Flux Dynamics and Time Dependent Effects in Superconducting MgB2

We have investigated the time dependent flux motion in a polycrystalline superconducting MgB₂ sample by means of current-voltage (I?CV curves) and transport relaxation (V?Ct curves) measurements. The transport measurements were carried out as functions of temperature (T), transport current (I), and applied magnetic field (H). The time effects in the I?CV curves were investigated with the help of the sweep rate of transport current (dI/dt). The I?CV curves exhibit nearly reversible behavior for both slow and high current sweep rates (dI/dt) upon cycling the transport current. It was observed that the evolution of the I?CV curves is nearly independent of dI/dt. The small instabilities, voltage jump, and drops appearing at low dissipation levels were interpreted as a kind of plastic flow of the vortices evolving in the form of stripe or cluster in different sizes and local fluctuations in the superconducting order parameter. It was shown that the experimental I?CV curves are in good agreement with a power law behavior, V(I)??I ⁿ . The pinning potential U ₀ extracted from the fitting procedure is found to be approximately independent of dI/dt. In addition to the standard procedure, a reverse procedure was employed to study how the transport current (or associated vortices) penetrates from the surface into the sample. It was found that there is no marked difference between the I?CV curves obtained in the standard and reverse procedures. These results suggest that, in polycrystalline MgB₂, the weak links between grains (standard procedure) and surface weak links (reverse procedure) do not have a considerable effect on the evolution of the I?CV curves and also on the other transport measurements. In order to understand better the flux dynamics, the experimental results for polycrystalline MgB₂ sample were compared to previous similar studies on superconducting Y₁Ba₂Cu₃O_7?x (YBCO) and Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O _x (BSCCO) polycrystalline samples.