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A low-cost, high-performance magnetic field sensor for applications such as biomagnetism and nondestructive evaluation can be fabricated by integrating a superconducting quantum interference device (SQUID) and a gradiometer on a single chip. Conventionally, the gradiometric pick-up loop would have a rectangular outline divided symmetrically about the midpoint of its length so that its spatial response was also symmetrical. However, it is also possible to divide the same outline asymmetrically, maintaining the field rejection order of the gradiometer by adding an extra crossover. The spatial response of this arrangement will also be asymmetric, which may be exploited to reduce the effects of the nearby SQUID as a magnetic anomaly or to enhance the sensitivity of the device to magnetic sources at a particular distance. The techniques to calculate the crossover positions are well established. Here we outline how different designs may be evaluated theoretically and report on first experimental results for three simple designs. Several devices have been fabricated using a well established Nb/Al-Al 2 O 3 /Nb trilayer process with high yields. The measurement of the spatial response of an asymmetric first-order gradiometer shows the expected magnetometer characteristics for a magnetic dipole source in the near field and first-order gradiometric characteristics for a far-field source. The balance of the integrated gradiometer appears to be better than one part in 10 4 , and the magnetic field gradient sensitivity has been measured to be 100 fT cm −1 Hz −1/2 .

We describe an eddy current nondestructive evaluation systeq using a low-temperature superconductor magnetic field sensor 1 in an electromagnetically unshielded environment. The sensor lcomprises a niobium dc superconducting quantum interference device (SQUID) integrated with a first-order gradiometric pickup coil to reject spatially uniform interference fields but remain sensitive to flaw induced fields. We demonstrate its use in locatihg and mapping subsurface flaws in a multilayered aluminum structure using an eddy current excitation field at a frequency 06' 70 Hz, with no magnetic shielding around the specimen or cryostat.

Fabrication and magnetocardiography application of the second-order superconducting quantum interference device gradiometer made from a single-layer of YBa 2 Cu 3 O 7 film Axial high-temperature superconducting-quantum-interference-device gradiometer composed of magnetometers with a monolithic feedback and compensation coil

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