Vortex pinning and slow creep in high-J_c MgB₂ thin films: A magnetic and transport study

We have investigated the pinning of vortices in high-J_c 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 μm 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.