A NUMERICAL AND EXPERIMENTAL INVESTIGATION OF PLANAR ASYMMETRIC SQUIDGRADIOMETER CHARACTERISTICS

A low-cost, high-performance magnetic field sensor for applications su ch 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 t he midpoint of its length so that its spatial response was also symmet rical. However, it is also possible to divide the same outline asymmet rically, maintaining the field rejection order of the gradiometer by a dding an extra crossover. The spatial response of this arrangement wil l also be asymmetric, which may be exploited to reduce the effects of the nearby SQUID as a magnetic anomaly or to enhance the sensitivity o f the device to magnetic sources at a particular distance. The techniq ues to calculate the crossover positions are well established. Here we outline how different designs may be evaluated theoretically and repo rt on first experimental results for three simple designs. Several dev ices have been fabricated using a well established Nb/Al-Al2O3/Nb tril ayer process with high yields. The measurement of the spatial response of an asymmetric first-order gradiometer shows the expected magnetome ter 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 pa rt in 10(4), and the magnetic field gradient sensitivity has been meas ured to be 100 fT cm(-1) Hz(-1/2).