Investigation of Material Effects With Micro-Sized SQUID Sensors

Micro-sized SQUID sensors have been designed and fabricated in conventional Nb/AlO_x/Nb junction technology. Two types are presented here: a microSQUID gradiometer with two small loops, each with a diameter of 3 μm , and an integrated susceptometer having 30 μm or 60 μm circular loops and on-chip field coils. Both sensor types have been read out by a SQUID array, acting as a low-noise preamplifier. The microSQUIDs were operated in a temperature range of 4.2 K to 7 K, whereas the susceptometers are rather intended for operation below 4.2 K. Important SQUID parameters such as flux noise, maximum operation field, and nonlinearity were measured at different temperatures. The best white flux noise level achieved with a microSQUID was 135 nΦ₀/Hz^1/2 at 4.2 K. Micro-sized magnetic beads were investigated with microSQUIDs, whereas susceptometers were used to measure the susceptibility of thin wires up to 100 kHz.