Superconducting thin-film gradiometer

Abstract

We describe the design, fabrication, and performance of planar thin-film dc SQUID’s and planar gradiometers in which a dc SQUID is incorporated as a null detector. Each gradiometer was fabricated on a planar substrate and measured an off-diagonal component of changes in the magnetic field gradient. The gradiometer with the highest sensitivity had 127×33-mm loops that could be connected in parallel or in series: The sensitivities were 2.1×10−13 and 3.7×10−13 T m−1 Hz−1/2, respectively. The intrinsic balance of the gradiometers was about 100 ppm for fields parallel to their plane, and a balance of about 1 ppm could be achieved for fields perpendicular to their plane. When the series-loop gradiometer was rotated through 360° in the earth’s field, the output returned to its initial value to within an amount corresponding to a balance of 1 ppm. Possible improvements in sensitivity are discussed.

Squid Fundamentals

“…The alternating voltage across the SQUID is coupled to a low-noise preamplifier, usually at room temperature, via either a cooled transformer [32] or a cooled LC series-resonant circuit [31]. The first of these options presents an impedance N2Rd to the preamplifier, and …”

SQUID Concepts and Systems

“…In particular, the Nb input terminals enable one to connect different input circuits in a straightforward way. A typical device has a white noise energy of 5x10- 29 JHz-1 , with a l!f noise energy of perhaps 1 o- 28 lliz-1 at 0.1 Hz.…”

Thin-Film-Based Ultralow Noise SQUID Magnetometer

“…For application in ULF MRI these planar pickup loop may be replaced e.g by an axial-gradiometric thin-film version as for example presented in [9]. The SQUID current sensor is based on a cloverleaf structure with four main washers, as proposed in [10]. They have been fabricated in our cross-type Josephson junction technology [11].…”