247 research outputs found
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
Controlling orbital moment and spin orientation in CoO layers by strain
We have observed that CoO films grown on different substrates show dramatic
differences in their magnetic properties. Using polarization dependent x-ray
absorption spectroscopy at the Co L edges, we revealed that the
magnitude and orientation of the magnetic moments strongly depend on the strain
in the films induced by the substrate. We presented a quantitative model to
explain how strain together with the spin-orbit interaction determine the 3d
orbital occupation, the magnetic anisotropy, as well as the spin and orbital
contributions to the magnetic moments. Control over the sign and direction of
the strain may therefore open new opportunities for applications in the field
of exchange bias in multilayered magnetic films
Spin-valve behaviour of anti-ferromagnetic boundaries in ultrathin magnetite films
Magneto-resistance (MR) measurements on epitaxial Fe3O4 films grown on polished MgO have been performed. The measurements presented here are interpreted by a model that describes the MR behaviour as spin-polarised transport across antiferromagnetic (AF) interfaces. The Fe3O4 films consist of structural domains, separated by anti-phase boundaries where an AF coupling is present. These AF interfaces enhance the resistance of the films. Upon application of a magnetic field the AF-spins rotate towards each other and the resistance decreases. The AF interfaces are thus behaving as spin-valves. In agreement with the model, the observed magneto-resistance is negative and shows linear and quadratic field dependence up to the anisotropy field for fields applied parallel and perpendicular to the film plane respectively. Above the anisotropy field, the slopes of the two MR curves are expected to be equal, which is observed at 60 K. Above the Verwey transition, the shape of the normalised MR curves is independent of temperature. Below the Verwey transition the MR curve becomes more linear with decreasing temperature. A large difference between parallel and perpendicular MR is observed at the Verwey transition.
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
Charge injection instability in perfect insulators
We show that in a macroscopic perfect insulator, charge injection at a
field-enhancing defect is associated with an instability of the insulating
state or with bistability of the insulating and the charged state. The effect
of a nonlinear carrier mobility is emphasized. The formation of the charged
state is governed by two different processes with clearly separated time
scales. First, due to a fast growth of a charge-injection mode, a localized
charge cloud forms near the injecting defect (or contact). Charge injection
stops when the field enhancement is screened below criticality. Secondly, the
charge slowly redistributes in the bulk. The linear instability mechanism and
the final charged steady state are discussed for a simple model and for
cylindrical and spherical geometries. The theory explains an experimentally
observed increase of the critical electric field with decreasing size of the
injecting contact. Numerical results are presented for dc and ac biased
insulators.Comment: Revtex, 7pages, 4 ps figure
Magnetic versus crystal field linear dichroism in NiO thin films
We have detected strong dichroism in the Ni x-ray absorption
spectra of monolayer NiO films. The dichroic signal appears to be very similar
to the magnetic linear dichroism observed for thicker antiferromagnetic NiO
films. A detailed experimental and theoretical analysis reveals, however, that
the dichroism is caused by crystal field effects in the monolayer films, which
is a non trivial effect because the high spin Ni ground state is not
split by low symmetry crystal fields. We present a practical experimental
method for identifying the independent magnetic and crystal field contributions
to the linear dichroic signal in spectra of NiO films with arbitrary
thicknesses and lattice strains. Our findings are also directly relevant for
high spin and systems such as LaFeO, FeO,
VO, LaCrO, CrO, and Mn manganate thin films
Giant positive magnetoresistance in metallic VOx thin films
We report on giant positive magnetoresistance effect observed in VOx thin
films, epitaxially grown on SrTiO3 substrate. The MR effect depends strongly on
temperature and oxygen content and is anisotropic. At low temperatures its
magnitude reaches 70% in a magnetic field of 5 T. Strong electron-electron
interactions in the presence of strong disorder may qualitatively explain the
results. An alternative explanation, related to a possible magnetic
instability, is also discussed.Comment: 4 pages, 5 figures included in the text, references update
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