Vortex instability phase of Bi2Sr2CaCu2O8+y single crystal

Using single crystals of Bi2Sr2CaCu2O8+y, we performed detailed magnetization measurements in the low temperature region below the vortex solid-glass transition temperature, a region, which has not received any attention well to date. We observed that thermoremnant magnetization showed dramatic, random oscillations in both the real part and the imaginary part of the AC magnetic susceptibility at temperatures below approximately 20 K. The drastic change of AC susceptibility is usually observed in the magnetic phase transition, indicating the presence of a new magnetic state in the region below the vortex solid-glass transition temperature. We also measured the long-time relaxation effect of thermoremnant magnetization below and above the anomalous temperature. From the analysis of decay of thermoremnant magnetization based on weak collective pinning and vortex glass theory, activation energy of vortices has sharp decline at the temperature where the AC susceptibility anomaly occurs. From the result of AC susceptibility and decay of the thermoremnant state, we consider the boundary of vortex motion in Bi2Sr2CaCu2O8+y at 20 K. (c) 2005 Elsevier B.V. All rights reserved.