Charge order to remove orbital degeneracy in triangular antiferromagnet AgNiO2
08 Sep 2009



The layered hexagonal magnet AgNiO2 realizes a unique example of a triangular antiferromagnet with both spin and orbital moment (Ni3+ with one eg electron) and with metallic conductivity.

(a) Charge order on the Ni site leads to displacements of the surrounding oxygen ions, directly observed by high resolution diffraction measurements. (b) Magnetic order:+/- are spins in/out of plane, dots are unordered sites.

Both the spin and orbital order are frustrated in a triangular geometry and the electronic ground state when electrons are mobile is not well understood. Using high-resolution diffraction on Osiris and HRPD we have uncovered a supercell crystal structure where expanded NiO6 octahedra are surrounded by a honeycomb network of contracted sites (see figure). Band-structure calculations indicate that this structural modulation is naturally explained by a spontaneous charge order on the Ni sites: expanded sites are electron rich Ni2+ and contracted sites are electron depleted Ni3.5+. Our results show that in weakly-delocalized metallic systems the orbital degeneracy can be lifted by charge order as opposed to the Jahn-Teller distortions found in insulating systems. At base temperature the honeycomb sites are nonmagnetic and the expanded Ni sites order in an unusual collinear pattern of alternating stripes on a triangular lattice (see figure).

E Wawrzyñska, R Coldea (University of Bristol), EM Wheeler (University of Oxford), II Mazin, MD Johannes (Naval Research Lab, Washington, USA), T Sörgel, M Jansen (Max-Planck Institut für Festkörperforschung, Stuttgart, Germany), RM Ibberson, PG Radaelli (ISIS)

Research date: December 2007

Further Information

E Wawrzyñska et al., Phys. Rev. Lett., 99 157204 (2007).​