Influence of YAlIG characteristics on cryogenic L-band circular performance

Absorption (x") tests are described which allow the selection of an optimum saturation magnetization (4πMs) for cryogenic ferrite circulators operating in the below resonance mode at L-band frequencies. The 1.5 GHz absorption spectra of a series of YAIIG ferrimagnets were measured at 300 K and 18 K as a function of dc magnetic field in a stripline structure which resembles the actual circulator configuration. At 300 K the samples behave as predicted by the Kittel equation. At 18 K the absorption curves are broadened and depend strongly on 4πMsin an unexpected manner. The below resonance x" shows a minimum for an intermediate value of 4πMs(278 gauss at 292 K). Care was taken to ensure that the nonlinear RF threshold, which was measured to be about 1 microwatt, was not exceeded. Several cryogenic circulators using these materials were fabricated to demonstrate device performance at 18 K. In agreement with the x" data, the circulator insertion loss exhibits a strong dependence on the saturation magnetization with a minimum loss occurring at an intermediate 4πMs(290 gauss at 292 K). The increased loss at 4πMsvalues less than 300 gauss may be attributed to a broadened resonance loss, while the increased loss at higher 4πMsvalues can be attributed to low-field loss.