A Magnetic-Field-Fluctuation Thermometer for the mK Range Based on SQUID-Magnetometry

We are developing a compact and easy-to-use thermometer for the temperature range of about 10 mK to 4 K based on the measurement of magnetic noise above the surface of a metal body which acts as a temperature sensor. The metal body is thermally anchored to the temperature to be measured. The magnetic field fluctuations arise from the thermally agitated motion of electric charges and can be related to the temperature of the metal via Nyquist's relation of the noise in a conductor. We measure the magnetic field fluctuations with a highly sensitive low-Tc dc SQUID magnetometer which is at the same temperature stage as the metal body and in close vicinity to the metal surface. The temperature to be measured is extracted from the spectrum of thermal magnetic noise detected by the magnetometer. The SQUID magnetometer used is a miniature multiloop magnetometer with maximized field sensitivity and low power dissipation. The spectrum of thermal magnetic noise detected by the magnetometer is significantly affected by the configuration of the metal sensor and the magnetometer. We discuss considerations regarding the configuration of an integrated magnetic-field-fluctuation thermometer and present measurements of its sensitivity and speed.