Vortex pinning and slow creep in high-J_c MgB_2 thin films: a magnetic and transport study

摘要

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 mu m thick MgB_2 by post-annealing in the presence of Mg vapour at 890 deg 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 / with time t was determined unconventionally with the sample fixed in position, by monitoring the SQUID feedback voltage cnJ versus time. The logarithmic decay rate S =-d ln(/)/dln(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 E oc J_c and used to obtain the corresponding creep rate S = I/(n - I). Effective values for n approach and often significantly exceed 100. From these results, we estimate the effective energy C/o for vortex pinning, as a function of magnetizing field H.