We present the study of magnetization, thermal expansion, specific heat, resistivity, and ac susceptibility of ${\left({\text{Pr}}_{1-y}{\text{Eu}}_y\right)}_{0.7}{\text{Ca}}_{0.3}{\text{CoO}}_3$ 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 (${}^{16}\text{O}$ by ${}^{18}\text{O}$) 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 $\left(y>0.18\right)$ is rather strong but it is much weaker for the transition to a ferromagnetic state $\left(y<0.18\right)$. The ferromagnetic ordering in the low-Eu-doped samples is shown to be promoted by the ${\text{Co}}^{4+}$ ions, which favor the formation of the intermediate-spin state of neighboring ${\text{Co}}^{3+}$ ions.

• Received 22 July 2009

DOI:https://doi.org/10.1103/PhysRevB.81.134427