Waveguide-Based Characterization of Magnetically-Biased Soft Ferrites for Nonreciprocal Devices at Sub-THz Frequencies

Ferrites, traditionally used for non-reciprocal microwave devices, have so far mainly been limited to sub- 100 GHz frequencies due to lack of low-loss high-frequency ferrite materials, and due to missing high-frequency characterization data of recently appearing sub-THz soft-magnetic materials. In this study, we investigate the electromagnetic properties of the commercially available soft ferrite TT2-111 in the 69-110 GHz frequency range, with and without a magnetic bias, by mapping a parameterized model to the measurement data by minimizing a multi-variable error function over the whole frequency band. We are using, to the best of our knowledge for the first time in this frequency range for magnetic materials, a waveguide-based characterization method, which, as shown in this paper, is superior to free-space quasi-optical material characterization methods previously used in this frequency range. The material was characterized to a relative permittivity of 12.6, a dielectric loss tangent of 0.006, and a transverse permeability ranging from 0.86 to 0.99 in the investigated frequency band, which asserts the suitability of soft magnetic materials for this frequency range. The parameter-matched simulation model is excellently able to reproduce the behavior of the measured data.