Effective flux-creep activation energy and anisotropy in a Bi2Sr2CaCu2Ox single crystal

Abstract

Magnetic relaxation studies have been performed on a Bi2Sr2CaCu2Op single crystal for field orientations parallel and perpendicular to the c-axis. The current density dependence of the effective flux-creep activation energy is derived from the flux-creep data through an expansion of the effective activation energy U about a magnetization current Jo. Although the current densities are much lower in the H∥ab orientation, the overall U(J) behaviors are similarly logarithmic for both field orientations. We discuss the nature of the effective activation energy and the so-called “apparent activation energy”, Uo, contrasting them to the actual activation energy, Up, and our data is used to illustrate problems pith the definition of Uo. A value ≈ 764 is found for the ratio of the critical vortex current densities in the tpo field orientations, {Jc}ab{Jc}c.