Magneto-Optical Observation of the Magnetic Flux Structure in Superconducting Niobium

Abstract

The magnetic flux structure in superconducting niobium has been studied using the high specific Faraday rotation in thin films of a mixture of EuS and EuF2 at low temperatures. The magneto-optical detection system had a resolution of about 20 μm. The specimens were niobium foils 18-μm-thick and vacuum-deposited niobium films. In the foils, after raising the magnetic field and returning it to zero, the trapped flux structure consists of large domains of flux with opposite direction in the specimen. The arrangement of the domains is closely related to the sample shape. As in a ferromagnetic body, the domains are established to reduce the total free energy of the system. The Nb films show a very imhomogeneous magnetic flux distribution consisting of regions with very large local magnetic fields.