Study of locally frozen magnetic field in a high-Tc superconducting ceramic

Abstract

The properties of a locally frozen (in a region of diameter 0.5mm) magnetic field in a YBa2Cu3O7−x slab 0.5μm thick are investigated as a function of the value of the excitation field, the regime of freezing, and the transport current through the sample. The first regime is cooling of the ceramic to 77K in the excitation field with a subsequent turning off of the excitation field, and the second regime is cooling in the Earth’s magnetic field with a subsequent turning on and off of the excitation field. At an excitation field up to 2000A∕m in these regimes two different types of macroscopic current vortex structures, which generate the frozen field, are formed. The local critical field of excitation when the vortex structure is formed in the second regime exceeds the uniform perpendicular critical field of the slab by a factor of 10 and equals 1700A∕m. On the other hand, the vortex structure of the first type can be formed by practically any weak excitation field, including fields smaller than the critical field of the vortex structure of the second type. In a representation of the ceramic as a Josephson medium, physical models of the two types of vortex structures are proposed which correspond most fully to the results of experiments. The displacement of the vortex structure of the first type upon the passage of transport current through the slab, as a result of the action of the Lorentz force on that structure, is registered. This makes it possible to calculate the pinning force Fp and to estimate the value of the viscosity η for the motion of such a vortex structure in the ceramic: Fp=6×10−8N, η=6×10−5kg∕s.