Critical fields and vortex structure of YBa sub 2 Cu sub 3 O sub 7 minus. delta. derived from reversible magnetization

The magnetic-field and temperature dependence of the Ginzburg-Landau order parameter {l angle}{ital f}{r angle} is derived from magnetization studies for grain-aligned YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} in the region of thermodynamic reversibility, based on the Hao-Clem model. The value of {l angle}{ital f}{r angle} at 5 T and 89 K is still 52.3% of the zero-field value. Fitting the magnetization data to the Hao-Clem theory reveals that {kappa}{sub {ital c}} is a slowly decreasing function of temperature near {ital T}{sub {ital c}}. {ital H}{sub {ital c}}({ital T}) and {ital H}{sub {ital c}2}({ital T}) were determined with high precision along with the {kappa}{sub {ital c}}({ital T}) values. The detailed shape of the order parameter is derived from {kappa}{sub {ital c}}({ital T}) and {ital H}{sub {ital c}}({ital T}) data. Characteristic variables derived from the data are {ital H}{sub {ital c}}(0)=1.18{times}10{sup 4} Oe, {lambda}{sub {ital a}{ital b}}(0)=1.31{times}10{sup 3} A, and {xi}{sub {ital a}{ital b}}(0)=18 A. The BCS fitting of {ital H}{sub {ital c}} reveals that the value of {minus}{ital dH}{sub {ital c}2}/{ital dT} decreases from 2.25 T/K at {ital T}=82 to 1.9 T/K at {ital T}{sub {ital c}} of 92.4 K.