SQUID-gradiometers for arrays of integrated low temperature magnetic micro-calorimeters

We designed first-order integrated planar SQUID gradiometers especially for readout of signals from magnetic micro-calorimeters used for the detection of single x-ray quanta. These SQUIDs must operate in magnetic fields up to 5 mT and at temperatures down to 10 mK. Various new designs of eight-pixel gradiometer SQUID arrays on one chip were developed. The dimensions of the gradiometers are adapted to the magnetic thermometer of the micro-calorimeter. The intrinsic noise of the SQUID gradiometers measured at 4.2 K is lower than 2.5μΦ₀/Hz¹2/. They withstand external magnetic fields up to 7 mT without degradation of their performance. A new concept for thermal contact of the detector to the environment based on normal-conducting metallic wiring on the chip is introduced. Field coils with a persistent current switch were integrated on chip to produce the necessary magnetizing field. Appropriate changes in niobium technology compared to our previous work were introduced to achieve necessary critical current of the coil. Thermal coupling between the switch heater and the superconducting short was optimized. A persistent current of up to 80 mA can be injected in the field coil using a heater power of 4 mW. This current corresponds to magnetizing field of 6 mT at the paramagnetic sensor. During the heat pulse the SQUID remains superconducting.