Magnetic relaxation in a polycrystalline ${\mathrm{Hg}}_{0.8}$${\mathrm{Tl}}_{0.2}$${\mathrm{Ba}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{8+\mathrm{\delta }}$ [(Hg,Tl)-1223] superconductor was measured in the temperature range 1.85–20 K with different magnetic fields applied during the cooling process. The relaxation curves show a nearly perfect linear ln(t) behavior. The temperature dependence of the normalized relaxation rate, R≡‖d(M/${\mathit{M}}_0$)/dln(t)‖, was studied for two applied magnetic fields. With the lower applied field (${\mathit{H}}_{\mathit{a}}$=3 kOe), a temperature-independent R is found below 2.1 K and explained in terms of quantum tunneling of vortices. As the applied field increases (${\mathit{H}}_{\mathit{a}}$=10 kOe), the transition from the thermal to the quantum regime is not found in the experimentally accessible temperature range above 1.85 K. The field dependence of R at 2.8 K is studied in order to analyze the dimensionality of the flux-line lattice.

• Received 28 March 1995

DOI:https://doi.org/10.1103/PhysRevB.52.1325