New type of second-order high-Tc SQUID planar gradiometer

We have proposed a novel type of high-Tc superconducting quantum interference device (SQUID) planar gradiometer that could measure the mixed partial derivative of magnetic fields ∂2Bz/∂x∂y. The device consists of two parts, one is the signal pickup antenna formed by four square pickup loops, and another one is a SQUID directly coupled to the common line which separates the antenna loops. The four pickup loops were arranged to a two-dimensional array. The loops in the diagonal direction are connected in series. The total current in the common line in this configuration is directly proportional to the mixed partial derivative of the magnetic field. We have made a device of such design on a STO bicrystal substrate and tested the device in gradient field