Dependence of the flux-creep activation energy on the magnetization current for a La1.86Sr0.14CuO4 single crystal.

Abstract

Magnetic relaxation and hysteresis studies have been performed for a c-axis-oriented single crystal of ${\mathrm{La}}_{1.86}$${\mathrm{Sr}}_{0.14}$${\mathrm{CuO}}_{4}$. We demonstrate that the effective activation energy for flux creep, ${\mathit{U}}_{\mathrm{eff}}$, is a strongly nonlinear function of the current density J (as given by the irreversible magnetization). For fixed fields of 0.5, 1.0, 2.0, and 3.0 T and temperatures between 4 and 15 K (as consistent with full field penetration of the sample), ${\mathit{U}}_{\mathrm{eff}}$(J) is seen to vary approximately logarithmically with J. A scaling relationship, ${\mathit{U}}_{\mathrm{eff}}$(J)=[-${\mathit{U}}_{1}$g(T)/${\mathit{H}}^{\mathit{n}}$]ln(J/${\mathit{J}}_{\mathit{c}}$), is shown to be obeyed for this crystal with n\ensuremath{\simeq}0.9 (H in T), ${\mathit{U}}_{1}$\ensuremath{\simeq}180${\mathit{k}}_{\mathit{B}}$, and ${\mathit{J}}_{\mathit{c}}$(1 T)\ensuremath{\simeq}3.2\ifmmode\times\else\texttimes\fi{}${10}^{5}$ A/${\mathrm{cm}}^{2}$. Several functional forms of g(T) have been explored and shown to give reasonable fits if monotonically decreasing in temperature over the range where flux creep is observed. We also explore the ramifications of this functional dependence in explaining an observed exponential temperature dependence of the hysterically determined critical current density. Finally, we show that comparison of the temperature dependence of logarithmic relaxation rate A=dM/d ln(t) and the derivative of the magnetization with respect to ln(T) at fixed time, ${\mathit{dM}}_{0}$/d ln(T), can be used to set a scale of the attempt frequency for flux motion consistent with the other measurements.