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

The oxygen isotope effect on the magnetic structure and charge ordering in (La(0.25)Pro(0.75))(0.7)Ca0.3MnO3 was studied by means of neutron powder di ffraction. lit first it was found that the two investigated samples, one co ntaining the natural mixture of isotopes (99.7% O-16, metallic at T less th an or equal to 100 K), and the other one enriched by O-18 in 75% (insulatin g in the whole temperature range), are identical at room temperature. At th e temperature lowering the sample with O-16 undergoes subsequent antiferrom agnetic (T-AFM=150 K) and ferromagnetic (T-FM=110 K) transitions, while in the sample with O-18 pure AFM ordering (T-AFM =150 K) is found. The tempera ture dependencies of the neutron diffraction peak intensities associated wi th charge ordering are also quite different in the samples with O-16 and O- 18 and correlate with the behavior of the electrical resistivity and the ma gnetic structure. The temperature behavior of the intensities of the CO and AFM peaks may indicate the presence of phase segregation of the sample wit h O-16 onto AFM-insulating and FM-metallic phases at low temperature, thoug h no indications of a macroscopic phase segregation were found. [S0163-1829 (99)13025-X].