Measurement of the permeability tensor in nanophases of granular metal oxides and field-induced magnetic anisotropy

Abstract

We have investigated the composition and frequency dependence of the permeability tensor in heterostructures composed of metal oxides nanoparticulate powders. Samples consist of grains of the magnetic phase $(\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3})$ homogeneously dispersed in a nonmagnetic insulating background (ZnO). A measurement capability has been used to investigate the permeability tensor components of the ferrimagnetic samples under an externaly magnetic field, as a function of composition. The samples exhibit a previously unobserved field-dependent anisotropic contribution to the permeability tensor. The gyromagnetic resonance frequency decreases with ${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ content and increases with applied magnetic field. The frequency-dependent response in the microwave frequency range can be fit reasonably well using a single parameter whose value is consistent with the determination from a ferromagnetic resonance experiment and can be rationalized by means of a recently developed coarse-grained multiscale model. The model is based on multiscale averaging techniques and takes into account magnetostatic intergranular interactions via a mean-field approximation. These observations are aimed at developing a more realistic description of magnetic nanophases that takes into account details of the microstructure and provide salient input to micromagnetic simulations.