Magnetization by rotation: The barnett effect in a superconductor

Abstract

The magnetic field due to the rotation of a superconductor has been measured for a number of hollow and solid tin cylinders of varying purity. The field generated within a hollow cylinder agrees with the predictions of the London theory:B=−(2mc/e)ω. The value of−2mc/e determined from the experiments was 1.17 × 10−7 G-sec, which agrees with the theoretical value of 1.137 × 10−7 G-sec within the experimental error of ±5%. The reversibility of the production of the field was investigated for both hollow and solid cylinders. In the case of the hollow cylinder, effects analogous to flux trapping were observed: If the specimen is cooled belowTc while in rotation, then the “London field” appeared only when the rotation speed was changed. In the case of the solid cylinder, effects analogous to those associated with “frozen in flux” were observed: the London field appeared reversibly only in those specimens sufficiently pure to exhibit a Meissner effect at fields of the order of 10−5 G. In one run a ferrite cylinder was substituted for the superconductor. A Barnett effect was observed; the magnitude of the field generated by rotation was consistent with the permeability measured at audio frequencies by conventional means. A novel heat-transfer effect connected with the rotation of the sample was also observed during the course of measurements on the hollow cylinders. A discussion of the electrodynamics of the rotating superconductor is given in an appendix.