Ferromagnetic resonance in granular thin films

Abstract

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 Hr 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 Hr(f) dependence at f=fp≈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>fp. 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=fp consists in a discontinuous jump of the Hr(f) slope. Otherwise, the FMR data for granular Fe–SiO2 films reveal a discontinuous jump in Hr itself at f≈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.