Melting transition of vortex matter in YBa2Cu3Oy with various oxygen contents

The effect of oxygen content on the vortex phase diagram of untwinned YBa2C u3Oy (YBCO) single crystals has been studied by magnetization and resistivi ty measurements in high magnetic fields up to 30 T. The cantilever magnetom etry with a capacitance torque meter has been developed for precise magneti zation measurement in high magnetic field. As for fully oxidized and optima lly doped YBCO, the magnetic properties near the first-order vortex lattice melting transition line are discussed in comparison with the vortex phase diagram obtained from the resistivity measurement. (C) 2001 Elsevier Scienc e B.V. All rights reserved.