Phase diagram of the (Pr1−ySmy)0.7Ca0.3CoO3 system: Valence and spin state transition versus ferromagnetism

Abstract

The valence and spin-state transition is a very specific transition found in some cobalt oxides which involves a coupled modification in the Co valence and the ${\mathrm{Co}}^{3+}$ spin state. In the phase diagram of these compounds, one can thus expect a competition of this transition with the ferromagnetism associated with ${\mathrm{Co}}^{4+}$ low spin/${\mathrm{Co}}^{3+}$ high spin double exchange. Here, we study the phase diagram of ${({\mathrm{Pr}}_{1\ensuremath{-}y}{\mathrm{Sm}}_{y})}_{0.7}{\mathrm{Ca}}_{3}\mathrm{Co}{\mathrm{O}}_{3}$ and ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Co}{\mathrm{O}}_{3}$ to shed some light on the competition between these two phenomena. To this end, we combine results obtained with a wide variety of techniques: magnetization, heat capacity, resistivity, neutron diffraction, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism. The origin of the various transition lines in the phase diagram of ${({\mathrm{Pr}}_{1\ensuremath{-}y}{\mathrm{Sm}}_{y})}_{0.7}{\mathrm{Ca}}_{3}\mathrm{Co}{\mathrm{O}}_{3}$ as well as that of the critical value ${y}_{\mathrm{crit}}\ensuremath{\sim}0.18$ are discussed. Particular attention is paid to the nature of the ground state magnetism which is an issue strongly debated so far in this type of cobaltate.