Assessment of magnetic materials for use in quasi-optical non-reciprocal devices operating at frequencies above 90 GHz

High-performance quasi-optical non-reciprocal devices are required for use in many current and future measurement systems operating at millimetre-wave/submillimetre-wave frequencies. In such a device, free-space quasi-collimated linearly-polarised signal-beams pass normally through a plane plate of a magnetic material that is uniformly magnetised perpendicularly to its surfaces and is of a thickness that will give a 45-degree Faraday rotation of a linearly polarised signal beam in a single pass. To find materials having acceptable magneto-static and rf properties for use in mm-wave/submm non-reciprocal devices has therefore involved in screening samples of permanent-magnet materials from many suppliers. There is now scope for the development of improved materials for operation over wider millimetre-wave bandwidths, or at higher submm-wave frequencies, because the chemical compositions, and the forming processes, of the presently available hexaferrites were not selected with a view to minimising mm-wave/submm-wave attenuation.