Second-order dc SQUID gradiometer from single layer of high temperature superconducting film

We developed a new design of the planar-type single-layer second-order SQUID gradiometer. The device consisted of 3 parallel-connected pickup loops, each of which is directly coupled to the step-edge junction dc SQUID. The device was made from a single layer of YBa/sub 2/Cu/sub 3/O/sub 7/ film patterned by photolithography with ion milling technique. Response of the device to various field distributions was tested, with 3 identical wire-wound coils coupled to each loop. Balancing was achieved by using a small piece of superconductor placed in the center loop. The gradiometer responded sensitively to the second-order field gradient, but did not respond to a uniform field or the first-order gradient. Measured off-balance was about 0.6% for the uniform field and 1.4% for the first order gradient.