Time-logarithmic magnetic-relaxation measurements have been obtained in an epitaxial ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ thin film for the zero-field cooled and remanent magnetization, with low and moderate applied magnetic fields. With low fields (${\mathit{H}}_{\mathit{a}}$=50 Oe), the normalized magnetic relaxation rates, S≡‖d(M/${\mathit{M}}_{\mathit{i}}$)/d lnt‖, are temperature independent below T≃3.4 K, and the transition from the thermal to the quantum regime appears to be quite sharp. With moderate fields (${\mathit{H}}_{\mathit{a}}$=500 Oe), the transition becomes smooth and takes place at temperatures higher than with low fields, T≃7 K. This seems to indicate that the mechanism responsible for low-temperature relaxation would be nondissipative Hall tunneling of two-dimensional vortices, rather than dissipative vortex motion.

• Received 13 June 1994

DOI:https://doi.org/10.1103/PhysRevB.50.9439