Ultrasensitive Broad Band SQUID Microsusceptometer for Magnetic Measurements at Very Low Temperatures

In this work we report the development and calibration of an ultrasensitive SQUID susceptometer capable of operating over an extremely wide frequency range (0.001 Hz-1 MHz) at extremely low temperatures (13 mK). Starting with 2-stage SQUID sensors fabricated at PTB-Berlin, an integrated susceptometer with experimental spin sensitivity of 10⁴ Bohr magneton/Hz^1/2 is obtained by rerouting some SQUID input circuit connections. Modification of the chips is carried out using Focused Ion Beam Induced Deposition (FIBID) of amorphous W, using W(CO)₆ as precursor gas. We have demonstrated that superconducting connections between W FIBID lines and Nb films can be fabricated and that they operate at 4.2 K, providing a powerful technique for fixing errors in device designs, repairing damaged circuits, or modifying existing ones. The microsusceptometer offers new possibilities in the study of thin films or even monolayers of nanosized magnets, and has potential applications in diverse fields such as quantum computing, high-density information storage or on-chip magnetic refrigeration.