Measurements of thermally fluctuating magnetic fields of normal metals at low temperatures by using SQUID magnetometer

The authors measured the thermal noise power spectrum (NPS) from various normal metal rods as a function of temperature by using a SQUID magnetometer. The thermally fluctuating magnetic noise flux from the rod is inductively coupled to the superconducting input coil of the SQUID system. The impedance of a coil wound around a normal metal rod was calculated. By applying the Nyquist theorem to this circuit, the thermal noise was estimated. The observed NPS from normal metal rods agreed very well with their calculation. The NPS from the semi-infinite half space filled with normal metal was measured by using a similar measuring system and compared with the NPS from a rod