Spin-state crossover and low-temperature magnetic state in yttrium-doped Pr_0.7Ca_0.3CoO₃

The structural and magnetic properties of two mixed-valence cobaltites with a formal population of 0.30 Co4⁺ ions per f.u., (Pr _1-yY_y)_0.7Ca_0.3CoO₃ (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 Co3 ⁺ and low-spin Co4⁺ (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 Pr3⁺ 4f to cobalt 3d states takes place simultaneously. Taking into account the nonmagnetic character of LS Co3 ⁺, such a valence shift electronic transition causes a magnetic dilution, formally to 0.12 LS Co4⁺ 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 Co3⁺ species stabilized near oxygen vacancies.