Conventional and torque magnetometry on disc-shaped high-T c superconductor samples : II. A dynamical theory of relaxation and transient phenomena

Abstract

The nonlinear steady-state electrodynamic theory of conventional and torque magnetometry on HTSC discs and thin films with oblique time-dependent external magnetic field H 0( t) recently given by the author is extended to relaxation and transient phenomena, assuming the movement of H 0( t) is stopped or reversed, respectively. It is shown that in the limiting case of discs with large aspect ratio and thickness much smaller than the nonlinear skin depth, short-time flux redistribution effects fail, the irreversible part of the magnetic momentum m irr( t) is strictly axial and described by a model with one degree of freedom only. The relaxation is governed by the conductivity tensor σ, but m irr( t) observed cannot, in general, absolutely support any model regarding the thermally activated vortex motion because m irr( t) becomes strongly modified also due to the complicated superposition of different principal values of σ which may account for different (isotropic and anisotropic) pinning mechanisms, depending upon the arrangement chosen. Transitional effects do not allow one to draw detailed conclusions about σ because of its strong nonlinearity but may support the underlying dynamical model. A striking double zero transition of the torque observed if H 0( t) crosses the disc normal is explained as a combined relaxation and transitional effect.