Readout System for NanoSQUID Sensors Using a SQUID Amplifier

NanoSQUID sensors (Superconducting Quantum Interference Devices with nanoscale dimensions) are regarded as a useful tool for sensitive magnetic measurements and have achieved a low noise level. The nanoSQUIDs comprising nanobridge junctions are fabricated by photolithography followed by focused ion beam patterning. While previously such self-shunted nanobridge junctions have shown hysteretic characteristics at 4.2 K, we propose an approach to increase the operational temperature in order to reduce the critical current of the device. We report the design of a readout system, comprising all necessary components to achieve a low noise readout, which is limited by the nanoSQUID noise. It consists of a variable temperature (5-10 K) liquid helium probe stick, a coil system and a SQUID preamplifier. The orthogonal coil system enables particle magnetization up to 100 mT in parallel to and a flux bias of up to 28 mT perpendicular to the nanoSQUID, to set the working point. The nanoSQUID achieves a low white noise level of 0.23 μΦ₀/√Hz including a readout contribution of 0.10 μΦ₀/√Hz.