR data for granular Fe-SiO2 films(15) reveal a discontinuous jump in H-r itself at f approximate to 0.28. The latter can indicate a 1st kind-like magnetic percolation transition in that material. Thus FMR studies can effectively probe the internal structural processes in granular magnetic systems. (C) 1999 American Institute of Physics. [S0021-8979(99)65608-0].