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Field dependent creep characteristics in Tl2Ba2CuO6 single crystals

The isothermal magnetization hysteresis loops were measured for Tl2Ba2CuO6 (Tl-2201) single crystals by using a SQUID magnetometer and a micro Hall sensor. In the temperature window from 15 to 30 K, the Hall sensor measurements for M(B) showed a second magnetization peak (SMP) at a peak field, Bsp, and an onset field, Bon. In this temperature region, the second peak appeared as a shoulder in the M(H) plots when measured by using SQUID. At a temperature of 20 K, time relaxation measurements for the time interval of 1–104 s were carried out at different fields by using a Hall sensor. From these relaxation data, for both flux exit and entry, the activation barrier, U0, and the creep exponent, μ, were separately calculated as a function of local field, Bz, by using the weak collective pinning theory. The variation of μ, and U0 as a function B indicates that below the onset of the second peak field, Bon, the creep mechanism is an elastic process, but above it, a gradual transition to plastic creep takes place. At higher fields, μ and U0 reduce sharply. This has been interpreted as a smooth transition to a 2D collective pinning state. These results are compared with that obtained in a double layer Tl compound, Tl2Ba2CaCu2O8 (Tl-2212),as well as other high-Tc materials.