Effect of oxygen isotope substitution and crystal micro-structure on magnetic ordering and phase separation in {rm (La_(1-y)Pr_(y))_(0.7)Ca_(0.3)MnO₃}

The crystal and magnetic structures of the specified CMR manganite system have been studied as a function of $y=(0.2-1)$ across the metal-insulator (MI) transition, and of the oxygen mass ($^{16}$O, $^{18}$O). We quantitatively show how the polaronic narrowing of the carrier bandwidth and the crystal lattice micro-strains control the volume fractions of the mesoscopic ferro- and antiferromagnetic clusters. A well-defined dip in the transition temperatures and the suppression of all the types of long range ordering seen near the MI-transition at $y\simeq0.9$ indicate a key role of the quenched disorder for the formation of the long-scale phase separated state.