Oxygen isotope effect on Neel temperature in various antiferromagnetic cuprates

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