Ferromagnetic Resonance on Micro- and Nanoferrites in Millimeter Waves

Complex magnetic permeability and dielectric permittivity of micro- and nano-sized powdered barium ferrite (BaFe$_{12}$O$_{19})$ and strontium ferrite (SrFe$_{12}$O$_{19})$ have been studied in a broadband millimeter wave frequency range for the first time. Transmittance measurements have been performed using a free space quasi-optical millimeter wave spectrometer, equipped with a set of high power backward wave oscillators. Backward wave oscillators have been used as sources of tunable coherent radiation at each individual $Q$-, $V$- and $W$- frequency bands. 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 the magnetic permeability have been obtained from Schl\"{o}mann's equation for partially magnetized ferrites. Tunable millimeter wave absorber, based on micro- and nano-sized powdered ferrite materials is presented.