MAGNETIZATION AND FLUX-PINNING IN YBA2CU3OY SINGLE-CRYSTALS

We have studied the temperature and magnetic field dependence of the m agnetic hysteresis of YBa2Cu3Oy Single crystals with different oxygen contents. Different behaviors in the hysteresis loop were observed bet ween oxygenated and oxygen-deficient crystals. Although the hysteresis loop of the latter exhibits a scaling feature, the second peak in the former collapses by fast relaxation of the magnetization. A small jum p in the M-T curve of the former crystal was found at a certain temper ature T-j just below the irreversibility line. The value of T-j in mag netic field H obeys an expression H proportional to (T-c - T-j)(2). Th is magnetization jump indicates a discontinuous change in vortex densi ty. These results are discussed on the basis of the first-order vortex -lattice melting transition.