AVS 50th International Symposium
    Surface Science Thursday Sessions
       Session SS3-ThM

Invited Paper SS3-ThM3
Resonant Photoemission of Cobaltite Perovskites and Double Perovskites

Thursday, November 6, 2003, 9:00 am, Room 328

Session: Oxide Structure, Growth, and Defects
Presenter: W.R. Flavell, UMIST, UK
Authors: W.R. Flavell, UMIST, UK
A.G. Thomas, UMIST, UK
D.T. Tsoutsou, UMIST, UK
A.K. Mallick, UMIST, UK
M. North, UMIST, UK
E.A. Seddon, CLRC Daresbury Lab, UK
C. Cacho, CLRC Daresbury Lab, UK
A.E.R. Malins, CLRC Daresbury Lab, UK
R. Stockbauer, Louisiana State U.
R.L. Kurtz, Louisiana State U.
P.T. Sprunger, Louisiana State U.
S.N. Barilo, Belarus Academy of Sciences
S.V. Shiryaev, Belarus Academy of Sciences
G.L. Bychkov, Belarus Academy of Sciences
Correspondent: Click to Email
Double perovskites based on Co(III), e.g. GdBaCo@sub2@O@sub5.5@, show GMR behaviour and a range of poorly understood spin-state transitions. Whereas the Co(III) ion occupies only octahedral sites in perovskites such as LaCoO@sub 3@, in double perovksites, Co is present in two environments, octahedral and pyramidal. In order to investigate the electronic structure of these oxides, resonant photoemission of single crystals of the double perovskites GdBaCo@sub 2@O@sub 5.5@ and DyBaCo@sub 2@O@sub 5.5@ and of the perovksite HoCoO@sub 3@ has been carried out at the UK Synchrotron Radiation Source. Co 3p->3d resonance photoemission has been shown to be a powerful diagnostic of the LS (low spin) state of d@super 6@ Co(III) in cobaltites. The resonance onset position of features associated with the LS state lies typically around 2.5 eV higher than those associated with Co (III) in HS (high spin) or IS (intermediate spin) states. This is thought to be because the t@sub 2g@ states are full in the LS state so the Co 3p -> 3d transitions are delayed until the e@sub g@ states, which lie ~2-3 eV higher, can be occupied. This diagnostic test is used to explore the Co spin fluctuations in double perovskites with temperature. In contrast with previous data for LaCoO@sub 3@, we see no delayed resonance for GdBaCo@sub 2@O@sub 5.5@ or DyBaCo@sub 2@O@sub 5.5@ at temperatures as low as 50 K, consistent with the idea that the Co spin in the pyramidal sites does not fluctuate with temperature. However, a delayed resonance is observed at temperatures up to 400 K in HoCoO@sub 3@. This resonance delay disappears as the temperature is raised, as does a low binding energy feature of the valence band associated with LS Co(III). These observations are consistent with a LS to IS transition in octahedral Co(III) in HoCoO@sub 3@. The giant RE 4d->4f resonances are also probed, and used to identify the 4f DOS. We show that the DOS closest to the Fermi energy for all materials is of TM 3d/O2p character, with no RE 4f contribution. Comparison is made with recent LDA+U calculations.