Effect of oxygen isotope substitution on magnetic structure of (La0.25Pr0.75)0.7Ca0.3MnO3

The oxygen isotope effect on the magnetic structure and charge ordering in (La0.25Pr0.75)0.7Ca0.3MnO3 was studied by neutron powder diffraction. At first it was found that two investigated samples, one of those contained the natural mixture of isotopes (99.7 % 16O, metallic at T ≤ 100 K), and the other one enriched by 18O in 75 % (insulating in all temperature range) are identical at room temperature. At the temperature lowering the sample with 160 undergoes subsequent antiferromagnetic (TAFM = 150 K) and ferromagnetic (TFM = 110 K) transitions, resulting in the noncollinear FM structure, while in the sample with 18O the pure AFM ordering (TAFM = 150 K) is found. The temperature dependencies of the diffraction peak intensities associated with charge ordering are also quite different in the samples with 16O and 18O and correlate with the behaviour of the electrical resistivity and the magnetic structure. No indications of long-range phase segregation at low temperature were found