FLUCTUATION DIAMAGNETISM AND MASS ANISOTROPY OF YBA2CU3O6+X

We have measured the fluctuation magnetization of large samples of YBa 2CU3O6+x with a variety of oxygen concentrations. Magnetization is mea sured for several isotherms near T-c as a function of applied magnetic field parallel to the c-axis, in fields up to 7 T. The fluctuation ma gnetization data are consistent with the predictions of 3D-XY scaling over the full doping range studied, x = 0.7-0.9. Within the anisotropi c-effective-mass model, scaling of the temperature and field dependenc e of the magnetization can be used to determine the mass anisotropy an d also the separate variations of the in-plane and out-of-plane cohere nce lengths with doping level. We show that the increase of anisotropy with decreasing oxygen content is dominated by changes in xi(ab)(0), With little change in xi(c)(0).