New methods for magnetic susceptibility measurements of solid3He at ultralow temperatures

An ac mutual inductance bridge for magnetic susceptibility measurements of solid³He in a metallic cell at ultralow temperatures is described. A null detector for the bridge was composed of an rf-biased superconducting quantum interference device (rf-SQUID) system and a lock-in amplifier that avoided strong fields of the nuclear demagnetization magnets. A design of a sample cell to reduce eddy currents and allow large skin depths associated with the ac measurements while securing high-thermal conductance is also described. The magnetic susceptibility of solid³He with a hexagonal close-packed structure was measured with this system down to 90 µK. The signal-to-noise ratio was 300 at 300 µK and 1 at 90 mK. The quantity of heat evolved in the susceptibility measurements was less than 3 pW.