The structural and magnetic properties of two mixed-valence cobaltites with a formal population of 0.30 Co${}^{4+}$ ions per f.u., (Pr${}_{1-y}$Y${}_y$)${}_{0.7}$Ca${}_{0.3}$CoO${}_3$ ($y=0$ and 0.15), have been studied down to very low temperatures by means of high-resolution neutron diffraction, SQUID magnetometry, and heat-capacity measurements. The results are interpreted within the scenario of the spin-state crossover from a room-temperature mixture of the intermediate-spin Co${}^{3+}$ and low-spin Co${}^{4+}$ (IS/LS) to the LS/LS mixture in the sample ground states. In contrast to the yttrium-free $y=0$ that retains the metallic-like character and exhibits ferromagnetic (FM) ordering below 55 K, the doped system $y=0.15$ undergoes a first-order metal-insulator transition at 132 K, during which not only the crossover to low-spin states but also a partial electron transfer from Pr${}^{3+}$ $4f$ to cobalt 3d states takes place simultaneously. Taking into account the nonmagnetic character of LS Co${}^{3+}$, such a valence shift electronic transition causes a magnetic dilution, formally to 0.12 LS Co${}^{4+}$ or 0.12 $t_{2g}$ hole spins per f.u., which is the reason for an insulating, highly nonuniform magnetic ground state without long-range order. Nevertheless, even in that case there exists a relatively strong molecular field distributed over all the crystal lattice. It is argued that the spontaneous FM order in $y=0$ and the existence of strong FM correlations in $y=0.15$ apparently contradict the single $t_{2g}$ band character of LS/LS phase. The explanation we suggest relies on a model of the defect-induced, itinerant hole-mediated magnetism, where the defects are identified with the magnetic high-spin Co${}^{3+}$ species stabilized near oxygen vacancies.

• Received 17 September 2013

DOI:https://doi.org/10.1103/PhysRevB.88.224412