Nonreciprocal polycrystalline antiferromagnetic devices for millimeter and submillimeter wavelengths

The effective susceptibility tensor for polycrystalline uniaxial antiferromagnetic materials has the Polder form, suggesting the possibility of using these materials for nonreciprocal devices at millimeter and submillimeter wavelengths. Figures of merit for these devices have been calculated from perturbation theory, assuming RF fields uniform throughout the material. A geometrical figure of merit is presented which takes into account the nonuni-fortuity of the RF field within the material, assumed to be an ideal nonreciprocal absorber. Geometrical figures of merit, along with those obtained from perturbation theory, indicate that, with regard to nonreciprocity, polycrystalline antiferromagnetic devices at millimeter and submillimeter wavelengths should perform as well as typical ferrite devices at microwave frequencies. Experimental observations at 2-mm wavelengths in powdered MnTiO3resulted in reverse to forward loss ratios greater than 10 to 1. The reverse attenuation was not sufficient for practical device application, but procedures to increase this are suggested.