Low-frequency noise of dc SQUID magnetometers having slotted structure along the grain boundary junction

Abstract

In order to operate high-Tc SQUID magnetometers in an environment with no or weak magnetic shielding, we incorporated slots along the wide (3 mm) grain boundary junction in the pickup coil and the flux dam of direct-coupled SQUID magnetometers. It was confirmed that the slots formed along the wide grain boundary junction suppressed vortex motion and prevented a large increase in low-frequency noise. When we applied external fields above a threshold value of the flux dam after zero-field cooling, the flux-locked-loop output fluctuated, and the low-frequency field noise Bn increased but became stable after a time. The relaxation time after field application was longer for higher fields. A slight increase in the low-frequency noise remained, which was attributed to the fluctuation of the flux trapped in the slots having grain boundaries.This paper was presented at the 8th International Superconductive Electronics Conference, Osaka, Japan, 19–22 June 2001.