A Magnetic Thermometer for High-Resolution 10 mK Scale Thermometry

An innovative thin-film magnetic thermometer with integrated superconducting quantum interference device (SQUID) readout is described for fast, precision temperature measurements in the 10 mK range. The compact 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 losses that would otherwise 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 in the paramagnet and reduces the inductance of the pickup coil, both of which improve the sensitivity and signal to noise ratio as compared with previous miniature susceptometer designs. The innovative magnetic thermometer design is expected to achive a temperature resolution well below 1 µK/Hz½.