Phase diagram and isotope effect in (PrEu)_0.7Ca_0.3CoO_3 cobaltites exhibiting spin-state transitions

We present the study of magnetization, thermal expansion, specific heat, resistivity, and a.c. susceptibility of (Pr$_{1-y}$Eu$_y$)$_{0.7}$Ca$_{0.3}$CoO$_3$ cobaltites. The measurements were performed on ceramic samples with $y = 0.12 - 0.26$ and $y = 1$. Based on these results, we construct the phase diagram, including magnetic and spin-state transitions. The transition from the low- to intermediate-spin state is observed for the samples with $y > 0.18$, whereas for a lower Eu-doping level, there are no spin-state transitions, but a crossover between the ferromagnetic and paramagnetic states occurs. The effect of oxygen isotope substitution along with Eu doping on the magnetic/spin state is discussed. The oxygen-isotope substitution ($^{16}$O by $^{18}$O) is found to shift both the magnetic and spin-state phase boundaries to lower Eu concentrations. The isotope effect on the spin-state transition temperature ($y > 0.18$) is rather strong, but it is much weaker for the transition to a ferromagnetic state ($y < 0.18$). The ferromagnetic ordering in the low-Eu doped samples is shown to be promoted by the Co$^{4+}$ ions, which favor the formation of the intermediate-spin state of neighboring Co$^{3+}$ ions.Comment: 13 pages, including 11 figures, to be published in Phys. Rev.

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