Evaluation of Circular Complex Permeability in Single-Crystal Yttrium Iron Garnet at Cryogenic Temperatures

The operation of superconducting qubits requires a sensitive readout circuit at cryogenic temperatures, driving the demand for cryogenic non-reciprocal microwave components such as circulators. However, evaluating these components at low temperatures presents significant challenges for companies and institutions without specialized measurement systems. In the development of such cryogenic non-reciprocal components, the temperature dependence of ferrite's magnetic properties is the most critical factor. Therefore, an evaluation technique for accurately assessing these properties at cryogenic temperatures is essential. In this study, we develop a measurement method to characterize low-loss ferrite materials over a temperature range of 300 K to 2 K. The use of the circularly polarized resonance mode (TE11n) enables the direct estimation of circular complex permeability and the determination of key material parameters, including saturation magnetization and damping constant - both essential for assessing the performance of ferrite materials in circulator applications. Without the need for device fabrication, we demonstrate that single-crystal Yttrium Iron Garnet (YIG) can effectively function as a circulator down to 2 K. This approach offers a promising pathway for the development of cryogenic circulators.