| A major challenge in the design and operation of high transition temperature (Tc) Superconducting Quantum Interference Devices (SQUIDS) is their potential to exhibit substantially higher levels of noise at low frequency f when exposed to earth's magnetic field. To investigate this problem, we studied the noise of high-Tc SQUIDs, directly coupled magnetometers and multilayer magnetometers in both static and changing magnetic fields. The directly coupled magnetometer consists of a do SQUID connected to a large area pickup loop in parallel. The multilayer magnetometer involves a multiturn flux transformer inductively coupled to a do SQUID on a separate substrate. All the devices are made of thin films of the high-Tc superconductor YBa2Cu3O7−δ, patterned into 4 μm linewidths.; After cooling in a magnetic field, the devices showed no increase in 1/f noise for fields up to threshold values well above the earth's magnetic field. The devices were also cooled in a magnetic field that was subsequently turned off. The 1/f noise of bare SQUIDs was unchanged for fields up to 12 μT. The addition of the flux transformer containing flux dams increased the sensitivity to magnetic field by a factor of 43 while reducing the threshold field only moderately, to 5 μT. This result implies that the multilayer magnetometer can be rotated in the earth's magnetic field through an angle of up to 26° without increasing the low frequency noise.; The results of these studies were incorporated into a 5-channel high-T c magnetocardiography system involving two first-derivative SQUID gradiometers and three reference SQUIDs. Each planar gradiometer consists of a directly coupled SQUID magnetometer inductively coupled to the smaller coil of an asymmetric, two-loop flux transformer. The reference SQUIDs are patterned into 4 μm lines. The outputs of the five channels were subtracted in software to form a second-derivative gradiometer. Its long baseline, 48 mm, and good intrinsic balance, better than 1 part in 100, make the gradiometer an excellent candidate for measuring heart signals in an unshielded environment. Using this system, we obtained adult magnetocardiograms in ambient environments of sufficient quality for clinical applications. |
