In situ XPS analysis of various iron oxide films grown by NO2-assisted molecular-beam epitaxy

We report on a systematic analysis of x-ray photoelectron spectroscopy (XPS) core- and valence-level spectra of clean and well-characterized iron oxide films, i.e., a-Fe2O3, y-Fe2O3, Fe3- dO4, and Fe3O4. All iron oxide films were prepared epitaxially by NO2-assisted molecular-beam epitaxy on single crystalline MgO(100) and a-Al2O3(0001) substrates. The phase and stoichiometry of the films were controlled precisely by adjusting the NO2 pressure during growth. The XPS spectrum of each oxide clearly showed satellite structures. These satellite structures were simulated using a cluster-model calculation, which could well reproduce the observed structures by considering the systematic changes in both the Fe 3d to O 2p hybridization and the d-d electron-correlation energy. The small difference in the satellite structures between a-Fe2O3 and y-Fe2O3 resulted mainly from changes in the Fe-O hybridization parameters, suggesting an increased covalency in g-Fe2O3 compared to a-Fe2O3. With increasing reduction in the y-Fe2O3-Fe3O4 system, the satellite structures in XPS became unresolved. This was not only due to the formation of Fe21 ions, but also to nonhomogeneous changes in the hybridization parameters between octahedral and tetrahedral Fe^3+ ions

Anti-phase domains and magnetism in epitaxial magnetite layers

Recent studies show that the magnetic properties of epitaxial thin films of magnetite (Fe3O4) deviate strongly from bulk behavior: it is difficult to saturate thin films, ultrathin films may become super paramagnetic, their saturation magnetization drops to zero, and the local magnetic moments are oriented out of plane in zero field. The possible relationship between this anomalous behavior and the occurrence of anti-phase boundaries (APBs) is discussed. Transmission electron microscopy images confirm that APBs are present in our Fe3O4 films grown by molecular beam epitaxy on MgO(100). Only APBs with out-of-plane shift vectors are visible. The much higher APB density found in sputtered films suggests that preparation conditions are important. To explain the deviating saturation and the superparamagnetic behavior of thin Fe3O4 films at the same time, the magnetic coupling over the APB must be dramatically reduced due to spin disorder along the boundaries

Correlative transmission electron microscopy and electrical properties study of switchable phase-change random access memory line cells

Phase-change memory line cells, where the active material has a thickness of 15 nm, were prepared for transmission electron microscopy (TEM) observation such that they still could be switched and characterized electrically after the preparation. The result of these observations in comparison with detailed electrical characterization showed (i) normal behavior for relatively long amorphous marks, resulting in a hyperbolic dependence between SET resistance and SET current, indicating a switching mechanism based on initially long and thin nanoscale crystalline filaments which thicken gradually, and (ii) anomalous behavior, which holds for relatively short amorphous marks, where initially directly a massive crystalline filament is formed that consumes most of the width of the amorphous mark only leaving minor residual amorphous regions at its edges. The present results demonstrate that even in (purposely) thick TEM samples, the TEM sample preparation hampers the probability to observe normal behavior and it can be debated whether it is possible to produce electrically switchable TEM specimen in which the memory cells behave the same as in their original bulk embedded state

Growth and properties of strained VOx thin films with controlled stoichiometry

We have succeeded in growing epitaxial films of rocksalt VOx on MgO(001) substrates. The oxygen content as a function of oxygen flux was determined using 18O2-RBS and the vanadium valence using XAS. The upper and lower stoichiometry limits found are similar to the ones known for bulk material (0.8<x<1.3). From the RHEED oscillation period a large number of vacancies for both vanadium and oxygen were deduced, i.e. ~16% for stoichiometric VO. These numbers are, surprisingly, very similar to those for bulk material and consequently quite strain-insensitive. XAS measurements reveal that the vacancies give rise to strong low symmetry ligand fields to be present. The electrical conductivity of the films is much lower than the conductivity of bulk samples which we attribute to a decrease in the direct overlap between t2g orbitals in the coherently strained layers. The temperature dependence of the conductivity is consistent with a variable range hopping mechanism.Comment: 12 pages, 16 figures included, revised versio

In situ XPS spectra of nonstoichiometric Fe3-deltaO4(100) films

XPS core- and valence-level spectra of various nonstoichiometric Fe3−δO4 films were measured systematically as a function of the δ value. The films were prepared epitaxially on MgO(100) substrates by NO2-assisted molecular beam epitaxy and characterized in situ with RHEED, LEED, and XPS. Stoichiometry of the films was controlled precisely by adjusting the NO2 pressure during deposition and analyzed ex situ by conversion electron Mössbauer spectroscopy

Nonlocal screening effects in 2p x-ray photoemission spectroscopy of NiO(100)

We report on the layer thickness dependence of Ni 2p core-level line shapes of epitaxially grown, in a layer-by-layer fashion, NiO on a single-crystal MgO (100) substrate. The results demonstrate the sensitivity of the core-level line shape to the nearest as well as next-nearest-neighbor coordination number. The results are consistent with a recent theoretical study of nonlocal screening effects

Composition of MBE-grown iron oxide films

A wide range of iron oxides have been grown epitaxially on MgO(100) substrates using a dual beam technique in which the deposited iron is oxidised by a beam of NO2 particles. At high fluxes magnetite (Fe3-deltaO4) phases with compositions between near-stoichiometric magnetite (Fe3O4, delta = 0) and maghemite (gamma-Fe2O3, delta = 1/3) are obtained and at low fluxes wustite (Fe1-xO1-yNy) phases. The nature and composition of these phases was determined by combining the results of Mossbauer spectroscopy, RHEED, XPS and ion scattering techniques