Effect of Eu Doping and Partial Oxygen Isotope Substitution on Magnetic Phase Transitions in (Pr1-yEuy)0.7Ca0.3CoO3 Cobaltites

We study experimentally and theoretically the effect of Eu doping and partial oxygen isotope substitution on the transport and magnetic characteristics and spin-state transitions in (Pr1-yEuy)(0.7)Ca0.3CoO3 cobaltites. The Eu doping level y is chosen in the range of the phase diagram near the crossover between the ferromagnetic and spin-state transitions (0.10 < y < 0.20). We prepared a series of samples with different degrees of enrichment by the heavy oxygen isotope O-18, namely, with 90, 67, 43, 17, and 0% of O-18. Based on the measurements of the ac magnetic susceptibility chi(T) and electrical resistivity rho(T), we analyze the evolution of the sample properties with a change of the Eu and O-18 content. It is demonstrated that the effect of increasing the O-18 content on the system is similar to that of increasing the Eu content. The band structure calculations of the energy gap between t(2g) and e(g) bands including the renormalization of this gap due to the electron-phonon interaction reveals the physical mechanisms underlying this similarity.