Tech Briefs

Cryogenic High-Sensitivity Magnetometer

Sensitivity would be about a million times that of a flux-gate magnetometer.

A proposed magnetometer for use in a cryogenic environment would be sensitive enough to measure a magnetic-flux density as small as a picogauss (10-16 Tesla). In contrast, a typical conventional flux-gate magnetometer cannot measure a magnetic-flux density smaller that about 1 microgauss (10-10 Tesla).

One version of this device, for operation near the low end of the cryogenic temperature range, would include a piece of a paramagnetic material on a platform, the temperature of which would be controlled with a periodic variation. The variation in temperature would be measured by use of a conventional germanium resistance thermometer. A superconducting coil would be wound around the paramagnetic material and coupled to a superconducting quantum interference device (SQUID) magnetometer.

The SQUID magnetometer would be used to measure the change in current in the coil as a result of the change in temperature measured by the germanium resistance thermometer. The ratio between the current change and the temperature change would be computed, then used to infer the ambient magnetic field. This inference would be drawn from a lookup table established by prior calibration measurements performed at the same mean operating temperature.

In an alternative version of this magnetometer, for operation at a temperature near the high end of the cryogenic range, the coil and the SQUID magnetometer would be made from a high-temperature superconductor and the coil would be in the form of a thin film deposited on the same substrate as that of the SQUID. The paramagnetic material would be inserted in a hole at the center of the coil. The temperature of the whole substrate would then be modulated during measurements of the type described above.

Because of the oscillatory temperature excitation, this magnetometer would exhibit very little drift. The highest sensitivity and the lowest noise would be achieved by careful selection of the paramagnetic material and operating near the Curie temperature of that material.

Although the SQUID magnetometer and the superconducting coil must be kept cold, it would be possible to measure the magnetic field of a warm environment. For this purpose, the magnetometer could be kept at the required low temperature in a nonmagnetic Dewar flask, which could be brought to the warm measurement location.

While conventional SQUID magnetometers are routinely used to measure the change in magnetic flux density with similar sensitivity, the proposed implementation allows the full sensitivity of SQUID to be used to measure a small static magnetic flux density. Another implementation method involves flipping the pickup coil of a SQUID to measure the ambient static field. Although such a method had been used before, it involves cumbersome mechanical actuators to flip the coil at cryogenic temperatures and the wire of the coil can work harden and break after repeated flipping. Because of the absence of moving parts in the proposed magnetometer, reliability is improved.

This work was done by Peter Day, Talso Chui, and David Goodstein of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category. NPO-40748