Magnetization damping in two-component metal oxide micropowder and nanopowder compacts by broadband ferromagnetic resonance measurements

Abstract

The microwave damping mechanisms in magnetic inhomogeneous systems have displayed a richness of phenomenology that has attracted widespread interest over the years. Motivated by recent experiments, we report an extensive experimental study of the Gilbert damping parameter of multicomponent metal oxides micro- and nanophases. We label the former by $M$ samples, and the latter by $N$ samples. The main thrust of this examination is the magnetization dynamics in systems composed of mixtures of magnetic $(\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3})$ and nonmagnetic (ZnO and epoxy resin) materials fabricated via powder processing. Detailed ferromagnetic resonance (FMR) measurements on $N$ and $M$ samples are described so to determine changes in the microwave absorption over the $6--18\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}$ frequency range as a function of composition and static magnetic field. The FMR linewidth and the field dependent resonance were measured for the $M$ and $N$ samples, at a given volume fraction of the magnetic phase. The asymmetry in the form and change in the linewidth for the $M$ samples are caused by the orientation distribution of the local anisotropy fields, whereas the results for the $N$ samples suggest that the linewidth is very sensitive to details of the spatial magnetic inhomogeneities. For $N$ samples, the peak-to-peak linewidth increases continuously with the volume content of magnetic material. The influence of the volume fraction of the magnetic phase on the static internal field was also investigated. Furthermore, important insights are gleaned through analysis of the interrelationship between effective permeability and Gilbert damping constant. Different mechanisms have been considered to explain the FMR linewidth: the intrinsic Gilbert damping, the broadening induced by the magnetic inhomogeneities, and the extrinsic magnetic relaxation. We observed that the effective Gilbert damping constant of the series of $N$ samples are found to be substantially smaller in comparison to $M$ samples. This effect is attributed to the surface anisotropy contribution to the anisotropy of ${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ nanoparticles. From these measurements, the characteristic intrinsic damping dependent on the selected material and the damping due to surface/interface effects and interparticle interaction were estimated. The inhomogeneous linewidth (damping) due to surface/interface effects decreases with diminishing particle size, whereas the homogeneous linewidth (damping) due to interactions increases with increasing volume fraction of magnetic particles (i.e., reducing the separation between neighboring magnetic phases) in the composite.