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.