New method for measurement of complex magnetic permeability in the millimeter-wave range, part II: Hexaferrites

Magneto-optical methods were applied for the first time in millimeter wavelength range for characterization of anisotropic ferrites, The principles of the free-space magnetooptical method are presented and the new experimental procedures leading to the determination of millimeter-wave permeability and permittivity in anisotropic ferrite materials are described. The measurements were performed with a computer-controlled W-band (70-120 GHz) quasi-optical-waveguide bridge. A backward-wave oscillator was used as a source of tunable millimeter wave radiation. The oriented Sr-hexaferrite ceramic was selected for the verification of millimeter-wave magnetooptical method. The magneto-optical measurements in transverse configuration revealed strong anomalous dispersion in the millimeter-wave refractive index spectrum for Sr hexaferrite, mainly due to the frequency variation of magnetic permeability, Computer simulations revealed a good agreement of measured parameters with known data for Sr hexaferrite. It was shown that the free-carrier absorption (sigma similar to 0.05 Omega(-1) cm(-1)) and magnetic permeability contribute to the relatively high millimeter-wave losses in hexaferrite ceramics. The frequency dependence of Faraday rotation in the millimeter-wave range was measured, and the experimental results are discussed. The results presented in this paper demonstrate that this new magneto-optical method is capable of providing accurate dielectric and magnetic data in the millimeter-wavelength range.