Free-Space Characterization of Magneto-Optical Hexaferrites in the Submillimeter-Wave Range

We present the quasi-optical free-space characterization of magneto-optical hexagonal ferrites-crystalline BaFe
₁₂
O
₁₉
and both ceramic and crystalline SrFe
₁₂
O
₁₉
-in the range between 325 and 500 GHz using a vector network analyzer. This method provides us the full complex spectrum of the 2 × 2 S-matrix element of the sample. Transmission through free air and reflection of a perfect mirror have been used as references. We determined the complex optical indices, permittivities, and permeabilities of all samples by fitting a calculated model to the measured data. The perturbing effects of multiple reflections between setup components were filtered out by inverse fast Fourier transform of the measured signal and subsequent time gating in the obtained time-domain signal. The results presented in this paper show that this processing method reliably deals with all perturbing effects in free-space measurement and therefore can be applied for characterization of various types of magneto-optical samples with different sizes and significant surface roughness as well. Furthermore, it provides convincing indications regarding the usefulness of the different types of hexaferrites for nonreciprocal devices (such as isolators) for submillimeter applications.