Valence and spin-state transition in cobaltates revisited by x-ray magnetic circular dichroism
Résumé
The compounds (Pr1−ySmy)0.7Ca0.3CoO3 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 Co3+ ions), hereafter referred to as VSST. The present study sheds light on the debated issues of the Co4+ and Co3+ spin states, by combining x-ray absorption spectroscopy (XAS) at Co and Pr edges and x-ray magnetic circular dichroism (XMCD) at Co L2,3 edges. XAS experiments at both L3 and M4,5 Pr edges attest to the appearance of Pr4+ below the VSST at T∗=106K, and allow a precise characterization of the evolution of the Co4+ content as a function of the temperature. XMCD at the Co L2,3 edges at 5 K, and in magnetic field up to 9 T, directly tackles the issue of the Co4+ spin state. It is found that the Co4+ ions are most likely in a low spin state, and experience ferromagnetic interactions at T≪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 Co3+ moments above 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.