A POSSIBLE VORTEX-DECOUPLING TRANSITION IN BI2SR2CA2CU3O10

Long-time (15-20 h) magnetic-relaxation measurements as a function of temperature are performed in a c-axis oriented bulk Bi2Sr2Ca2Cu3O10 co mpound sample in a magnetic field of H=1 T applied along the c-axis. I t is found that for observation times t greater than or equal to 1 h, M(t) dependences follow the power law M similar to t(-T/Uo), where U-o is the activation energy. A very pronounced peak in the U-o versus T dependence is found at the temperature T-p approximate to 20 K. It is argued that the low-temperature increase of U-0 with temperature is du e to thermally induced vortex depinning, while the reduction of U-o ab ove T-p results from a vortex-lattice decoupling phase transition.