Ferromagnetic Resonance on Micro- and Nano-Hexaferrites at Extremely High Frequencies

Millimeter wave transmittance measurements have been successfully performed on micro- and nano-sized BaFe$_{\mathrm{12}}$O$_{\mathrm{19}}$ and SrFe$_{\mathrm{12}}$O$_{\mathrm{19}}$ hexaferrite powders. Broadband transmittance measurements have been performed using free space quasi-optical spectrometer, equipped with a set of high power backward wave oscillators covering the frequency range of 30 -- 120 GHz. Real and imaginary parts of dielectric permittivity for both types of micro- and nanoferrites have been calculated using analysis of recorded high precision transmittance spectra. Frequency dependences of magnetic permeability of ferrite powders, as well as saturation magnetization and anisotropy field have been determined based on Schl\"{o}emann's theory for partially magnetized ferrites. Micro- and nano-sized ferrite powders have been further investigated by DC magnetization to assess magnetic behavior and compare with millimeter wave data. Consistency of saturation magnetization determined independently by both millimeter wave absorption and DC magnetization have been found for all ferrite powders. These materials seem to be quite promising as tunable millimeter wave absorbers and filters, based on size-dependent absorption.