Abstract
We here report an investigation of the electronic states in the $R\mathrm{F}{\mathrm{e}}_{2}{\mathrm{O}}_{4}(R=\mathrm{Lu},\phantom{\rule{0.16em}{0ex}}\mathrm{Yb},\phantom{\rule{0.16em}{0ex}}\mathrm{Tm},\phantom{\rule{0.16em}{0ex}}\mathrm{Y})$ mixed-valence ferrites by means of soft x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements together with ab initio theoretical calculations. The presence of ${\mathrm{Fe}}^{+2}$ and ${\mathrm{Fe}}^{+3}$ pure ionic species is discarded in the XAS spectra at the $\mathrm{O}\phantom{\rule{0.16em}{0ex}}K$ edge in both experimental data and simulations based on the multiple scattering theory. Similarly, no trace of $\mathrm{F}{\mathrm{e}}^{+2}/\mathrm{F}{\mathrm{e}}^{+3}$ contributions is detected in the XMCD spectra at the $\mathrm{Fe}\phantom{\rule{0.16em}{0ex}}K$ edge. On the other hand, the XAS and XMCD spectra at the $\mathrm{Fe}\phantom{\rule{0.16em}{0ex}}{L}_{2,3}$ edges can be well described in terms of $\mathrm{F}{\mathrm{e}}^{+2}/\mathrm{F}{\mathrm{e}}^{+3}$ contributions, and are also supported by multiplet calculations. This finding can be interpreted as the existence of a mixture of $3{d}^{5}/3{d}^{6}$ configurations at the Fe atoms. Alternative ferrimagnetic spin orderings based on a trimodal Fe valence distribution are also proposed and discussed. Finally, a possible explanation for the strong dependence of the $\mathrm{Fe}\phantom{\rule{0.16em}{0ex}}{L}_{2,3}$ edges XMCD signal magnitude on both the sample surface preparation and detection method is presented.
Highlights
Mixed-valence transition-metal oxides exhibit remarkable properties such as high temperature superconductivity, colossal magnetoresistance, and multiferroicity, which may give rise to important technological applications [1]
Fe+2/Fe+3 charge ordering (CO) leading to a polar configuration in LuFe2O4 lacks experimental support. These results suggest that the Fe atoms in the mixed-valence RFe2O4 compounds are in a homogeneous mixed-valence state instead of a heterogeneous mixed-valence state between two integer valences
This investigation adds to other recent published papers [10,38,39,40,41,42,43,44] to provide a framework for the physics of the mixed-valence RFe2O4 (R = Lu, Yb, Tm, Y) compounds, which in some aspects can be extended to other mixed-valence oxides
Summary
Mixed-valence transition-metal oxides exhibit remarkable properties such as high temperature superconductivity, colossal magnetoresistance, and multiferroicity, which may give rise to important technological applications [1]. Previous to these recent results, various x-ray magnetic circular dichroism (XMCD) experiments at the Fe L2,3 edges in LuFe2O4 [27,30,38] were interpreted using a spin ordering model in terms of the Fe+2/Fe+3 ionic species in which all Fe+2 as well as 1/3 of the Fe+3 moments are aligned parallel to the applied magnetic field and the remaining 2/3 of the Fe+3 in an antiparallel way In view of these results, a consistent description of the local electronic and magnetic structure of the RFe2O4 compounds in the new framework of homogeneous intermediate valence state for the Fe atoms is mandatory.
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