Abstract
Flux pinning and creep have been studied in textured composite Bi2Sr2CaCu2Ox superconductors containing MgO whiskers. The critical current density increases with MgO volume fraction and the irreversibility line is shifted to higher temperatures compared to a Bi2Sr2CaCu2Ox single crystal, indicating that the MgO whiskers provide pinning sites due to their large surface area and the sharp interface with the superconducting matrix. These pinning centers coexist with weak atomic-scale defects and alter the characteristic flux dynamics in the composite material. Magnetic-relaxation measurements and hysteresis-loop measurements were combined to construct the E-J characteristics over six orders of magnitude. The characteristics show a pronounced kink at low current densities which is not in agreement with the Anderson-Kim theory of flux creep. The characteristic flux dynamics in a system with two distinct types of pinning centre is discussed for this composite material and it is shown that by considering a model system the E-J characteristics could be interpreted by an extended Anderson-Kim flux-creep theory without the need to incorporate collective-creep or phase-transition concepts.
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