Development of a low temperature SQUID gradiometer for magnetic microcalorimeters

We will report on the performance of a first order SQUID gradiometer developed for the readout of the magnetic microcalorimeters. The SQUID is designed to operate at temperatures below 0.1 K with flux noise level lower than 1 μ Φ 0 / Hz in a magnetic field of 3.5 mT at its washers. It has a baseline of 500 μ m and washer size of 50 μ m in diameter. To produce an ambient magnetic field in a persistent current mode, field coils with a superconducting short equipped with a heater as a switch are integrated on the same chip. Care was taken to decrease the heat input generated by shunt resistors and cooling fins were attached to them to minimize the over-heating of the electrons in the shunts. In order not to degrade the performance by the readout circuit, the SQUID gradiometer is connected to and read out through a low noise series SQUID amplifier.