A thin-film magnetic thermometer with integrated, superconducting quantum interference device (SQUID) readout has been designed for fast, precision temperature measurements in the 10-mK range. The compact magnetic thermometer consists of a miniature DC SQUID susceptometer with a dilute paramagnetic alloy deposited in one of the two series-configured, gradiometric SQUID pickup loops that form the SQUID inductance. Directly sensing the magnetic signal with the SQUID eliminates coupling losses that occur by transformer-coupling the signal to a remotely located SQUID, usually operating at a higher temperature, and consequently, with a higher noise floor.

In addition, a novel superconducting flux concentrator deposited on top of the paramagnetic alloy homogenizes and concentrates the magnetic flux density to the paramagnet, and strongly suppresses the inductance of the pickup coil, both of which improve the sensitivity and signal-to-noise ratio.

The paramagnetic alloy is a noble metal with a dilute magnetic dopant concentration. Because the thermometric element is a good thermal conductor, and the measurement is magnetic rather than electrical, robust metal-to-metal thermal links from the thermometric element to the exterior thermometer housing can be used. This is a key advantage for thermometry in the 10-mK range, where thermal contact becomes increasingly problematic. The metallic paramagnet is also expected to offer excellent long-term stability with repeated thermal cycling. For typical thermometry applications, an AC readout mode will be used. Specifically, a low-frequency (≈20 Hz) excitation current applies a small sinusoidal magnetizing field via field coils that are integral with the device and concentric with the SQUID pickup loops. The resulting sinusoidal magnetization of the paramagnetic sample is measured with the SQUID, and the temperature is deduced from the ratio of the SQUID signal to the excitation current. Because the full magnetization of the sample is measured on every cycle, flux jumps have no effect on the calibration. Further, lock-in detection techniques strongly reduce measurement noise.

Projected performance specification is a temperature resolution of better than 1 K/Hz in the 10-mK range. Peripheral equipment required consists of flux-locked loop electronics to read out the SQUID. An existing commercial version will be modified to include the additional functionality required to operate the thermometer.

This work was done by Stephen Boyd of University of New Mexico and Robin Cantor of Star Cryoelectronics for Goddard Space Flight Center. GSC-16133-1

NASA Tech Briefs Magazine

This article first appeared in the January, 2015 issue of NASA Tech Briefs Magazine.

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