Ferromagnetic Resonance in Very Fine Co Particles forming Granular Co-Cu Films

Abstract

Fine magnetic particles have been investigated with ferromagnetic resonance (FMR) for many years [1]. With the growing interest in giant magnetoresistance in granular materials [2], FMR has been proved useful for revealing important information on the size and shape of particles [3]. There are several aspects which are important in description of FMR response in fine particle assemblies: (i) thermally activated magnetization reversal, i.e., superparamagnetism; (ii) magnetic anisotropies; (iii) interparticle interactions and (iv) particle size distribution. The essential factor which distinguishes FMR in fine magnetic particles from that in polycrystals is their superparamagnetic behaviour. If the sizes of particles are so small that thermal fluctuations of the magnetic moments temporarily disturb their equilibrium configuration, the static picture is no longer valid and a statistical approach is needed. Our experimental data will be interpreted in the framework of such an approach [4] extended to take into account dipolar interactions between particles as well as their size distribution. The description of FMR in fine particles will be limited here to the main concepts. A full discussion can be found in the original paper [4]. Let us consider a fine particle of volume v, magnetic moment μ= My and the uniaxial anisotropy Ku which is subjected to a slowly varying magnetic field