U. Klein et al., A NUMERICAL AND EXPERIMENTAL INVESTIGATION OF PLANAR ASYMMETRIC SQUIDGRADIOMETER CHARACTERISTICS, Superconductor science and technology, 9(4A), 1996, pp. 124-128
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).