Direct observation of charge order in triangular metallic AgNiO2 by single-crystal resonant X-ray scattering

We report resonant X-ray scattering measurements on the orbitally-degenerate triangular metallic antiferromagnet 2H-AgNiO2 to probe the spontaneous transition to a triple-cell superstructure at temperatures below 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 ~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.Comment: 5 pages, 4 figure

Antiferromagnetic order and the structural order-disorder transition in the Cd6Ho quasicrystal approximant

International audienceIt has generally been accepted that the orientational ordering of the Cd4 tetrahedron within the Cd6 R quasicrystal approximants is kinetically inhibited for R = Ho, Er, Tm and Lu by steric constraints. Our high-resolution X-ray scattering measurements of the Cd6Ho quasicrystal approximant, however, reveal an abrupt (first-order) transition to a monoclinic structure below T S = 178 K for samples that have 'aged' at room temperature for approximately one year, reopening this question. Using X-ray resonant magnetic scattering at the Ho L 3-edge we have elucidated the nature of the antiferromagnetic ordering below T N = 8.5 K in Cd6Ho. The magnetic Bragg peaks are found at the charge forbidden H + K + L = 2n + 1 positions, referenced to the high-temperature body-centred cubic structure. In general terms, this corresponds to antiferromagnetic arrangements of the Ho moments on adjacent clusters in the unit cell as previously found for Cd6Tb

Combined coherent x-ray micro-diffraction and local mechanical loading on copper nanocrystals

Coherent x-ray micro-diffraction and local mechanical loading can be combined to investigate the mechanical deformation in crystalline nanostructures. Here we present measurements of plastic deformation in a copper crystal of sub-micron size obtained by loading the sample with an Atomic Force Microscopy tip. The appearance of sharp features in the diffraction pattern, while conserving its global shape, is attributed to crystal defects induced by the tip

Structural and magnetic transitions in the crystalline approximant Cd6Sm.

International audienceElectrical resistivity, specific heat, magnetic susceptibility, and magnetization of the crystalline approximant Cd6Sm are investigated in a temperature range 1.8-300 K. Both structural and magnetic transitions are clearly evidenced. A sharp structural phase transition is observed at T-c = 179 K in the electrical resistivity as well as in the specific heat, which is attributed to orientational ordering of the Cd-4 tetrahedron located at the center of the icosahedral cluster. In addition, three successive magnetic transitions are found at T-M1 = 12.5, T-M2 = 10.2, and T-M3 = 6.5 K. The anomaly at T-M1 is attributed to an antiferromagnetic transition, whereas those at T-M2 and T-M3 are to ferrimagnetic transitions. The observation of the ferrimagnetic orders indicates that (pseudo) icosahedra made of localized Sm spins have a nonzero net magnetic moment, and the moments of the magnetic clusters form a long-range magnetic order

Antiferromagnetic order in the quasicrystal approximant Cd6Tb studied by x-ray resonant magnetic scattering

We have used x-ray resonant magnetic scattering at the Tb L2 edge to elucidate the nature of magnetic ordering in Cd6Tb, a 1/1 approximant closely related to the Cd-Mg-R (R = rare earth) icosahedral alloys. Below TN ≈ 24 K, the ordered moments associated with the icosahedral clusters at the corners and body center of the pseudocubic unit cell are antiferromagnetically correlated, and long-range magnetic order is realized

Structural and magnetic transitions in the crystalline approximant Cd(6)Sm

International audienceElectrical resistivity, specific heat, magnetic susceptibility, and magnetization of the crystalline approximant Cd(6)Sm are investigated in a temperature range 1.8-300 K. Both structural and magnetic transitions are clearly evidenced. A sharp structural phase transition is observed at T(c) = 179 K in the electrical resistivity as well as in the specific heat, which is attributed to orientational ordering of the Cd(4) tetrahedron located at the center of the icosahedral cluster. In addition, three successive magnetic transitions are found at T(M1) = 12.5, T(M2) = 10.2, and T(M3) = 6.5 K. The anomaly at T(M1) is attributed to an antiferromagnetic transition, whereas those at T(M2) and T(M3) are to ferrimagnetic transitions. The observation of the ferrimagnetic orders indicates that (pseudo) icosahedra made of localized Sm spins have a nonzero net magnetic moment, and the moments of the magnetic clusters form a long-range magnetic order