Changes in the magnetic structure of (La0.25Pr0.75)(0.7) Ca0.3MnO3 upon the isotopic substitution of O-18 for O-16

A giant isotope effect, wherein the low-temperature metallic state is repla ced by an insulator state, was recently observed when O-18 was substituted for O-16 in (La0.25Pr0.75)(0.7)Ca0.3MnO3 [N. A. Babushkina, L. M. Belova, O . Yu. Gorbenko et al., Nature (London) 391, 159 (1998)]. In the present wor k, the temperature evolution of the magnetic structure of two samples of th is compound is studied by neutron diffraction. One sample contained a natur al mixture of oxygen isotopes, 99.7% O-16, while the other was enriched to 75% with the isotope O-18. It is established that the samples are crystallo graphically identical at room temperature. As the temperature decreases, th e sample with O-16 undergoes successive antiferromagnetic (T-AFM = 150 K) a nd ferromagnetic (T-FM = 110 K) transitions, which lead to the establishmen t of a noncollinear ferromagnetic structure, while a purely antiferromagnet ic (T-AFM = 150 K) order arises in the sample with O-18. The temperature de pendences of the intensities of the diffraction peaks associated with the c harge ordering differ substantially in samples with O-16 and O-18, and they correlate with the behavior of their electric resistance and magnetic stru cture. (C) 1999 American Institute of Physics. [S0021-3640(99)01001-4].