ANISOTROPIC PROPERTIES OF THE ANOMALOUS 2ND PEAK IN THE MAGNETIZATION CURVES AND THE IRREVERSIBILITY FIELD OF YBA2CU3OY (6.6-LESS-THAN-OR-EQUAL-TO-Y-LESS-THAN-OR-EQUAL-TO-6.9) SINGLE-CRYSTALS

Anisotropic properties of the anomalous second peak in the magnetization curves and the irreversibility field have been investigated by magnetic measurement for YBa2Cu3Oy (6.6 less than or equal to gamma less than 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 peak appears is well described by the Ginzburg-Landau effective-mass model. This behavior results from the anisotropic vortex structure dominated 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 the influence of the flux creep on the irreversibility field. The anisotropic parameter gamma is consistently estimated from the angular dependence of both characteristic fields of H-p and H-irr. The gamma value increases with decreasing oxygen content indicating the suppression of the interlayer coupling between CuO2 superconducting layers by reducing oxygen.