Magnetic relaxation in Tl2Ba2CaCu2O8 single crystals by SQUID magnetometer and micro-Hall sensor

Abstract

Measurements of the isothermal magnetization hysteresis loops $M\left(H\right)\mathrm{}$ and the magnetic relaxation for a ${\mathrm{Tl}}_2{\mathrm{Ba}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ single crystal were carried out by using a superconducting quantum interference device magnetometer and a micro-Hall sensor. In the temperature window from 30 to 60 K, the measurements for $M\left(H\right)\mathrm{}$ show a second anomalous peak at a field $H_{\mathrm{sp}}$ and an onset field of $H_{\mathrm{on}}.$ From these relaxation data measured by two different techniques, the activation barrier $U_0$ and the creep exponent $\mu$ were separately calculated as functions of H based on the weak collective pinning theory. The variation of the normalized creep rate, $S=(|d\ln M/d\ln t|),$ with H is also presented. The results at $T=35\mathrm{}\mathrm{K}$ indicate that at a characteristic field $H^{*}$ lying between $H_{\mathrm{on}}$ and $H_{\mathrm{sp}},$ a minimum is observed in $S\left(H\right)\mathrm{}$ whereas a maximum is observed in $U_0\mathrm{}\left(H\right).$ These analyses also show that below $H^{*},$ the creep phenomenon is controlled by an elastic process. However, above $H_{\mathrm{sp}}$ a negative power law, $U_0\mathrm{}\left(H\right)\mathrm{}\propto H^{\nu }$ with $\nu \sim -0.9,$ is observed suggesting the existence of plastic creep. At a low temperature of $T=20\mathrm{}\mathrm{K},$ where no second peak is observed, both $S\left(H\right)\mathrm{}$ and $U_0\mathrm{}\left(H\right)\mathrm{}$ behave differently. A comparative study of the different creep parameters obtained from the two different measurement techniques is also discussed.