Temperature-Driven Self-Doping in Magnetite.

Magnetite is one of the most fascinating materials exhibiting the enigmatic first-order Verwey transition which is conventionally manipulated through chemical doping. Here, we show that heating magnetite results in a spontaneous charge reordering and, consequently, a hole self-doping effect at the octahedral sublattice. Core-level x-ray spectroscopy measurements combined with theory uncovers that there are three regimes of self-doping that map the temperature dependence of the electrical conductivity and magnetism up to the Curie temperature. Our results provide an elegant analogy between the effect of chemical doping and temperature-driven self-doping on trimerons in magnetite.

The five-analyzer point-to-point scanning crystal spectrometer at ESRF ID26.

X-ray emission spectroscopy in a point-to-point focusing geometry using instruments that employ more than one analyzer crystal poses challenges with respect to mechanical design and performance. This work discusses various options for positioning the components and provides the formulas for calculating their relative placement. Ray-tracing calculations were used to determine the geometrical contributions to the energy broadening including the source volume as given by the beam footprint on the sample. The alignment of the instrument is described and examples are given for the performance.

Relation among Oxygen Stoichiometry, Structure, and Co Valence and Spin State in Single-Layer La2-xAxCoO4±Î´ (A = Ca, Sr) Perovskites.

We have investigated the role of oxygen stoichiometry and structural properties in the modulation of Co valence and spin state in single-layer La2-xAxCoO4±Î´ (A = Sr, Ca; 0 ≤ x ≤ 1) perovskites as well as the interplay between their local structural properties and the magnetic and charge-ordering phenomena. We show the results of high angular resolution powder X-ray diffraction and Co K-edge X-ray absorption and emission spectroscopy experiments on polycrystalline and single-crystal samples. The different doping-induced changes in the Co valence and spin state by Ca (or Sr) substitution can be understood in terms of the evolving oxygen stoichiometry. For Ca doping, the interstitial oxygen excess around the La/Ca atoms in underdoped samples is rapidly lost upon increasing the Ca content. The creation of oxygen vacancies leads to the stabilization of a mixed-valence Co2.5+ independently of the Ca content. In contrast, Sr substitution leads to almost stoichiometric samples and a lower oxygen vacancy concentration, which allows higher mixed-valence states for Co up to Co2.9+. The Co mixed-valence state along the two series is fluctuating between two valence states, Co2.4+ as in La2CoO4.2 and Co2.9+ as in LaSrCoO3.91, that become periodically ordered for the charge-ordered phases around the half-doping. The X-ray emission derived spin states agree well with the Co fluctuating mixed-valence state derived from X-ray absorption spectroscopy on consideration of a distribution of high-spin Co2+ and low-spin Co3+. Furthermore, there is no quenching of the orbital contribution for the high-spin Co2+, as concluded from a comparison with macroscopic magnetization measurements. Doping holes are mainly located in the ab plane and have a strong oxygen 2p character. The major lattice distortions, which are different for Sr and Ca doping, occur along the c axis, where changes in the oxygen stoichiometry take place. Moreover, charge-order transitions are clearly shown from the anomalous increase of the c lattice parameter with an increase in the temperature above 500 K but there is no signature for a temperature-dependent spin-state transition.

New reflections on hard X-ray photon-in/photon-out spectroscopy.

Analysis of the electronic structure and local coordination of an element is an important aspect in the study of the chemical and physical properties of materials. This is particularly relevant at the nanoscale where new phases of matter may emerge below a critical size. X-ray emission spectroscopy (XES) at synchrotron radiation sources and free electron lasers has enriched the field of X-ray spectroscopy. The spectroscopic techniques derived from the combination of X-ray absorption and emission spectroscopy (XAS-XES), such as resonant inelastic X-ray scattering (RIXS) and high energy resolution fluorescence detected (HERFD) XAS, are an ideal tool for the study of nanomaterials. New installations and beamline upgrades now often include wavelength dispersive instruments for the analysis of the emitted X-rays. With the growing use of XAS-XES, scientists are learning about the possibilities and pitfalls. We discuss some experimental aspects, assess the feasibility of measuring weak fluorescence lines in dilute, radiation sensitive samples, and present new experimental approaches for studying magnetic properties of colloidal nanoparticles directly in the liquid phase.

Selective magnetometry of superparamagnetic iron oxide nanoparticles in liquids.

We show that the properties of superparamagnetic iron oxide nanoparticles suspended in liquids can be effectively studied using Magnetic Circular Dichroism in Resonant Inelastic X-ray Scattering. Analysis of the spectral shape and magnetic contrast produced by this experiment enables an assessment of the site distribution and magnetic state of metal ions in the spinel phase. The selective magnetization profile of particles as derived from the field dependence of dichroism empowers an estimation of particle size distribution. Furthermore, the new proposed methodology discriminates sizes that are below the detection limits of X-ray and light scattering probes and that are difficult to spot in TEM.

Chemical Sensitivity of Kß and Kα X-ray Emission from a Systematic Investigation of Iron Compounds.

K-fluorescence X-ray emission spectroscopy (XES) is receiving growing interest in all fields of natural sciences to investigate the local spin. The spin sensitivity in Kß (Kα) XES stems from the exchange interaction between the unpaired 3p (2p) and the 3d electrons, which is greater for Kß than for Kα. We present a thorough investigation of a large number of iron-bearing compounds. The experimental spectra were analyzed in terms of commonly used quantitative parameters (Kß1,3-first moment, Kα1-full width at half-maximum, and integrated absolute difference -IAD-), and we carefully examined the difference spectra. Multiplet calculations were also performed to elucidate the underlying mechanisms that lead to the chemical sensitivity. Our results confirm a strong influence of covalency on both Kß and Kα lines. We establish a reliable spin sensitivity of Kß XES as it is dominated by the exchange interaction, whose variations can be quantified by either Kß1,3-first moment or Kß-IAD and result in a systematic difference signal line shape. We find an exception in the Kß XES of Fe3+ and Fe2+ in water solution, where a new difference spectrum is identified that cannot be reproduced by scaling the exchange integrals. We explain this by strong differences in orbital mixing between the valence orbitals. This result calls for caution in the interpretation of Kß XES spectral changes as due to spin variations without a careful analysis of the line shape. For Kα XES, the smaller exchange interaction and the influence of other electron-electron interactions make it difficult to extract a quantity that directly relates to the spin.

Magnetic Contrast at Spin-Flip Excitations: An Advanced X-Ray Spectroscopy Tool to Study Magnetic-Ordering.

The determination of the local orientation and magnitude of the magnetization in spin textures plays a pivotal role in understanding and harnessing magnetic properties for technological applications. Here, we show that by employing the polarization dependence of resonant inelastic X-ray scattering (RIXS), we can directly probe the spin ordering with chemical and site selectivity. Applied on the prototypical ferrimagnetic mixed-valence system, magnetite ([Fe3+]A[Fe3+,Fe2+]BO4), we can distinguish spin-flip excitations at the A and B antiferromagnetically coupled Fe3+ sublattices and quantify the exchange field. Furthermore, it is possible to determine the orbital contribution to the magnetic moment from detailed angular dependence measurements. RIXS dichroism measurements performed at spin-flip excitations with nanometer spatial resolution will offer a powerful mapping contrast suitable for the characterization of magnetic ordering at interfaces and engineered spin textures.

A new type of antiferrodistortive structure in a double perovskite with Pb.

We have found a new structural transition in Pb(2)MnReO(6) at 410 K. Above this temperature, Pb(2)MnReO(6) is cubic with disordered and dynamic atomic displacements manifested in the large thermal parameters of Pb and O atoms. Below 410 K, the antiferrodistortive shift of 2/3 of Pb(2+) cations away from the high-symmetry cubic site produces a new type of monoclinic cell. The unit cell expands at the transition and the heat capacity shows a peak with thermal hysteresis. These features agree with a first order transition. The entropy content of the transition is quite low indicating that the structural disorder has not been completely removed in the low temperature phase. The monoclinic phase of Pb(2)MnReO(6) shows thermally activated conductivity which does not vary when an external magnetic field is applied. A change in the slope of the resistivity curve, observed at the structural phase transition temperature, is related to a slight difference in the activation energy between both phases. It suggests that the condensation of the distortions likely affects the conduction mechanism. The isothermal magnetization measurements reveal the presence of ferromagnetic contributions below 85 K. The ac magnetic susceptibility shows a dynamic peak at 50 K and, in addition, zero-field-cooled and field-cooled magnetization curves diverge strongly below 80 K. These features might be signature of magnetic inhomogeneity. Magnetic loops, obtained at 5 K, do not show saturation in fields up to 9 T. Furthermore, the measured coercivity increases sharply at low temperature indicating an abrupt change in the magnetic anisotropy. We show that all these magnetic properties point out to a ferrimagnetic ordering of Mn and Re atoms in an intermediate valence state.