Microwave properties of a high-temperature superconductor and ferromagnetic bilayer structure

Microwave properties for a bilayer structure made of the high-temperature superconducting and the ferromagnetic materials are theoretically investigated. The properties are explored through the effective surface impedance calculated by using the enhanced two- fluid model for high-temperature superconductors together with the transmission line theory. The calculated effective surface impedance will be numerically analyzed as a function of the frequency, the temperature, and the thicknesses of the constituent layers. It is found that, for a thinner superconducting film, the effective surface resistance is a strong function of the frequency, and the effective surface reactance exhibits a peak and a dip in the frequency-domain. In the study of the effect of thickness in ferromagnetic substrate, there is a peak frequency in the surface reactance for a thinner substrate. There is also a threshold thickness for the ferromagnetic substrate such that it behaves like a bulk substrate when its thickness is larger than this threshold value. In the temperature dependence of surface reactance, the peak near critical temperature is shifted to lower temperature and broadened as the film thickness decreases.