Miniature Thin-Film Superconducting Quantum Interference Device Susceptometer

We have developed a thin-film dc SQUID susceptometer for precision measurements of the magnetization of microscopic samples. The miniature SQUID susceptometer, fabricated using a Nb/Al - AlO_x/Nb Josephson junction process on a 2-mm times 3-mm chip, consists of two 40-mum ID pickup loops in a series, "figure-of-eight" gradiometric configuration that forms the SQUID inductance and two magnetizing field coils, one centered on each pickup loop. The magnetizing field coils, which apply equal and parallel magnetic fields to each pickup loop, may be operated in persistent current mode using an on-chip thin-film persistence switch. The sample to be measured is placed inside one of the pickup loops, and the SQUID gradiometer is used to sense the change in magnetization of the sample as the temperature is varied. The SQUID susceptometers can be operated using conventional locked-loop feedback electronics over the temperature range from 0 to around 6 K with a typical white flux noise of 3 muPhi₀/Hz^1/2 (rms) at 4 K. These susceptometers are currently being used to measure the magnetization of Mn- and Fe-doped Pd samples.