Peak effect and vortex phase diagram of YBa2Cu4O8

Magnetization measurements of Y124 single crystals have been carried out up to 16 T by means of the torque magnetometer in an inclined magnetic field from the c-axis by 5degrees. At temperatures near T-c, the peak line seems to start from a critical point (2200 G, 76 K). However, the melting line has been observed by means of the AC susceptibility in fields higher than 2200 G. The peak onset field similar to2200 G is rather independent of temperature at temperatures above 40 K while it increases rapidly at temperatures below 35 K. The peak line always increases as temperature decreases, and it shows anomaly at 25 K. These profiles are different from Y123 and Bi2212, for which the positive temperature derivative or the T-independent peak has been known so far. The nature of the melting transition remains to be clarified while the AC susceptibility is suggestive of the first-order transition. (C) 2003 Elsevier B.V. All rights reserved.