Vortex pinning and slow creep in high-Jc MgB2 thin films: a magnetic and transport study

We have investigated the pinning of vortices in high-Jc films of polycrystalline MgB2, by studying the dependence of current density J on electric field E using both magnetic and transport methods. Precursor films of amorphous boron, deposited on sapphire substrates, were converted to 0.6??m thick MgB2 by post-annealing in the presence of Mg vapour at 890??C for 1?h. In magnetic studies, a SQUID magnetometer was used conventionally to determine the induced current density by the Bean model. The decay of J with time t was determined unconventionally with the sample fixed in position, by monitoring the SQUID feedback voltage versus time. The logarithmic decay rate S = ?d ln(J)/d ln(t) was found to be very low in the H?T phase space away from the irreversibility line. Complementary four-probe transport studies of E(J) were analysed as a power law dependence of the form and used to obtain the corresponding creep rate S = 1/(n?1). Effective values for n approach and often significantly exceed 100. From these results, we estimate the effective energy U0 for vortex pinning, as a function of magnetizing field H.