Oxide electronics

Abstract:

We report resonant x-ray scattering measurements on a single crystal of the orbitally degenerate triangular metallic antiferromagnet 2H-AgNiO2 to probe the spontaneous transition to a triple-cell superstructure at temperatures below T-S = 365 K. We observe a strong resonant enhancement of the supercell reflections through the Ni K edge. The empirically extracted K-edge shift between the crystallographically distinct Ni sites of 2.5(3) eV is much larger than the value expected from the shift in final states, and implies a core-level shift of similar to 1 eV, thus providing direct evidence for the onset of spontaneous honeycomb charge order in the triangular Ni layers. We also provide band-structure calculations that explain quantitatively the observed edge shifts in terms of changes in the Ni electronic energy levels due to charge order and hybridization with the surrounding oxygens.

Abstract:

Two members of the isostructural R2CoGa8 intermetallic series, Er2CoGa8 and Tm2CoGa8, have been studied by powder neutron diffraction. Antiferromagnetic ordering of the rare-earth sublattices was confirmed to occur at 3.0 K and 2.0 K, respectively. Furthermore, determination of the critical exponent showed Er2CoGa8 to adopt a three-dimensional universality class. In spite of a common magnetic easy axis and similar structural characteristics, the antiferromagnetic structures were found to be different for the erbium- and thulium-based compounds. The corresponding magnetic space groups were determined to be P(2a)mmm’ and P(C)mmm. The difference in magnetic structures is discussed based on crystal electric field effects that are known to be prevalent in such materials.

Abstract:

In multiferroic TbMn2O5, the behavior of the terbium ions forms a crucial part of the magnetoelectric coupling. The result is a magnetically induced reversal of the electric polarization at 2 T. In this article we present the first direct measurement of the terbium magnetic structure under applied magnetic fields. Contrary to the current interpretation of the magnetic properties of TbMn2O5, we show that upon the electric polarization reversal the terbium sublattice adopts a canted antiferromagnetic structure with a large component of magnetic moment parallel to the a axis. Furthermore, we provide evidence for a coupling between the manganese 3d magnetic structure and the terbium 4f magnetism, which is of great significance in the elusive magnetoelectric mechanisms at play.