Quantum Interference Device Gradiometer Research Articles

Fabrication and magnetocardiography application of the second-order superconducting quantum interference device gradiometer made from a single-layer of YBa2Cu3O7 film

We have fabricated single-layer, second-order high Tc superconducting quantum interference device (SQUID) gradiometers on SrTiO3 substrates and investigated their noise properties and performance in magnetocardiography. The gradiometer consists of three parallel pickup loops that are directly coupled to a step-edge junction SQUID, in such a way that the coupling polarity of two side loops is opposite to that of the center loop. The overall size of the device is 17.6 mm×6 mm with a baseline of 5.8 mm. The measured gradient noises are 0.45 and 0.84 pT/cm2/√Hz at 1 Hz for the shielded and the unshielded cases, respectively, which correspond to equivalent field noises of 150 and 280 fT/√Hz, respectively. In spite of the short baseline of 5.8 mm, the high common-mode rejection ratio of the gradiometer, 103, allowed us to record a magnetocardiogram of a human subject, which demonstrates the feasibility of the design in biomagnetic studies.

Direct-coupled second-order superconducting quantum interference device gradiometer from single layer of high temperature superconductor

We developed a noble design of the planar-type single-layer second-order superconducting quantum interference device (SQUID) gradiometer, and demonstrated that the gradiometer did not respond to a uniform field or the first-order gradient of the field but responded sensitively to the second-order gradient. The device consisted of three parallel-connected pickup loops, each of which is directly coupled to the step-edge junction SQUID. The entire structure was made from a single layer of YBa2Cu3O7 patterned by photolithography with ion milling technique. Response of the device to the field was tested with three identical wire-wound coils coupled to each loop and balancing was achieved by using a small piece of superconductor placed in the center loop. Measured off balance was about 0.6% for the uniform field and 1.4% for the first-order gradient, which were believed not due to intrinsic imbalance but mostly due to slight difference in alignment of the test coils.