OXYGEN-ISOTOPE EFFECT ON NEEL TEMPERATURE IN VARIOUS ANTIFERROMAGNETIC CUPRATES

Oxygen isotope effect on Neel temperature (T(N)) has been studied in t he antiferromapetic cuprates La2CuO4, Gd2CuO4, Gd1.9Ce0.1CuO4, and CuO . For stoichiometric La2CuO4 with T(N) appproximately 310 K, the Neel temperature of the O-18 samples is approximately 1.8 K lower than the O-16 samples. For oxygen-annealed La2CuO4 samples (in 0.10 and 0.15 ba r oxygen partial pressures), the Neel temperatures of the O-18 samples are approximately 3.3-4.7 K lower than the O-16 samples. For La2CuO4 annealed in 0.6 bar oxygen, the Neel temperatures of the O-16 and O-18 samples are nearly equal due to segregation into oxygen-rich and oxyg en-poor phases. For the other antiferromagnetic cuprates (which lack a pical oxygen), we find a negligible oxygen isotope effect on T(N). We suggest that the observed oxygen isotope effect on T(N) of La2CuO4 ori ginates from the oxygen-mass dependences of J (in-plane antiferromagne tic exchange energy) due to the nonadiabatic motion of apical oxygen.