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

TL;DR

This study investigates the magnetic, thermal, and electronic properties of (PrEu)_0.7Ca_0.3CoO_3 cobaltites, revealing how Eu doping and oxygen isotope substitution influence spin-state and magnetic transitions, and constructing a detailed phase diagram.

Contribution

It provides the first comprehensive phase diagram of (PrEu)_0.7Ca_0.3CoO_3 cobaltites, highlighting the isotope effect on spin-state and magnetic phase boundaries, and elucidates the role of Co ions in ferromagnetic ordering.

Findings

Oxygen isotope substitution shifts phase boundaries to lower Eu concentrations.

Strong isotope effect on spin-state transition temperature for y > 0.18.

Ferromagnetism is promoted by Co^{4+} ions favoring intermediate-spin states.

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.