We present the results of experiments on the optical, electrical and magnetic properties and electronic structure and optical spectrum calculations of the Heusler alloys Fe 2 TiAl, Fe 2 VAl and Fe 2 CrAl. We find that the drastic transformation of the band spectrum, especially near the Fermi level, when replacing the Me element (Me = Ti, V, Cr), is accompanied by a significant change in the electrical and optical properties. The electrical and optical properties of Fe 2 TiAl are typical for metals. The abnormal behavior of the electrical resistivity and the optical properties in the infrared range for Fe 2 VAl and Fe 2 CrAl are determined by electronic states at the Fermi level. Both the optical spectroscopic measurements and the theoretical calculations demonstrate the presence of low-energy gaps in the band spectrum of the Heusler alloys. In addition, we demonstrate that the formation of Fe clusters may be responsible for the large enhancement of the total magnetic moment in Fe 2 CrAl.
We describe the design and show first results of a large solid angle x-ray emission spectrometer that is optimised for energies between 5.5 keV and 1.5 keV. The spectrometer is based on an array of eleven cylindrically bent Johansson crystal analysers arranged in a point-to-point Rowland circle geometry. The smallest achievable energy bandwidth is smaller than the core hole lifetime broadening of the absorption edges in this energy range. Energy scanning is achieved using an innovative design, maintaining the Rowland circle conditions for all crystals with only four motor motions. The entire spectrometer is encased in a high-vacuum chamber that allows fitting a liquid helium cryostat and provides sufficient space for in situ cells and operando catalysis reactors.
Cobalt-doped
anatase Ti1–x
Co
x
O2 (0 < x ≤
0.04) nanopowders (with a particle size of 30–40 nm) were produced
by the hydrothermal synthesis method. Morphology, structure, and thermal
stability of the synthesized compounds were examined using transmission
electron microscopy, infrared spectroscopy, and X-ray diffraction
analysis. Using X-ray photoelectron spectroscopy, cobalt ions are
shown to have an oxidation state of 2+, with titanium ions having
a tetravalent state of Ti4+. In the as-prepared state,
all investigated compounds of Ti1–x
Co
x
O2 are paramagnetic, with
the value of paramagnetic susceptibility growing in proportion to
cobalt content; with the spin of cobalt ion equal to S = 3/2. Analysis of the electron paramagnetic resonance spectra reveals
that doping TiO2 with cobalt (up to 2%) is accompanied
by a significant increase in the concentration of F+ centers.
Further growth of the cobalt content results in a relatively wide
line (nearly 600 Oe) in the spectrum, with a g-factor
of about 2.005, demonstrating exchange-coupled regions being formed,
the fraction of which increases with cobalt content, while the intensity
of F+-center signals is reduced appreciably. Annealing
of Ti0.96Co0.04O2 in vacuum at 1000
K is shown to have resulted in the substantial localization of cobalt
atoms in the subsurface layers, resulting in an approximately 3-fold
increase in the Co atoms content on the surface of nanoparticles as
compared with that in the bulk. This is shown to be accompanied by
appearance of spontaneous magnetization at room temperature, the value
of which depends on the cobalt content in TiO2 nanopowders.
The value of magnetic moment per Co atom decreases monotonically down
to a value of ≃1 μB with cobalt content increasing.
A core–shell model proposed to be the most adequate for describing
the magnetic properties of TiO2:Co after the reducing annealing.
A hypothesis is put forward suggesting that the defect surface enriched
with Co atoms and vacancies is described with itinerant type magnetism,
allowing for the delocalized nature of electrons near vacancies.
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