High-sensitivity paramagnetic salt second-sound detector

The authors describe a second sound detector based on the observation of temperature-dependent magnetization changes in paramagnetic materials. The sensitive element consists of a small pill of powdered paramagnetic salt placed in a region of high magnetic field and surrounded by a superconducting pickup loop connected to a SQUID magnetometer. At 10 Hz, the ac sensitivity was estimated to be 3 /times/ 10/sup /minus/9/ K/√Hz increasing to 1 /times/ 10/sup /minus/10/ K/√Hz for dc signals. The device is particularly useful for experiments near the lambda point, which can be approached much more closely than with superleak transducers