Compact noise thermometer for mK-temperatures based on integrated SQUID magnetometers

We report on the development and optimization of very compact noise thermometers for the temperature range of the PLTS-2000. They are based on the detection of the magnetic field fluctuations above the surface of a metal body by means of thin film SQUID magnetometers. The thermally driven Johnson noise currents inside a metal body produce fluctuating magnetic fields, which can be detected by highly sensitive low-Tc dc-SQUID magnetometers or gradiometers placed close to the metal surface. The fundamental fluctuation-dissipation theorem provides a direct relation between temperature and noise currents or field fluctuations to be measured: The power spectral density of the thermal magnetic flux density noise is strictly proportional to the temperature, provided the electrical conductivity does not change. Thus, the temperature of the metal body can be determined from the spectrum of the magnetic flux detected by the SQUID, making up a semi-primary thermometer. Since the spectrum of these fluctuations depends significantly on the configuration of pick-up coil and metal body, it must be optimized to achieve the largest noise signal (power) for a limited chip area. This has been done resulting in thin film miniature multi-loop SQUID gradiometers. We present measurements of the integrated magnetic field fluctuation thermometer characterizing its sensitivity and speed. (orig.)