A highly sensitive YBCO serial SQUID magnetometer with a flux focuser

We have designed and fabricated a low noise, high T_c superconducting quantum interference device (SQUID) magnetometer with an improved yield. In order to reduce the field noise level of the magnetometer, we increased the voltage modulation depth with a series SQUID array and enhanced the effective area with a flux focuser. Ten washer-type SQUIDs were connected in series, thereby increasing the voltage flux transfer function by a factor of 10. An enhancement in effective area by a factor of 5 was achieved by coupling the 10-SQUID array to a single-layered flux focuser. For the directly coupled SQUID array magnetometer, the yield was improved by selecting the best SQUIDs to have in series in order to reduce the flux noise. The magnetic field sensitivity of 40 fT Hz^-1/2 in the white regime and 100 fT Hz ^-1/2 at 1 Hz is demonstrated by using a single layer of high T _c film. The proposed high yield magnetometers would be suitable for building low noise multi-channel magnetocardiographs.