High-frequency characteristics of dc-SQUIDs with magnetic nanoparticle patterns

We studied high-frequency characteristics of dc-SQUIDs with magnetic-nanoparticle-film patterns, which would be applied to superconductor integrated circuits for reducing widths in passive transmission lines and for increasing coupling factors in transformers. The patterns were constructed by Fe₃O₄ nanoparticles (NPs) with an average diameter of 5 nm. The high-frequency characteristics were obtained by comparison in the resonant characteristics and threshold characteristics of the SQUIDs before and after formation of the patterns. The n-th resonant frequency (f_n) was calculated from the n-th resonant voltage step of the SQUID, giving the LC product at f_n. Here, L is the SQUID loop inductance, and C is the equivalent capacitance consisting of Josephson junction of the SQUID and the NP pattern. In SQUIDs with large NP patterns, the LC product increased from 8% to 260% at frequencies of 47 GHz or higher with the thickness of the pattern although the increase in inductance of the SQUID in threshold characteristics was from 0.9% to 5.5%. On the other hand, the increase in LC product caused by a small pattern was negligible regardless of a 7.6% increase in inductance in threshold characteristics.