Self-Biased Y-Junction Circulators Using Lanthanum- and Cobalt-Substituted Strontium Hexaferrites

In this paper, we propose the application of polycrystalline lanthanum- and cobalt-substituted strontium hexaferrites in the realization of self-biased circulators. These materials present a high anisotropy field, dependent on the substitution rate, which makes it possible to reach operating frequencies in the millimeter-wave range. A first demonstrator was successfully designed and realized using a 20% rate of substitution (Sr_0.8La_0.2Fe_11.8Co_0.2O₁₉). This circulator showed insertion losses of 1.79 dB and an isolation level of 28.1 dB at 41.4 GHz without magnets. Performances can be significantly improved by applying a low magnetic field (Happ=2100 Oe). According to the literature, increasing the substitution rate makes it possible to increase the anisotropy field, and thus, the internal field. Consequently, a 30% substituted strontium hexaferrite was tested. It appears that the anisotropy field was not higher in this case. However, magnetic losses are much lower and enabled us to halve insertion losses of the self-biased circulator (0.87 dB at 41 GHz).