Observations of flux motion in niobium films

A magnetic field trapped in a superconducting sphere was examined at temperatures from 4.6 K to 5.5 K. The sphere was the rotor of a precision gyroscope and was made of fused quartz and coated with a sputtered niobium film. The rotor diameter was 3.8 cm. The film thickness was 2.5 mu m. The tests were carried out at an ambient magnetic field of about 1 mG. Unexpected instability of the trapped field was observed. The experimental results and possible explanations are presented.