Off-diagonal 2nd-order SQUID gradiometer made from a single layer of high Tc superconductor film

We developed a transverse second-order SQUID gradiometer from a single layer of high T_c film, which can measure the off-diagonal element d²B_z/dxdy of the second-order field gradient. In this design, four-way ´clover-leaf´ pick-up loops are coupled directly to a 4-junction dc SQUID in such a way that the coupling polarity of the two diagonal loops is opposite to that of the other two loops. The pickup loops are intrinsically balanced for both uniform field and the 1st-order gradient. We made the device by using a pulsed-laser deposition and Ar ion-milling with photolithography. The entire structure was made on a single crystalline SrTiO₃ substrate. The gradiometer responded sensitively to the 2nd-order gradient. Induced field focused on the SQUID loop by the screening current flowing along the device perimeter is believed the major off-balancing factor, which can be balanced by adding a closed superconducting loop just outside the device perimeter or merging facing sides of neighboring loops.