Valence and spin-state transition in cobaltates revisited by x-ray magnetic circular dichroism

The compounds $({\mathrm{Pr}}_{1\ensuremath{-}y}{\mathrm{Sm}}_y{)}_0.7{\mathrm{Ca}}_0.3{\mathrm{CoO}}₃$ belong to a class of Pr-based cobaltates presenting a unique case of simultaneous valence (charge transfer between Pr and Co ions) and spin-state transition (of the ${\mathrm{Co}}³⁺$ ions), hereafter referred to as VSST. The present study sheds light on the debated issues of the ${\mathrm{Co}}⁴⁺$ and ${\mathrm{Co}}³⁺$ spin states, by combining x-ray absorption spectroscopy (XAS) at Co and Pr edges and x-ray magnetic circular dichroism (XMCD) at Co ${L}_2,3$ edges. XAS experiments at both ${L}₃$ and ${M}_4,5$ Pr edges attest to the appearance of ${\mathrm{Pr}}⁴⁺$ below the VSST at ${\mathrm{T}}^*=106\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, and allow a precise characterization of the evolution of the ${\mathrm{Co}}⁴⁺$ content as a function of the temperature. XMCD at the Co ${L}_2,3$ edges at 5 K, and in magnetic field up to 9 T, directly tackles the issue of the ${\mathrm{Co}}⁴⁺$ spin state. It is found that the ${\mathrm{Co}}⁴⁺$ ions are most likely in a low spin state, and experience ferromagnetic interactions at $\mathrm{T}\ensuremath{\ll}{\mathrm{T}}^*$. On the basis of temperature dependent XMCD at 9 T, the fingerprint of the VSST on the Co moments is isolated, and found to be consistent with bulk magnetization data when accounting for the rare-earth contributions derived from reference samples. These temperature dependent XMCD data are used to characterize the evolutions of the various valence/spin state of the Co species involved in the VSST. It appears that the ${\mathrm{Co}}³⁺$ moments above ${\mathrm{T}}^*$ are not consistent with a pure intermediate spin state, whereas they can be well reproduced by considering a low/high spin mixture. Finally, these XMCD results are compared to those derived from fitting of the XAS spectra recorded in zero field at various temperatures.