Hard Magnetic Dots for Flux Bias of DC SQUIDs

In this paper, we demonstrate that perpendicularly magnetized sub- and micrometer-sized magnetic dots, with a high coercivity and nearly complete remanence, can be used to provide the flux bias for dc superconducting quantum interference devices (SQUIDs). The radius of a submicrometer Co-Pd magnetic dot has been optimized with respect to the surrounding superconducting circuitry to generate the phase shift of approximately pi/2 in it. In this way, it is possible to bias a dc SQUID to the steepest part of its voltage-flux characteristic without using an additional coil and current source. Furthermore, it has been shown that the flux bias can also be achieved using a micrometer-sized Co-Pd dot, upon saturation, by placing a dc SQUID on top of the dot. Given that the underlying principles of flux biasing do not require any particular superconducting pairing scheme (s- or d-wave), these hybrid superconductor/ferromagnet structures can be used as to design a dc SQUID with a complete flux self-biasing