The fabrication and performance of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-x</sub> SQUID magnetometers

Abstract

To enhance the SQUID`s field sensitivity, it is coupled to a flux transformer, a closed superconducting circuit consisting of a pickup loop, to which a signal is applied, connected in series to an input coil, which is inductively coupled to the SQUID. To fabricate an optimal flux transformer, one must use more than one superconducting thin-film layer, each of which is patterned into narrow strips or wires. Some wires from different layers cross, yet remain electrically isolated, to form crossovers, while in other places there must be superconducting contact between wires from different layers. Together, the superconducting wire, superconducting-superconducting contact and the superconducting crossover constitute a superconducting interconnect or multilayer wiring technology. We discuss the development of an interconnect technology involving the high transition temperature ({Tc}) superconductor YBa{sub 2}Cu{sub 3}O{sub 7-x} (YBCO). Because of the need for epitaxial growth there are limits on materials for the insulating layer separating the YBCO films in multilayer structures, and on deposition and patterning techniques. We discuss the use of pulsed laser deposition in conjunction with patterning by shadow masks and later by photolithography to produce interconnects, multiturn input coils, and flux transformers. We also discuss the performance of SQUID magnetometers, in which amore » flux transformer fabricated on one substrate is coupled to a SQUID fabricated on another. The first magnetometers were hybrids -- high {Tc} transformers coupled to low {Tc} SQUIDS, while later ones had both high {Tc} transformers and SQUIDs and could operate immersed in liquid nitrogen. We report on a magnetometer with a magnetic field sensitivity at lHz of about 2pTHz{minus}{sup 1/2} at 77K, that was successfully used to perform magnetocardiograms on human subjects.« less