Temperature dependence of maximum fluxoid number in an Nb micro-ring

Using a newly developed highly sensitive static force magnetometry, which enables the observation of single fluxoids, we studied the temperature dependence of the maximum fluxoid number in an Nb superconducting micro-ring. For an 100 nm-thick ring with outer and inner radius of 4.0 $\mu$m and 2.0 $\mu$m, respectively, the maximum number ranged from 100 to 40 as the temperature increases from 4 K to 7 K, showing a slope of 2 fluxoids quantum per 0.1 K in a discrete way. Thermal escape of fluxoids and its temperature dependence were analyzed with a theoretical model.