Resonant photoemission of single-crystal RBaCo2O5+δ (R=Gd, Dy)

Abstract

Resonant photoemission of single crystals of the double perovskites $\mathrm{Gd}\mathrm{Ba}{\mathrm{Co}}_{2}{\mathrm{O}}_{5+\mathrm{\ensuremath{\delta}}}$ and $\mathrm{Dy}\mathrm{Ba}{\mathrm{Co}}_{2}{\mathrm{O}}_{5+\mathrm{\ensuremath{\delta}}}$ has been carried out at the UK Synchrotron Radiation Source at Daresbury. The resonance onset energy at the Co $3p\ensuremath{\rightarrow}3d$ threshold is used to explore the Co spin state in the double perovskites as a function of temperature. In contrast with the simple perovskites $\mathrm{La}\mathrm{Co}{\mathrm{O}}_{3}$ and $\mathrm{Ho}\mathrm{Co}{\mathrm{O}}_{3}$, an undelayed resonance is observed for $\mathrm{Gd}\mathrm{Ba}{\mathrm{Co}}_{2}{\mathrm{O}}_{5+\mathrm{\ensuremath{\delta}}}$ and $\mathrm{Dy}\mathrm{Ba}{\mathrm{Co}}_{2}{\mathrm{O}}_{5+\mathrm{\ensuremath{\delta}}}$ at temperatures as low as 50 K, consistent with the idea that the Co spin state in the pyramidal sites of the double perovskites does not fluctuate with temperature. The temperature variation of the data suggest that the phase transition observed in $\mathrm{Gd}\mathrm{Ba}{\mathrm{Co}}_{2}{\mathrm{O}}_{5+\mathrm{\ensuremath{\delta}}}$ at around 350 K is not associated with a sudden low spin-high spin switch in the octahedral sites. The giant rare earth $(R)$ $4d\ensuremath{\rightarrow}4f$ resonances are also probed, and are used to identify the $4f$ contributions to the valence band. This shows that the density of states close to the Fermi energy for all materials is of Co $3d∕\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ character, with no $R\phantom{\rule{0.2em}{0ex}}4f$ contribution. Comparison is made with $\mathrm{LSDA}+\mathrm{U}$ calculations. Our experiments indicate that the spin equilibrium in the double perovskites is significantly shifted in favor of higher spin multiplicities compared with materials such as $\mathrm{La}\mathrm{Co}{\mathrm{O}}_{3}$.