Effect of the 16O → 18O isotopic substitution and a magnetic field on the spin dynamics in the (La0.25Pr0.75)0.7Ca0.3MnO3 manganite according to the 139La NMR data

Abstract139La nuclear magnetic resonance spectra have been recorded and the spin-spin relaxation times in the paramagnetic region in external magnetic fields of 5 and 9.4 T have been measured for two types of manganites: initial (La0.25Pr0.75)0.7Ca0.3Mn16O3 and enriched with the 18O isotope ((La0.25Pr0.75)0.7Ca0.3Mn18O3). The manganite enriched with the heavier oxygen isotope exhibits disappearance of a signal in the charge-ordering region (T < Tco) in an external field of 5 T. This effect is related to the anomalous increase in the spin-spin relaxation rate. With an increase in the external magnetic field to 9.4 T, the difference in the behavior of the relaxation rate for the two samples in the charge-ordering region remains pronounced, although the magnitude of this effect becomes much smaller. The observed giant isotope effect is a bright evidence of the important role of oxygen motion in the formation of long-range magnetic order in the manganites under consideration.