ANISOTROPIC PROPERTIES OF THE ANOMALOUS 2ND PEAK IN THE MAGNETIZATIONCURVES AND THE IRREVERSIBILITY FIELD OF YBA2CU3OY LESS-THAN-OR-EQUAL-TO-Y-LESS-THAN-OR-EQUAL-TO-6.9) SINGLE-CRYSTALS

Anisotropic properties of the anomalous second peak in the magnetizati on curves and the irreversibility field have been investigated by magn etic measurement for YBa2Cu3Oy (6.6 less than or equal to gamma less t han or equal to 6.9) single crystals in magnetic fields up to 55 kOe. The angular dependence of the magnetic field H-p at which the second p eak appears is well described by the Ginzburg-Landau effective-mass mo del. This behavior results from the anisotropic vortex structure domin ated by the effective mass of superconducting electrons. However, the anisotropic behavior of the irreversibility field H-irr is given by an extended expression, H-irr ln H-irr proportional to 1/(cos(2) theta gamma(-2)sin(2) theta)(1/2), which is proposed by taking account of t he influence of the flux creep on the irreversibility field. The aniso tropic parameter gamma is consistently estimated from the angular depe ndence of both characteristic fields of H-p and H-irr. The gamma value increases with decreasing oxygen content indicating the suppression o f the interlayer coupling between CuO2 superconducting layers by reduc ing oxygen.