The relaxation of the magnetization due to thermally activated flux creep in a bulk sample of a nonideal type II superconductor is discussed theoretically. Firstly, an expression for the average volume of a flux bundle characterizing its coherent thermal hopping motion is derived by starting with an equation for the local force balance on pinned fluxoids. Secondly, a thermal hopping rate of flux bundles is derived from the Fokker-Planck equation of pinned flux bundles. These results are used to derive expressions for both the time dependence of the magnetization and the induced electric field due to flux creep. It is shown that the present theory can quantitatively explain the existing observed data of the flux-creep rate, not only in conventional superconductors such as Nb-Ta and Pb-Tl but also in strongly-pinning bulk samples of the high-Tc oxide superconductors such as Y-Ba-Cu-O.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics
- Energy Engineering and Power Technology
- Electrical and Electronic Engineering