Search

We have successfully grown centimeter sized layered RSrNiO4 single crystals under high oxygen pressures of 120 150 bar by the floating zone technique. This enabled us to perform neutron scattering experiments where we observe close to quarter integer magnetic peaks below amp; 8764;77 K that are accompanied by steep upwards dispersing spin excitations. Within the high frequency Ni O bond stretching phonon dispersion, a softening at the propagation vector for a checkerboard modulation can be observed. We were able to simulate the magnetic excitation spectra using a model that includes two essential ingredients, namely checkerboard charge disproportionation and nano phase separation. The results thus suggest that charge disproportionation is preferred instead of a Jahn Teller distortion even for this layered Ni3 system

International audience; We report on our investigation of the electronic structure of Ti2O3 using (hard) x-ray photoelectron and soft x-ray absorption spectroscopy. From the distinct satellite structures in the spectra, we have been able to establish unambiguously that the Ti-Ti c-axis dimer in the corundum crystal structure is electronically present and forms an a(1g)a(1g) molecular singlet in the low-temperature insulating phase. Upon heating, we observe a considerable spectral weight transfer to lower energies with orbital reconstruction. The insulator-metal transition may be viewed as a transition from a solid of isolated Ti-Ti molecules into a solid of electronically partially broken dimers, where the Ti ions acquire additional hopping in the a-b plane via the e(g)(pi) channel, the opening of which requires consideration of the multiplet structure of the on-site Coulomb interaction.

The first known magnetic mineral, magnetite, has unusual properties, which have fascinated mankind for centuries; it undergoes the Verwey transition around 120 K with an abrupt change in structure and electrical conductivity. The mechanism of the Verwey transition, however, remains contentious. Here we use resonant inelastic X-ray scattering over a wide temperature range across the Verwey transition to identify and separate out the magnetic excitations derived from nominal Fe2+ and Fe3+ states. Comparison of the experimental results with crystal-field multiplet calculations shows that the spin–orbital dd excitons of the Fe2+ sites arise from a tetragonal Jahn-Teller active polaronic distortion of the Fe2+O6 octahedra. These low-energy excitations, which get weakened for temperatures above 350 K but persist at least up to 550 K, are distinct from optical excitations and are best explained as magnetic polarons., The Verwey transition of magnetite is complex due to the coexistence of strong correlations and electron-phonon coupling. Here, the authors use resonant inelastic X-ray scattering to show evidence for magnetic polarons in magnetite and provide insight into the nature of the transition.

We have studied the magnetic properties and crystal structure of Sr\ud 3\ud CoIrO\ud 6\ud and Sr\ud 3\ud NiIrO\ud 6\ud as a function of temperature. Two characteristic temperatures, \ud T\ud 1\ud =\ud 90\ud K and \ud T\ud 2\ud =\ud 25\ud K for Sr\ud 3\ud CoIrO\ud 6\ud , and \ud T\ud 1\ud =\ud 85\ud K and \ud T\ud 2\ud =\ud 15\ud K for Sr\ud 3\ud NiIrO\ud 6\ud , were observed. Below \ud T\ud 1\ud a significant increase of magnetization and below \ud T\ud 2\ud a weak temperature dependence of magnetization in the field-cooled and practically zero magnetization values in the zero-field-cooled mode were detected for both compounds. The existence of \ud Ir\ud 4\ud +\ud in Sr\ud 3\ud CoIrO\ud 6\ud was confirmed by an Ir-\ud L\ud III\ud x-ray absorption measurement. Magnetoelastic effects have been observed in the temperature dependence of the lattice parameters of Sr\ud 3\ud CoIrO\ud 6\ud and Sr\ud 3\ud NiIrO\ud 6\ud . The magnetic structure of Sr\ud 3\ud CoIrO\ud 6\ud in zero fields can be described as a commensurate modulated antiferromagnet with a propagation vector \ud k\ud =\ud (\ud 0\ud ,\ud 0\ud ,\ud 1\ud )\ud . Neutron powder diffraction with polarized neutrons gave evidence of short-range magnetic order, above and below the magnetic ordering temperature.

International audience; We have investigated the valence, spin, and orbital state of the Co ions in the one-dimensional cobaltate Ca3 Co2 O6 using x-ray absorption and x-ray magnetic circular dichroism at the Co- L2,3 edges. The Co ions at both the octahedral Cooct and trigonal Cotrig sites are found to be in a 3+ state. From the analysis of the dichroism we established a low-spin state for the Cooct and a high-spin state with an anomalously large orbital moment of 1.7 μB at the Cotrig 3+ ions. This large orbital moment along the c -axis chain and the unusually large magnetocrystalline anisotropy can be traced back to the double occupancy of the d2 orbital in trigonal crystal field. © 2006 The American Physical Society.

The aim of this project was to study the influence of epitaxial strain on the electronic and magnetic structure of transition metal oxide layers. In the first part of the thesis the discovery of characteristic diffuse X-ray scattering patterns is reported. They are caused by the misfit dislocations, which appear in the layers if their thickness exceeds the so-called critical thickness. In the remaining part of the thesis it is shown using XAS spectroscopy that the spin direction in the CoO layers is determined by the sign of the strain. It is here also discussed how such CoO layers with controlled spin orientation can influence the spin orientation in adjacent MnO thin films.

Strong resonant enhancements of the charge-order and spin-order superstructure-diffraction intensities in La1.8Sr0.2NiO4 are observed when x-ray energies in the vicinity of the Ni L2,3 absorption edges are used. The pronounced photon-energy and polarization dependences of these diffraction intensities allow for a critical determination of the local symmetry of the ordered spin and charge carriers. We found that not only the antiferromagnetic order but also the charge-order superstructure resides within the NiO2 layers; the holes are mainly located on in-plane oxygens surrounding a Ni2+ site with the spins coupled antiparallel in close analogy to Zhang-Rice singlets in the cuprates., Comment: 4 pages, 3 figures

This thesis describes the efforts to stabilize chromium monoxide (CrO) as a thin epitaxial film on various substrates. The most stable and common oxidation phase for chromium is in chromium sesquioxide (Cr2O3). CrO does not exist as a bulk material in nature, therefore we have to grow it in a artificial manner as a single crystal film using molecular beam epitaxy (MBE). The motivation behind this project is that in CrO, Cr ion has the same electronic configuration as the manganese ion in the lanthanum manganates (LaMnO3), a material that doped with strontium or calcium (La1-xSrxMnO3) is the so-called colossal magneto resistance material. One may expect that CrO has equally fascinating magnetic and electrical properties as the manganates. ... Zie: Summary

K-doped C60 monolayers on polycrystalline Cu, Ag, and Au are studied by means of ultraviolet photoelectron spectroscopy. The polycrystalline character of the substrate allows an accurate subtraction procedure to isolate the photoemission signal of the monolayer. Charge transfers of 1.0–1.8 electrons per molecule are found for the undoped monolayer. On high K doping the hybridization with the substrate is shown to practically disappear, and the overlayer almost totally suppresses the substrate signal.

We report the measurement of the local Ni 3d spin polarization, not only below but also above the Curie temperature (T-C), using the newly developed spin-resolved circularly polarized 2p (L-3) resonant photoemission technique. The experiment identifies the presence of 3d(8) singlets at high energies and 3d(8) triplets at low energies extending all the way to the Fermi energy, both below and above T-C, showing that it is the orbital degeneracy of the 3d band and the Hund's rule splitting which is of utmost importance to understand Ni and other 3d ferromagnets.

Using a combination of photoelectron and inverse photoelectron spectroscopy, we show that the band gap in a monolayer of C-60 on a Ag surface is strongly reduced from the solid C-60 surface value. We argue that this is a result of the reduction of the on-site molecular Coulomb interaction due to the influence of image charges in the metal substrate. This result suggests that the physical properties of correlated insulators and semiconductors will be strongly modified if prepared in ultra thin form on metal substrates or sandwiched between metal layers.

We present temperature-dependent V-2p and O-1s x-ray-absorption spectra of LiVO2. The aim of this study is to monitor changes in electronic structure on going through the phase transition. The spectral changes turn out to be very small: the V-3d-O-2p hybridization does not change considerably, and the symmetry of the V-ion ground state (high-spin T-3(1)) is retained. To explain our results, together with the anomalously low magnetic susceptibility below the transition temperature, we propose a model in which a three-orbital sublattice is formed in the V (111) planes, which results in a total singlet state.

The Verwey transition in magnetite, Fe 3 O 4 , has been studied using temperature-dependent high-resolution photoemission spectroscopy. On heating through the transition temperature T V the band gap is not collapsed, but is merely reduced by ∼50 meV, showing that a metal-insulator transition does not occur. The change in the gap is perfectly consistent with the two orders of magnitude conductivity jump at T V . Thus even above T V short-range charge ordering rather than site equivalency dominates the single-particle excitations and the electrical properties. We also point out important implications for efforts to model the electrical transport above T V .

Soft X-ray magnetic circular dichroism (SXMCD) measurements are reported on the O K- and Mn L-absorption edges of La1-xSrxMnO3 (x = 0.15, 0.18, and 0.4) single crystals in their ferromagnetic regime. This has been done using a recently developed experimental technique for magnetizing the samples offering the possibility to get high-accuracy SXMCD effect measurements on bulk samples. The very strong doping-induced O K-edge intensity is proof of an enhanced O 2p hole component of the doped holes. O K soft X-ray magnetic circular dichroism spectra give evidence for an orbital magnetic moment of this O 2p component ferromagnetically coupled to the Mn moments.

A new effect on the X-ray absorption line shape is described which is proportional to the nearest-neighbor spin-spin correlation function. We present theory and demonstrate the use of linear polarized X-ray absorption spectroscopy to study the temperature dependence of the long-range order parameter and the nearest-neighbor spin-spin correlation function in antiferromagnetic NiO.

We report strong magnetic linear dichroism at the Fe L2,3 edge of the antiferromagnet Fe2O3 (hematite). The relative difference in absorption for light polarized parallel and perpendicular to the magnetic moment is as high as 40% at the Fe L2 edge. The spectra are in excellent agreement with calculations of magnetic dichroism for Fe3+ (3d5). The magnetic origin of this dichroism is demonstrated by the Morin transition at almost-equal-to 10-degrees-C, where the moments in Fe2O3 rotate by 90-degrees. Magnetic linear dichroism may be applied to measure the spin orientation in thin films and multilayers, and to image magnetic domains.