Local and long-range atomic/magnetic structure of non-stoichiometric spinel iron oxide nanocrystallites.

Spinel iron oxide nanoparticles of different mean sizes in the range 10-25 nm have been prepared by surfactant-free up-scalable near- and super-critical hydro-thermal synthesis pathways and characterized using a wide range of advanced structural characterization methods to provide a highly detailed structural description. The atomic structure is examined by combined Rietveld analysis of synchrotron powder X-ray diffraction (PXRD) data and time-of-flight neutron powder-diffraction (NPD) data. The local atomic ordering is further analysed by pair distribution function (PDF) analysis of both X-ray and neutron total-scattering data. It is observed that a non-stoichiometric structural model based on a tetragonal γ-Fe2O3 phase with vacancy ordering in the structure (space group P43212) yields the best fit to the PXRD and total-scattering data. Detailed peak-profile analysis reveals a shorter coherence length for the superstructure, which may be attributed to the vacancy-ordered domains being smaller than the size of the crystallites and/or the presence of anti-phase boundaries, faulting or other disorder effects. The intermediate stoichiometry between that of γ-Fe2O3 and Fe3O4 is confirmed by refinement of the Fe/O stoichiometry in the scattering data and quantitative analysis of Mössbauer spectra. The structural characterization is complemented by nano/micro-structural analysis using transmission electron microscopy (TEM), elemental mapping using scanning TEM, energy-dispersive X-ray spectroscopy and the measurement of macroscopic magnetic properties using vibrating sample magnetometry. Notably, no evidence is found of a Fe3O4/γ-Fe2O3 core-shell nanostructure being present, which had previously been suggested for non-stoichiometric spinel iron oxide nanoparticles. Finally, the study is concluded using the magnetic PDF (mPDF) method to model the neutron total-scattering data and determine the local magnetic ordering and magnetic domain sizes in the iron oxide nanoparticles. The mPDF data analysis reveals ferrimagnetic collinear ordering of the spins in the structure and the magnetic domain sizes to be ∼60-70% of the total nanoparticle sizes. The present study is the first in which mPDF analysis has been applied to magnetic nanoparticles, establishing a successful precedent for future studies of magnetic nanoparticles using this technique.

Impact of magnetization and hyperfine field distribution on high magnetoelectric coupling strength in BaTiO3-BiFeO3 multilayers.

Correlations were established between the hyperfine field distribution around the Fe atoms, the multiferroic properties, and the high magnetoelectric coefficient in BaTiO3-BiFeO3 multilayer stacks with variable BiFeO3 single layer thickness, down to 5 nm. Of key importance in this study was the deposition of 57Fe - enriched BiFeO3, which enhances the sensitivity of conversion electron Mössbauer spectroscopy by orders of magnitude. The magnetoelectric coefficient αME reaches a maximum of 60.2 V cm-1 Oe-1 at 300 K and at a DC bias field of 2 Tesla for a sample of 15 × (10 nm BaTiO3-5 nm BiFeO3) and is one of the highest values reported so far. Interestingly, the highest αME is connected to a high asymmetry of the hyperfine field distribution of the multilayer composite samples. The possible mechanisms responsible for the strong magnetoelectric coupling are discussed.

X-Ray Diffraction and Mössbauer Spectroscopy Studies of Pressure-Induced Phase Transitions in a Mixed-Valence Trinuclear Iron Complex.

The mixed-valence complex Fe3 O(cyanoacetate)6 (H2 O)3 (1) has been studied by single-crystal X-ray diffraction analysis at pressures up to 5.3(1) GPa and by (synchrotron) Mössbauer spectroscopy at pressures up to 8(1) GPa. Crystal structure refinements were possible up to 4.0(1) GPa. In this pressure range, 1 undergoes two pressure-induced phase transitions. The first phase transition at around 3 GPa is isosymmetric and involves a 60° rotation of 50 % of the cyanoacetate ligands. The second phase transition at around 4 GPa reduces the symmetry from rhombohedral to triclinic. Mössbauer spectra show that the complex becomes partially valence-trapped after the second phase transition. This sluggish pressure-induced valence-trapping is in contrast to the very abrupt valence-trapping observed when compound 1 is cooled from 130 to 120 K at ambient pressure.

Correlation between alkaline earth diffusion and fragility of silicate glasses.

We have studied the correlation between liquid fragility and the inward diffusion (from surface toward interior) of alkaline earth ions in the SiO(2)-Na(2)O-Fe(2)O(3)-RO (R = Mg, Ca, Sr, Ba) glass series. The inward diffusion is caused by reduction of Fe(3+) to Fe(2+) under a flow of H(2)/N(2) (1/99 v/v) gas at temperatures around the glass transition temperature (T(g)). The consequence of such diffusion is the formation of a silica-rich nanolayer. During the reduction process, the extent of diffusion (depth) decreases in the sequence Mg(2+), Ca(2+), Sr(2+), and Ba(2+), whereas the fragility increases in the same sequence. It is found that the ratio of the activation energy of the inward diffusion E(d) near T(g) to the activation energy for viscous flow E(eta) at T(g) increases with increasing fragility of the liquid. The inward cationic diffusion can be enhanced by lowering the fragility of glass systems via varying the chemical composition.

Indication of drier periods on Mars from the chemistry and mineralogy of atmospheric dust.

The ubiquitous atmospheric dust on Mars is well mixed by periodic global dust storms, and such dust carries information about the environment in which it once formed and hence about the history of water on Mars. The Mars Exploration Rovers have permanent magnets to collect atmospheric dust for investigation by instruments on the rovers. Here we report results from Mössbauer spectroscopy and X-ray fluorescence of dust particles captured from the martian atmosphere by the magnets. The dust on the magnets contains magnetite and olivine; this indicates a basaltic origin of the dust and shows that magnetite, not maghemite, is the mineral mainly responsible for the magnetic properties of the dust. Furthermore, the dust on the magnets contains some ferric oxides, probably including nanocrystalline phases, so some alteration or oxidation of the basaltic dust seems to have occurred. The presence of olivine indicates that liquid water did not play a dominant role in the processes that formed the atmospheric dust.