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We report on a combined soft x-ray absorption and magnetic circular dichroism (XMCD) study at the Co-L3,2 edge on the hybrid 3d/5d solid state oxide Sr2Co0.5Ir0.5O4 with K2NiF4 structure. Our data indicate unambiguously a pure high spin state (S=2) for the Co3+ (3d6) ions with a significant unquenched orbital moment Lz/2Sz=0.25 despite the sizable elongation of the CoO6 octahedra. Using quantitative model calculations based on parameters consistent with our spectra, we have investigated the stability of this high spin state with respect to the competing low spin and intermediate spin states. [ABSTRACT FROM AUTHOR]

The coupled valence and spin state transition (VSST) taking place in (Pr0.7Sm0.3)0.7Ca0.3CoO3 was investigated by soft x-ray absorption spectroscopy (XAS) experiments carried out at the Pr-M4,5, Co-L2,3, and O-1s edges. This VSST is found to be composed of a sharp Pr/Co valence and Co spin state transition centered at T* ~ 89.3 K, followed by a smoother Co spin-state evolution at higher temperatures. At T < T*, we found that the praseodymium displays a mixed valence Pr3+/Pr4+ with about 0.13 Pr4+/f.u., while all the Co3+ is in the low-spin (LS) state. At T~T*, the sharp valence transition converts all the Pr4+ to Pr3+ with a corresponding Co3+ to Co4+ compensation. This is accompanied by an equally sharp spin state transition of the Co3+ from the low to an incoherent mixture of low and high-spin (HS) states. An involvement of the intermediate-spin (IS) state can be discarded for the Co3+. While above T* and at high temperatures the system shares rather similar properties as Sr-doped LaCoO3, at low temperatures, it behaves much more like EuCoO3 with its highly stable LS configuration for the Co3+. Apparently, the mechanism responsible for the formation of Pr4+ at low temperatures also helps to stabilize the Co3+ in the LS configuration despite the presence of Co4+ ions. We also found out that that the Co4+ is in an IS state over the entire temperature range investigated in this study (10-290 K). The presence of Co3+ HS and Co4+ IS at elevated temperatures facilitates the conductivity of the material. [ABSTRACT FROM AUTHOR]