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Electronic and magnetic properties of the charge ordered phase of LuFe2 O4 are investigated by means of x-ray spectroscopic and theoretical electronic structure approaches. LuFe2 O4 is a compound showing fascinating magnetoelectric coupling via charge ordering. Here, we identify the spin ground state of LuFe2 O4 in the charge ordered phase to be a 2:1 ferrimagnetic configuration, ruling out a frustrated magnetic state. An enhanced orbital moment may enhance the magnetoelectric coupling. Furthermore, we determine the densities of states and the corresponding correlation potentials by means of x-ray photoelectron and emission spectroscopies, as well as electronic structure calculations. © 2009 The American Physical Society.

Nd0.52Sr0.48MnO3 films of various thicknesses have been prepared by dc magnetron sputtering on single crystal LaAlO3 (001) substrates. Reducing the film thickness leads to a significant suppression of ferromagnetic (FM) ordering and the Curie point falls below the antiferromagnetic (AFM) transition temperature. When this occurs, a huge rise of the magnetoresistance ratio from 400 to 60 000% is observed in an applied magnetic field of 5 T. We surmise that this new kind of the enhanced colossal magnetoresistance effect originates in the FM/AFM competition and the collapse of the charge-ordered state at high magnetic fields, rather than in the regular double-exchange mechanism.

Electronic and magnetic properties of the charge ordered phase of LuFe2O4 are investigated by means of x-ray spectroscopic and theoretical electronic structure approaches. LuFe2O4 is a compound showing fascinating magnetoelectric coupling via charge ordering. Here, we identify the spin ground state of LuFe2O4 in the charge ordered phase to be a 2:1 ferrimagnetic configuration, ruling out a frustrated magnetic state. An enhanced orbital moment may enhance the magnetoelectric coupling. Furthermore, we determine the densities of states and the corresponding correlation potentials by means of x-ray photoelectron and emission spectroscopies, as well as electronic structure calculations.

Here we report the magnetic properties of the layered cobalt oxide system, LixCoO2, in the whole range of Li composition, 0 = 0.40), on the other hand, various phase transitions are observed. For x = 0.40, a susceptibility hump is seen at 30 K, suggesting the onset of static AF order. A magnetic jump, which is likely to be triggered by charge ordering, is clearly observed at Tt = 175 K in samples with x = 0.50 (= 1/2) and 0.67 (= 2/3), while only a tiny kink appears at T = 210 K in the sample with an intermediate Li composition, x = 0.60. Thus, the phase diagram of the LixCoO2 system is complex, and the electronic properties are sensitively influenced by the Li content (x)., Comment: 29 pages, 1 table, 9 figures

We study the dynamical magnetization process in the ordered ground-state phase of the transverse Ising model under sweeps of magnetic field with constant velocities. In the case of very slow sweeps and for small systems studied previously [H. De Raedt , Phys. Rev. B 56, 11761 (1997)], nonadiabatic transitions at avoided level-crossing points give the dominant contribution to the shape of magnetization process. In contrast, in the ordered phase of this model and for fast sweeps, we find significant size-independent jumps in the magnetization process. We study this phenomenon in analogy to the spinodal decomposition in classical ordered state and investigate its properties and its dependence on the system parameters. An attempt to understand the magnetization dynamics under field sweep in terms of the energy-level structure is made. We discuss a microscopic mechanism of magnetization dynamics from a viewpoint of local cluster flips and show that this provides a picture that explains the size independence. The magnetization dynamics in the fast-sweep regime is studied by perturbation theory and we present a perturbation scheme based on interacting Landau-Zener-type processes to describe the local cluster flip dynamics.

International audience; We report the observation of a static long-range 2Ni(3+)-> Ni3+delta+Ni3-delta charge redistribution in PrNiO3 below T-MI=130 K by means of high-resolution powder neutron diffraction. In contrast to previous structural studies, these present data show that the symmetry of the low-temperature insulating phase is not orthorhombic but monoclinic P2(1)/n. The two available Ni sites host contracted and expanded NiO6 octahedra which alternate along the three pseudocubic perovskite axes, suggesting the existence of a Ni3+delta/Ni3-delta charge order similar to that reported for the nickelates with heavier lanthanide ions. The difference between the average Ni-O distances in the two Ni sites at T/T-MI=0.08 is 0.052(2) A, which is almost two times smaller than the value reported for LuNiO3 at T/T-MI=0.50 [0.084(4) A]. This result suggests a progressive decrease in delta by approaching the itinerant limit and gives further support to the charge disproportionation mechanism as an origin of the metal-insulator transition in the whole RNiO3 family.

Electronic and magnetic properties of the charge ordered phase of LuFe2O4 are investigated by means of x-ray spectroscopic and theoretical electronic structure approaches. LuFe2O4 is a compound showing fascinating magnetoelectric coupling via charge ordering. Here, we identify the spin ground state of LuFe2O4 in the charge ordered phase to be a 2:1 ferrimagnetic configuration, ruling out a frustrated magnetic state. An enhanced orbital moment may enhance the magnetoelectric coupling. Furthermore, we determine the densities of states and the corresponding correlation potentials by means of x-ray photoelectron and emission spectroscopies, as well as electronic structure calculations.

Among quasi-two-dimensional organic conductors, (BEDT-TTF)2X, what form the two column structure of α or θ is thought to show a charge-ordering insulating state. The α' modification is the other candidate for a charge-ordering state. To determine the existence of a charge-ordering state and electron magnetism in the α' phase, which has a symmetry that differs from that of the α and θ phases, we utilized C-13-NMR to examine α'-(BEDT-TTF)2IBr2 in which one side of the central carbon on the BEDT-TTF molecule is substituted with C-13 nuclei. We observed changes in its spectrum at 200 K. The angular dependence of the NMR shift at 15 K indicated that the charge-ordering pattern is intracolumn disproportionation and the ratio of charge disproportionation on the spin-poor site and spin-rich sites is almost 1:0. The angle dependence of NMR spectrum at 60 K suggests that the hyperfine coupling tensor in the paramagnetic charge-ordering state is determined not only by the charge density of on-site molecules but also by the contribution of off-site molecules. The change in hyperfine coupling tensor must be taken into account in NMR studies of the paramagnetic CO state.

The quasi-one-dimensional metallic system BaVS3 with a metal-insulator transition at T_MI=70 K shows large changes of the optical phonon spectrum, a central peak, and an electronic Raman scattering continuum that evolve in a three-step process. Motivated by the observation of a strongly fluctuating precursor state at high temperatures and orbital ordering and a charge gap at low temperatures we suggest a concerted action of the orbital, electronic, and lattice subsystems dominated by electronic correlations., Comment: 4 pages, 4 figures