Anisotropic properties of the magnetization and flux dynamics of a spherical single crystal of La 1.8Sr 0.2CuO 4− δ

Abstract

Single crystals of La 1.8Sr 0.2CuO 4− δ grown by the traveling solvent floating zone method have been ground to spherical shape for studies of anisotropic supercondcucting properties by SQUID magnetometry. Here, we report on magnetization measurements parallel and perpendicular to the c-axis of one of these crystals. At low enough temperatures and fields the sphere is perfectly shielding (susceptibility −1.5 [SI]) and thus magnetically isotropic. Magnetization vs field experiments reveal a large difference in the hysteresis behavior along the two directions and a surprising enhancement of the critical current density through the CuO planes compared to in-plane at higher fields. Magnetization relaxation curves at low fields and temperatures close to the critical temperature ( T c≈29.5 K) show, in both directions, a distorted logarithmic relaxation and a similar power law field dependence of the relaxation rate at low fields followed by a broad maximum. The relaxation rate shows a field and direction dependent maximum at temperatures below T c.