Highly sensitive and easy-to-use SQUID sensors

We have developed a novel family of low-noise superconducting quantum interference devices (SQUIDs) to cover a wide range of applications. These sensors are robust and easy to use without compromising noise performance. They are optimized for operation with our high-speed. direct-coupled flux-locked loop (FLL) electronics XXF-1. For the readout of cryogenic detectors, series arrays of 16 SQUIDs with less than or similar to 3 nH input inductance were designed which can be cooled down and operated in the Earth's field without magnetic shield. A compact gradiometric design allows the chips to be mounted directly on a Cu block at the cold stage of a mK cryostat without degradation in noise. A current noise level of 9 pA/root Hz or 5 pA/root Hz is achieved at 4.2 K or 300 mK, respectively. Ultra-high-speed operation was demonstrated by using a cold FLL electronics prototype in close proximity to the SQUID array. An extremely high FLL bandwidth of 350 MHz was achieved which outperforms any flux-locked SQUID before by more than an order of magnitude. For applications requiring a larger input inductance of up to 2 mu H, integrated two-stage sensors were developed consisting of a single front-end SQUID with double-transformer coupling read out by a 16-SQUID array. These sensors are very convenient to use as their voltage-flux characteristic is essentially single-SQUID-like. Devices optimized for 4.2 K operation have a coupled energy resolution around 50 times Planck's constant h. Heavily shunted devices for low-temperature operation typically achieve 80 h or 8 h at 4.2 K or 300 mK, respectively. An optional current limiter (Q-spoiler) at the input reduces the input current in pulsed applications like nuclear magnetic resonance or magnetorelaxometry. Integrated rf filters and resistor-capacitor shunts across the input coil result in smooth, well-behaved voltage-flux characteristics even at low temperatures << 4.2 K. For magnetic field sensing applications, integrated miniature multiloop magnetometers were designed with maximized field resolution. For a 3 mm x 3 mm chip size, a noise level of 3.6 fT/root Hz is obtained at 4.2 K.