289 research outputs found
Investigation of CoFeSi: The Heusler compound with Highest Curie Temperature and Magnetic Moment
This work reports on structural and magnetic investigations of the Heusler
compound CoFeSi. X-Ray diffraction and M\"o\ss bauer spectrometry indicate
an ordered structure. Magnetic measurements by means of X-ray magnetic
circular dichroism and magnetometry revealed that this compound is, currently,
the material with the highest magnetic moment () and Curie-temperature
(1100K) in the classes of Heusler compounds as well as half-metallic
ferromagnets
Half-metallic ferromagnetism with high magnetic moment and high Curie temperature in CoFeSi
CoFeSi crystallizes in the ordered L2 structure as proved by X-ray
diffraction and M\"o\ss bauer spectroscopy. The magnetic moment of CoFeSi
was measured to be about at 5K. Magnetic circular dichroism spectra
excited by soft X-rays (XMCD) were taken to determine the element specific
magnetic moments of Co and Fe. The Curie temperature was measured with
different methods to be ()K. CoFeSi was found to be the Heusler
compound as well as the half-metallic ferromagnet with the highest magnetic
moment and Curie temperature.Comment: conference contribution, MMM200
Seebeck coefficients of half-metallic ferromagnets
In this report the Co2 based Heusler compounds are discussed as potential
materials for spin voltage generation. The compounds were synthesized by
arcmelting and consequent annealing. Band structure calculations were performed
and revealed the compounds to be half-metallic ferromagnets. Magnetometry was
performed on the samples and the Curie temperatures and the magnetic moments
were determined. The Seebeck coefficients were measured from low to ambient
temperatures for all compounds. For selected compounds high temperature
measurements up to 900 K were performed.Comment: accepted contribution o the Special Issue "Spin Caloritronics" of
Solid State Communication
Correlation in the transition metal based Heusler compounds CoMnSi and CoFeSi
Half-metallic ferromagnets like the full Heusler compounds with formula
XYZ are supposed to show an integer value of the spin magnetic moment.
Calculations reveal in certain cases of X = Co based compounds non-integer
values, in contrast to experiments. In order to explain deviations of the
magnetic moment calculated for such compounds, the dependency of the electronic
structure on the lattice parameter was studied theoretically. In local density
approximation (LDA), the minimum total energy of CoFeSi is found for the
experimental lattice parameter, but the calculated magnetic moment is about 12%
too low. Half-metallic ferromagnetism and a magnetic moment equal to the
experimental value of are found, however, only after increasing the
lattice parameter by more than 6%.
To overcome this discrepancy, the LDA scheme was used to respect on-site
electron correlation in the calculations. Those calculations revealed for
CoFeSi that an effective Coulomb-exchange interaction in the
range of about 2eV to 5eV leads to half-metallic ferromagnetism and the
measured, integer magnetic moment at the measured lattice parameter. Finally,
it is shown in the case of CoMnSi that correlation may also serve to
destroy the half-metallic behavior if it becomes too strong (for CoMnSi
above 2eV and for CoFeSi above 5eV). These findings indicate that on-site
correlation may play an important role in the description of Heusler compounds
with localized moments.Comment: submitted to Phys. Rev.
Epitaxial Co2Cr0.6Fe0.4Al thin films and magnetic tunneling junctions
Epitaxial thin films of the theoretically predicted half metal
Co2Cr0.6Fe0.4Al were deposited by dc magnetron sputtering on different
substrates and buffer layers. The samples were characterized by x-ray and
electron beam diffraction (RHEED) demonstrating the B2 order of the Heusler
compound with only a small partition of disorder on the Co sites. Magnetic
tunneling junctions with Co2Cr0.6Fe0.4Al electrode, AlOx barrier and Co counter
electrode were prepared. From the Julliere model a spin polarisation of
Co2Cr0.6Fe0.4Al of 54% at T=4K is deduced. The relation between the annealing
temperature of the Heusler electrodes and the magnitude of the tunneling
magnetoresistance effect was investigated and the results are discussed in the
framework of morphology and surface order based of in situ STM and RHEED
investigations.Comment: accepted by J. Phys. D: Appl. Phy
Exotic magnetism in the alkali sesquoxides Rb4O6 and Cs4O6
Among the various alkali oxides the sesquioxides Rb4O6 and Cs4O6 are of
special interest. Electronic structure calculations using the local
spin-density approximation predicted that Rb4O6 should be a half-metallic
ferromagnet, which was later contradicted when an experimental investigation of
the temperature dependent magnetization of Rb4O6 showed a low-temperature
magnetic transition and differences between zero-field-cooled (ZFC) and
field-cooled (FC) measurements. Such behavior is known from spin glasses and
frustrated systems. Rb4O6 and Cs4O6 comprise two different types of dioxygen
anions, the hyperoxide and the peroxide anions. The nonmagnetic peroxide anions
do not contain unpaired electrons while the hyperoxide anions contain unpaired
electrons in antibonding pi*-orbitals. High electron localization (narrow
bands) suggests that electronic correlations are of major importance in these
open shell p-electron systems. Correlations and charge ordering due to the
mixed valency render p-electron-based anionogenic magnetic order possible in
the sesquioxides. In this work we present an experimental comparison of Rb4O6
and the related Cs4O6. The crystal structures are verified using powder x-ray
diffraction. The mixed valency of both compounds is confirmed using Raman
spectroscopy, and time-dependent magnetization experiments indicate that both
compounds show magnetic frustration, a feature only previously known from d-
and f-electron systems
Probing Exfoliated Graphene Layers and Their Lithiation with Microfocused X-rays
X-ray diffraction is measured on individual bilayer and multilayer graphene single-crystals and combined with electrochemically induced lithium intercalation. In-plane Bragg peaks are observed by grazing incidence diffraction. Focusing the incident beam down to an area of about 10 μm × 10 μm, individual flakes are probed by specular X-ray reflectivity. By deploying a recursive Parratt algorithm to model the experimental data, we gain access to characteristic crystallographic parameters of the samples. Notably, it is possible to directly extract the bi/multilayer graphene c-axis lattice parameter. The latter is found to increase upon lithiation, which we control using an on-chip peripheral electrochemical cell layout. These experiments demonstrate the feasibility of in situ X-ray diffraction on individual, micron-sized single crystallites of few- and bilayer two-dimensional materials
Large Anomalous Hall and Nernst Effects in High Curie-Temperature Iron-Based Heusler Compounds
Abstract The interplay between topology and magnetism has recently sparked the frontier studies of magnetic topological materials that exhibit intriguing anomalous Hall and Nernst effects owning to the large intrinsic Berry curvature (BC). To better understand the anomalous quantum transport properties of these materials and their implications for future applications such as electronic and thermoelectric devices, it is crucial to discover more novel material platforms for performing anomalous transverse transport studies. Here, it is experimentally demonstrated that low-cost Fe-based Heusler compounds exhibit large anomalous Hall and Nernst effects. An anomalous Hall conductivity of 250?750 S cm?1 and Nernst thermopower of above 2 µV K?1 are observed near room temperature. The positive effect of anti-site disorder on the anomalous Hall transport is revealed. Considering the very high Curie temperature (nearly 1000 K), larger Nernst thermopowers at high temperatures are expected owing to the existing magnetic order and the intrinsic BC. This work provides a background for developing low-cost Fe-based Heusler compounds as a new material platform for anomalous transport studies and applications, in particular, near and above room temperature
Reverse Bisimulations on Stable Configuration Structures
The relationships between various equivalences on configuration structures,
including interleaving bisimulation (IB), step bisimulation (SB) and hereditary
history-preserving (HH) bisimulation, have been investigated by van Glabbeek
and Goltz (and later Fecher). Since HH bisimulation may be characterised by the
use of reverse as well as forward transitions, it is of interest to investigate
forms of IB and SB where both forward and reverse transitions are allowed. We
give various characterisations of reverse SB, showing that forward steps do not
add extra power. We strengthen Bednarczyk's result that, in the absence of
auto-concurrency, reverse IB is as strong as HH bisimulation, by showing that
we need only exclude auto-concurrent events at the same depth in the
configuration
Evidence for Localized Moment Picture in Mn-based Heusler Compounds
X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism
(XMCD) were used to probe the oxidation state and element specific magnetic
moments of Mn in Heusler compounds with different crystallographic structure.
The results were compared with theoretical calculations, and it was found that
in full Heusler alloys, Mn is metallic (oxidation state near 0) on both
sublattices. The magnetic moment is large and localized when octahedrally
coordinated by the main group element, consistent with previous theoretical
work, and reduced when the main group coordination is tetrahedral. By contrast,
in the half Heusler compounds the magnetic moment of the Mn atoms is large and
the oxidation state is +1 or +2. The magnetic and electronic properties of Mn
in full and half Heusler compounds are strongly dependent on the structure and
sublattice, a fact that can be exploited to design new materials.Comment: 15 pages, 4 figure
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