Argon annealing of the oxygen-isotope-exchanged manganite La0.8Ca0.2MnO3+y

We have resolved a controversial issue concerning the oxygen-isotope shift of the ferromagnetic transition temperature T-C in the manganite La0.8Ca0.2MnO3+y. We show that the giant oxygen-isotope shift of T-C observed in the normal oxygen-isotope exchanged samples is indeed intrinsic, while a much smaller shift observed in the argon annealed samples is an artifact. The argon annealing causes the O-18 sample to partially exchange back to the O-16 isotope due to a small O-16(2) contamination in the Ar gas. Such a contamination is commonly caused by the oxygen outgas that is trapped in the tubes, connectors, and valves. The present results thus umambiguously demonstrate that the observed large oxygen isotope effect is an intrinsic property of manganites, and places an important constraint on the basic physics of these materials.