Effect of oxygen stoichiometry on the out-of-plane anisotropy of YBa2Cu3O7-delta single crystals near optimal doping

We have performed magnetic measurements on pure detwinned YBa2Cu3O7-delta s ingle crystals in order to investigate the influence of variations in the o xygen stoichiometry on the out-of-plane anisotropy gamma(ck) in the region close to optimal doping (6.90 < 7-delta < 6.98). The anisotropy is estimate d using the anisotropic London model from the period of the commensurabilit y oscillations of the hysteretic magnetization as observed for fields appli ed parallel to the a-b-planes. For a variation in the oxygen deficiency del ta by as much as 300%, we find that gamma(ck) decreases but only slightly ( less than 12%) with decreasing oxygen deficiency. The possibility of a temp erature-dependent gamma(ck) is also discussed.