Operation of a high-T$_{c}$ SQUID gradiometer with a two-stage Joule-Thomson micro-cooler

Practical applications of high-T$_{c}$ SQUIDs require cheap, simple in operation, and cryogen-free cooling. Mechanical cryo-coolers are generally not suitable for operation with SQUIDs due to their inherent magnetic and vibrational noise. In this work, we utilized a Joule-Thomson microfluidic cooling system to operate our high-T$_{c}$ SQUIDs [1]. The micro-cooler system is based on a commercial desktop CryoLab unit from DEMCON kryoz [2]. It contains a two-stage MEMS micro-cooler with a base temperature of 75 K, gross cooling power of 75 mW@80 K, and temperature stability ± 50 mK.
Our high-TC dc SQUID gradiometers were fabricated from YBa$_{2}$Cu$_{3}$O$_{7-x}$ thin films grown by pulsed laser deposition on 10 mm × 10 mm SrTiO$_{3}$ bicrystal substrates with 24° misorientation angle. The SQUID chip was glued onto a 0.3 mm thick silicon wafer chip carrier that was attached to the second stage of the cold head. The vacuum housing of the cold stage was made from non-magnetic material (polyethylene terephthalate, PET) and evacuated to a base pressure below 2x10$^{-3}$ mbar. The vacuum chamber features a 0.3 mm thick sapphire window that is placed above the sensor/cold stage. ... mehrWe demonstrated that the equivalent magnetic flux noise of the high-T$_{c}$ SQUID gradiometer is largely unaffected by the micro-cooler setup. The cut-off frequency of the 1/f noise in our SQUID measured on the micro-cooler was around 10 Hz. This indicates that the micro-cooler does not introduce significant magnetic fields in the vicinity of the cold stage. We thus demonstrate that such a microfluidic cooling system is a promising technology for cooling of high-T$_{c}$ SQUIDs in practical applications.
We also used the micro-cooler system to build a prototype a magnetic ac susceptibility (ACS) system for detection of specific binding reactions between DNA target molecules and functionalized magnetic nanoparticles (fMNP) in liquid solution. The detection principle relies on changes in Brownian rotation dynamics of fMNPs. We present the results of experiments with various concentrations of magnetic nanoparticles and discuss further development of the portable magnetic bioassay system for detection of influenza virus using oligonucleotide-tagged magnetic nanoparticles with sub-picomolar sensitivity.

[1] A. Kalabukhov et al., Supercond. Sci. Technol. 29 095014 (2016).
[2] http://kryoz.nl/portfolio-item/cryolab-msg/