Dynamic contribution to the fishtail effect in a twin-free DyBa 2Cu 3O 7−δ single crystal

Abstract

Superconducting current densities j s and dynamic relaxation rates Q  d ln j s/d In(d B e/d t), where d B e/d t is the sweep rate of the external magnetic field B e, were measured as a function of temperature (5 K < T < 65 K) in magnetic fields up to 7 T on a twin-free DyBa 2Cu 3O 7−δ single crystal by means of a high-sensitivity capacitance torque magnetometer. Above 15 K, we observe a “fishtail” effect, i.e. a pronounced minimum in the j s( B e) curve at fields around B e = 1 T. The relaxation rate Q shows an anomalous increase at low fields which is correlated to the minimum in the j s( B e) curve. Both the j s versus B e and Q versus B e data are used as input parameters into the generalized inversion scheme developed by Schnack et al. [Phys. Rev. B 48 (1993) 13178] to calculate the true critical current density j c which is by definition independent of relaxation effects. Interestingly, the j c( B e, T) curves derived in this way do not show a minimum. This points clearly to a dynamic contribution to the fishtail effect. The true critical current density j c( B e, T) decreases weakly with increasing B e over the entire measured temperature and field range, as expected for single-vortex pinning. This indicates that the observed fishtail effect is not caused by a crossover from single-vortex pinning to pinning of flux bundles. The temperature dependence of j c is in good agreement with the predictions of a model based on single-vortex pinning caused by spatial fluctuations in the charge-carrier mean free path.