Temperature dependence of the effective sensing area of planar high-T-c dc SQUID magnetometers

Due to the temperature dependence of the effective sensing area, temperature fluctuations lead to an apparent magnetic field noise in case of SQUID operation in a background magnetic field. We have studied this temperature dependence for directly coupled and monolithic flux transformer coupled high-T-c dc SQUID magnetometers. In particular the directly coupled dc SQUIDs show a strong temperature dependence, due to the relatively large influence of the London penetration depth, via the kinetic inductance, on the coupling of the signal to the SQUID. For the flux transformer coupled SQUID the temperature dependence is weaker and arises primarily from the temperature dependence of the flux focusing capability of the SQUID washer. Based on the presented measurements, requirements for temperature stabilisation are formulated for magnetocardiography in an unshielded environment, with the earth magnetic field reduced by 40 dB.