Dielectric and magnetic losses of microwave electromagnetic radiation in granularstructures with ferromagnetic nanoparticles

Abstract

We have studied dielectric and magnetic losses in granular structures constituted byferromagnetic nanoparticles (Co, Fe, B) in an insulating amorphous a-SiO2matrix at microwave frequencies, in relation to metal concentration, substratetemperatures and gas content, in the plasma atmosphere in sputteringand annealing. The magnetic losses are due to fast spin relaxation ofnanoparticles, which becomes more pronounced with decreasing metalcontent and occur via simultaneous changes in the granule spin directionand spin polarization of electrons on exchange-split localized statesin the matrix (spin-polarized relaxation mechanism). The differencebetween the experimental values of the imaginary parts of magneticpermeability for granular structures prepared in Ar and Ar + O2atmospheres is determined by different electron structures of argon and oxygenimpurities in the matrix. To account for large dielectric losses in granularstructures, we have developed a model of cluster electron states (CESs).Cluster states are formed by s-electrons of nanoparticles and by electronsin localized states in intergranule regions of the matrix. The observedsharp increase in dielectric losses in the percolation region, the differentvalues of losses for structures sputtered in the Ar and Ar + O2atmospheres and reduction of dielectric losses upon annealing, are caused byelectric dipole polarization of electrons in CESs.