Magnetism in SQUIDs at Millikelvin Temperatures

TL;DR

This study investigates the temperature dependence of flux in dc SQUIDs at millikelvin temperatures, revealing a 1/T flux increase linked to surface spins and trapped vortices, shedding light on flux noise origins.

Contribution

It provides new experimental evidence connecting flux noise in SQUIDs to surface spin polarization and vortex trapping effects at millikelvin temperatures.

Findings

Flux increases as 1/T at lower temperatures.

Flux change correlates with trapped vortex density.

Evidence of spin-glass freezing at low temperatures.

Abstract

We have characterized the temperature dependence of the flux threading dc SQUIDs cooled to millikelvin temperatures. The flux increases as 1/T as temperature is lowered; moreover, the flux change is proportional to the density of trapped vortices. The data is compatible with the thermal polarization of surface spins in the trapped fields of the vortices. In the absence of trapped flux, we observe evidence of spin-glass freezing at low temperature. These results suggest an explanation for the "universal" 1/f flux noise in SQUIDs and superconducting qubits.