An investigation was made of 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 B, deposited on sapphire substrates, were converted to 0.6µm thick MgB2 by post-annealing in the presence of Mg vapour at 890C for 1h. 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 ( J) versus time. The logarithmic decay rate S = -dln[J]/dln[t] was found to be very low in the H–T phase space away from the irreversibility line. Complementary 4-probe transport studies of E(J) were analyzed as a power-law dependence of the form, E  Jn, and used to obtain the corresponding creep rate S = 1/(n-1). Effective values for n approach and often significantly exceeded 100. From these results, the effective energy U0 for vortex pinning was estimated as a function of the magnetizing field H.

Vortex Pinning and Slow Creep in High-JC MgB2 Thin Films - a Magnetic and Transport Study. J.R.Thompson, K.D.Sorge, C.Cantoni, H.R.Kerchner, D.K.Christen, M.Paranthaman: Superconductor Science and Technology, 2005, 18, 970-6