  An influence of the 16O-18O isotope substitutions on magnetic state of
perovskite-type manganite (La0.25Pr0.75)0.7Ca0.3MnO3 is studied by 55Mn NMR.
Successive cycling with an isochronal exposure at different magnetic fields up
to H=8T is used to study the field-induced transition from antiferromagnetic
insulating (AFI) state to the ferromagnetic metal (FMM) one in the 18O-enriched
sample. After exposure at H>H_{cr} \sim 5.3T the NMR spectrum of the 18O-sample
evidences for magnetic phase separation (PS) resulted in the coexisting AFI and
FMM domains. Further increase of exposing field leads to a progressive growth
of the FMM phase at the expense of AFI domains.
  Its relative fraction can be controlled by external magnetic field and the
resulting magnetic structure in the PS region is discussed. Anomalous
T-dependence of the 55Mn nuclear spin-lattice relaxation rate is revealed in
the FMM state of both 16O-and 18O-enriched samples. The manifestation of the Pr
magnetic ordering at T \sim 40K is considered.
