Tantalum Passive Persistence Shunts for On-Chip Current Trapping in Metallic Magnetic Calorimetry

Ultrahigh resolution photon detectors based on metallic magnetic calorimeters (MMCs) employ a weakly magnetized paramagnetic sensor to measure the energy of the absorbed particles. MMCs can require large on-chip magnetizing currents of order ∼100 mA to achieve optimal performance. To minimize noise injected from room-temperature current supplies, it is useful to trap these currents in on-chip persistent superconducting loops. These loops have so far used electrically heated persistent current switches. However, wire count can be reduced and design flexibility increased by using a passive superconducting persistent current switch with a $T_{{\rm{c}}}$ intermediate between $T_{{\rm{c}}}$ of the Nb loop and the operating temperature of the MMC. In addition, it is desirable for the $T_{{\rm{c}}}$ of the switch to be above the regeneration temperature on single-shot adiabatic demagnetization refrigerators (ADRs). We present passive persistent current switch measurements obtained with Ta film grown on a 100 A Nb base layer. We have demonstrated trapping of up to 150 mA with no evidence of flux creep over 20 h, and persistence of 100 mA trapped current through several regeneration cycles of our ADR with a regeneration temperature of ∼2 K.