Application of single-crystal scandium substituted barium hexaferrite for monolithic millimeter-wavelength circulators

Single-crystal scandium-substituted barium hexaferrite crystals have been characterized and used in the fabrication of miniature monolithic circulators on silicon-glass wafers. The single crystal has an anisotropy field of 8700 Oe, saturation magnetization of 3900 G, and ferromagnetic resonance linewidth of 100 Oe. Microstrip impedance transformers were fabricated on the surrounding glass substrate to facilitate the testing and make fabrication compatible with semiconductor processing. Through-reflect-line (TRL) was used to calibrate the insertion loss and reflection due to the exponentially tapered microstrip transformer. The insertion losses are reasonably low for both unsaturated and saturated bias magnetic field conditions. Operation of the circulator has been measured at Ka-band frequencies both above and below the ferromagnetic resonance frequency.