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

Abstract

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.

13 pages, including 11 figures, to be published in Phys. Rev. B