Effect of oxygen stoichiometry on the out-of-plane anisotropy of YBa₂Cu₃O_7- single crystals near optimal doping

We have performed magnetic measurements on pure detwinned YBa₂Cu₃O_7- single crystals in order to investigate the influence of variations in the oxygen stoichiometry on the out-of-plane anisotropy _ck in the region close to optimal doping (6.90 < 7- < 6.98). The anisotropy is estimated using the anisotropic London model from the period of the commensurability oscillations of the hysteretic magnetization as observed for fields applied parallel to the a-b-planes. For a variation in the oxygen deficiency by as much as 300%, we find that _ck decreases but only slightly (less than 12%) with decreasing oxygen deficiency. The possibility of a temperature-dependent _ck is also discussed.