Early stages of magnetization relaxation in superconductors

Abstract

Magnetic flux dynamics in type-II superconductors is studied within the model of a viscous nonlinear diffusion of vortices for various sample geometries. We find that time dependence of magnetic moment relaxation after the field is switched off can be accurately approximated by $m(t)\propto 1-\sqrt{t/\tilde{\tau }}$ in the narrow initial time interval and by $m(t)\propto (1+t/\tau {)}^{-1}$ at later times before the flux creep sets in. The characteristic times $\tilde{\tau }$ and $\tau$ are proportional to the viscous drag coefficient $\eta$. Quantitative agreement with available experimental data is obtained for both conventional and high-temperature superconductors with $\eta$ exceeding by many orders of magnitude the Bardeen-Stephen coefficient for free vortices. Huge enhancement of the drag, as well as its exponential temperature dependence, indicates a strong influence of pinning centers on the flux diffusion. Notwithstanding complexity of the vortex motion in the presence of pinning and thermal agitation, we argue that the initial relaxation of magnetization can still be considered as a viscous flux flow with an effective drag coefficient.

arXiv comment5 pages, 7 figures

References in corpus (7)

• The Flux-Line Lattice in Superconductors
• Reliable determination of vortex parameters from measurements of the microwave complex resistivity
• Collective flux creep: beyond the logarithmic solution
• Microwave Surface Impedance Measurements of the Electronic State and Dissipation of Magnetic Vortices in Superconducting Iron-Based LiFeAs Single Crystals
• Local mapping of dissipative vortex motion
• Vortex Dynamics in Ferromagnetic Superconductors: Vortex Clusters, Domain Walls and Enhanced Viscosity
• Study of locally frozen magnetic field in a high-Tc superconducting ceramic

Cited by in corpus (2)

• Low frequency electrodynamics in the mixed state of superconducting NbN and a-MoGe films using two-coil mutual inductance technique
• Averaged effective pinning potential in YBCO single crystals close to Tc