73 research outputs found
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
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
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
Electronic structure of Co_xTiSe_2 and Cr_xTiSe_2
The results of investigations of intercalated compounds Cr_xTiSe_2 and
Co_xTiSe_2 by X-ray photoelectron spectroscopy (XPS) and X-ray emission
spectroscopy (XES) are presented. The data obtained are compared with
theoretical results of spin-polarized band structure calculations. A good
agreement between theoretical and experimental data for the electronic
structure of the investigated materials has been observed. The interplay
between the M3d--Ti3d hybridization (M=Cr, Co) and the magnetic moment at the M
site is discussed. A 0.9 eV large splitting of the core Cr2p{3/2} level was
observed, which reveals a strong exchange magnetic interaction of 3d-2p
electrons of Cr. In the case of a strong localization of the Cr3d electrons
(for x<0.25), the broadening of the CrL spectra into the region of the states
above the nominal Fermi level was observed and attributed to X-ray re-emission.
The measured kinetic properties are in good accordance with spectral
investigations and band calculation results.Comment: 14 pages, 11 figures, submitted to Phys.Rev.
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
Supramolecular ionics: electric charge partition within polymers and other non-conducting solids
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