X-ray absorption and emission spectroscopy study of Mn and Co valence and spin states inTbMn1−xCoxO3

The valence and spin state evolution of Mn and Co on $\mathrm{TbM}{\mathrm{n}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{o}}_{x}{\mathrm{O}}_{3}$ series is precisely determined by means of soft and hard x-ray absorption spectroscopy (XAS) and $\mathrm{K}\ensuremath{\beta}$ x-ray emission spectroscopy (XES). Our results show the change from $\mathrm{M}{\mathrm{n}}^{3+}$ to $\mathrm{M}{\mathrm{n}}^{4+}$ both high-spin (HS) together with the evolution from $\mathrm{C}{\mathrm{o}}^{2+}$ HS to $\mathrm{C}{\mathrm{o}}^{3+}$ low-spin (LS) with increasing $x$. In addition, high energy resolution XAS spectra on the K pre-edge region are interpreted in terms of the strong charge transfer and hybridization effects along the series. These results correlate well with the spin values of Mn and Co atoms obtained from the $\mathrm{K}\ensuremath{\beta}$ XES data. In this paper, we determine that Co enters into the transition metal sublattice of $\mathrm{TbMn}{\mathrm{O}}_{3}$ as a divalent ion in HS state, destabilizing the Mn long-range magnetic order since very low doping compositions ($x\ensuremath{\le}0.1$). Samples in the intermediate composition range ($0.4\ensuremath{\le}x\ensuremath{\le}0.6$) adopt the crystal structure of a double perovskite with long-range ferromagnetic ordering which is due to $\mathrm{M}{\mathrm{n}}^{4+}$-O-$\mathrm{C}{\mathrm{o}}^{2+}$ superexchange interactions with both cations in HS configuration. Ferromagnetism vanishes for $x\ensuremath{\ge}0.7$ due to the structural disorder that collapses the double perovskite structure. The spectroscopic techniques reveal the occurrence of $\mathrm{M}{\mathrm{n}}^{4+}$ HS and a fluctuating valence state $\mathrm{C}{\mathrm{o}}^{2+}$ HS/$\mathrm{C}{\mathrm{o}}^{3+}$ LS in this composition range. Disorder and competitive interactions lead to a magnetic glassy behavior in these samples.