Charge disproportionation and collinear magnetic order in the frustrated triangular antiferromagnetAgNiO2

Abstract

We report a high-resolution neutron diffraction study of the crystal and magnetic structure of the orbitally degenerate frustrated metallic magnet AgNiO 2 . At high temperatures the structure is hexagonal with a single crystallographic Ni site, low-spin Ni 3+ with spin 1/2 and twofold orbital degeneracy, arranged in an antiferromagnetic triangular lattice with frustrated spin and orbital order. A structural transition occurs upon cooling below 365 K to a tripled hexagonal unit cell containing three crystallographically distinct Ni sites with expanded and contracted NiO 6 octahedra, naturally explained by spontaneous charge order on the Ni triangular layers. No Jahn-Teller distortions occur, suggesting that charge order occurs in order to lift the orbital degeneracy. Symmetry analysis of the inferred Ni charge order pattern and the observed oxygen displacement pattern suggests that the transition could be mediated by charge fluctuations at the Ni sites coupled to a soft oxygen optical phonon breathing mode. At low temperatures the electron-rich Ni sublattice ͑assigned to a valence close to Ni 2+ with S =1͒ orders magnetically into a collinear stripe structure of ferromagnetic rows ordered antiferromagnetically in the triangular planes. We discuss the stability of this uncommon spin order pattern in the context of an easy-axis triangular antiferromagnet with additional weak second-neighbor interactions and interlayer couplings.

Mean-Field Study of Charge, Spin, and Orbital Orderings in Triangular-Lattice CompoundsANiO2(A= Na, Li, Ag)

“…9) We also note that the CO stabilization energy, which is estimated from the energy difference between the √ 3 × √ 3 COM phase and a para phase, is ∼ 0.05t for the present parameters γ br and λ br : This result leads a rough estimate that the CO stabilization energy is ∼ 0.01 -0.02 eV, which is in the same order of magnitude as the CO transition temperature observed in AgNiO 2 (365K 26,27) ). Finally, we make a brief comment on the absence of any explicit ordering in LiNiO 2 .…”

“…A similar differentiation was proposed for the low-T state of AgNiO 2 , 26,27) although the calculated spin pattern does not fully agree with the experimental result. We note that importance of interlayer coupling is experimentally suggested for the magnetic ordering, [26][27][28] which is not taken into account in our model. We will discuss the nature of this COM phase in Sec.…”

“…7,25) It shows a structural transition associated with a √ 3 × √ 3 charge ordering at 365K and antiferromagnetic transition at 20K. 26,27) The system remains metallic down to the lowest T . It was claimed that in the low-T phase the system separates into rather localized spins at 1/3 Ni sites and itinerant electrons at the remaining sites.…”

“…It was claimed that in the low-T phase the system separates into rather localized spins at 1/3 Ni sites and itinerant electrons at the remaining sites. 26,27) Magnetic properties at low T were analyzed by considering the competing nearest-and second-neighbor exchange couplings between localized spins.…”

The contribution of Diamond Light Source to the study of strongly correlated electron systems and complex magnetic structures

“…One interesting example, which was studied in detail on the Materials and Magnetism beamline (I16) by a group led by Coldea (Oxford), is that of the hexagonal AgNiO 2 nickelate [89]. AgNiO 2 has several unusual features which were previously explored by the same group using neutron diffraction and other techniques [89,90]. The most significant feature is that, unlike the case for other isostructural and isoelectronic nickelates such as NaNiO 2 , the Jahn-Teller mechanism does not seem to be active in AgNiO 2 so that it remains hexagonal and metallic at all temperatures.…”