336 research outputs found
The half-metallic ferromagnet Co2Mn0.5Fe0.5Si
Electronic structure calculation were used to predict a new material for
spintronic applications. Co2Mn0.5Fe0.5Si is one example which is stable against
on-site correlation and disorder effects due to the position of the Fermi
energy in the middle of the minority band gap. Experimentally the sample were
made exhibiting L21 structure and a high magnetic order.Comment: 5 pages, 2 Figures, J. Magn. Magn. Mater. accepte
Slater-Pauling Rule and Curie-Temperature of Co-based Heusler compounds
A concept is presented serving to guide in the search for new materials with
high spin polarization. It is shown that the magnetic moment of half-metallic
ferromagnets can be calculated from the generalized Slater-Pauling rule.
Further, it was found empirically that the Curie temperature of Co based
Heusler compounds can be estimated from a seemingly linear dependence on the
magnetic moment. As a successful application of these simple rules, it was
found that CoFeSi is, actually, the half-metallic ferromagnet exhibiting
the highest magnetic moment and the highest Curie temperature measured for a
Heusler compound
Properties of the quaternary half-metal-type Heusler alloy CoMnFeSi
This work reports on the bulk properties of the quaternary Heusler alloy
CoMnFeSi with the Fe concentration . All samples, which
were prepared by arc melting, exhibit long range order over the complete
range of Fe concentration. Structural and magnetic properties of
CoMnFeSi Heusler alloys were investigated by means of X-ray
diffraction, high and low temperature magnetometry, M{\"o\ss}bauer
spectroscopy, and differential scanning calorimetry. The electronic structure
was explored by means of high energy photo emission spectroscopy at about 8 keV
photon energy. This ensures true bulk sensitivity of the measurements. The
magnetization of the Fe doped Heusler alloys is in agreement with the values of
the magnetic moments expected for a Slater-Pauling like behavior of
half-metallic ferromagnets. The experimental findings are discussed on the hand
of self-consistent calculations of the electronic and magnetic structure. To
achieve good agreement with experiment, the calculations indicate that on-site
electron-electron correlation must be taken into account, even at low Fe
concentration. The present investigation focuses on searching for the
quaternary compound where the half-metallic behavior is stable against outside
influences. Overall, the results suggest that the best candidate may be found
at an iron concentration of about 50%.Comment: 26 pages, 9 figures Phys. Rev. B accepte
Observation of the Fresnel and Arago laws using the Mach-Zehnder interferometer
An experimental study is conducted to determine the effect of polarization on the interference of light waves. By using the temporal coherence property of light in a Mach-Zehnder interferometer, we verified the four important Fresnel and Arago laws for linearly polarized and circularly polarized light. This experiment provides a simple method for undergraduates to study the phenomena of interference and polarization. (C) 2008 American Association of Physics Teachers
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
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.
Ferrimagnetism and disorder in epitaxial Mn(2-x)Co(x)VAl thin films
The quaternary full Heusler compound Mn(2-x)Co(x)VAl with x = 1 is predicted
to be a half-metallic antiferromagnet. Thin films of the quaternary compounds
with x = 0...2 were prepared by DC and RF magnetron co-sputtering on heated MgO
(001) substrates. The magnetic structure was examined by x-ray magnetic
circular dichroism and the chemical disorder was characterized by x-ray
diffraction. Ferrimagnetic coupling of V to Mn was observed for Mn2VAl (x = 0).
For x = 0.5, we also found ferrimagnetic order with V and Co antiparallel to
Mn. The observed reduced magnetic moments are interpreted with the help of band
structure calculations in the coherent potential approximation. Mn2VAl is very
sensitive to disorder involving Mn, because nearest-neighbor Mn atoms couple
anti-ferromagnetically. Co2VAl has B2 order and has reduced magnetization. In
the cases with x >= 0.9 conventional ferromagnetism was observed, closely
related to the atomic disorder in these compounds.Comment: 10 pages, 4 figure
First-principles indicators of metallicity and cation off-centricity in the IV-VI rocksalt chalcogenides of divalent Ge, Sn, and Pb
We use first-principles density functional theory to calculate the phonon frequencies, electron localization lengths, Born effective charges, dielectric response, and conventional electronic structures of the IV-VI chalcogenide series. The goals of our work are twofold: first, to determine the detailed chemical composition of lone pairs and, second, to identify the factors that cause lone pairs to favor high- or low-symmetry environments. Our results show that the traditional picture of cation s-p mixing causing localization of the lone pair lobe is incomplete, and instead the p states on the anion also play an important role. In addition these compounds reveal a delicate balance between two competing instabilities-structural distortion and tendency to metallicity-leading, at the same time, to anomalously large Born effective charges as well as large dielectric constants. The magnitude of the LO-TO splitting, which depends on the relative strength of both instabilities, shows a trend consistent with the structural distortions in these compounds
Kitaev interactions between j=1/2 moments in honeycomb Na2IrO3 are large and ferromagnetic: insights from ab initio quantum chemistry calculations
NaIrO, a honeycomb 5 oxide, has been recently identified as a
potential realization of the Kitaev spin lattice. The basic feature of this
spin model is that for each of the three metal-metal links emerging out of a
metal site, the Kitaev interaction connects only spin components perpendicular
to the plaquette defined by the magnetic ions and two bridging ligands. The
fact that reciprocally orthogonal spin components are coupled along the three
different links leads to strong frustration effects and nontrivial physics.
While the experiments indicate zigzag antiferromagnetic order in NaIrO,
the signs and relative strengths of the Kitaev and Heisenberg interactions are
still under debate. Herein we report results of ab initio many-body electronic
structure calculations and establish that the nearest-neighbor exchange is
strongly anisotropic with a dominant ferromagnetic Kitaev part, whereas the
Heisenberg contribution is significantly weaker and antiferromagnetic. The
calculations further reveal a strong sensitivity to tiny structural details
such as the bond angles. In addition to the large spin-orbit interactions, this
strong dependence on distortions of the IrO plaquettes singles out the
honeycomb 5 oxides as a new playground for the realization of
unconventional magnetic ground states and excitations in extended systems.Comment: 13 pages, 2 tables, 3 figures, accepted in NJ
Effect of Bleaching on Color Change and Surface Topography of Composite Restorations
This study was conducted to determine the effect of 15% carbamide peroxide bleaching agent on color change and surface topography of different composite veneering materials (Filtek Z350 (3M ESPE), Esthet X (Dentsply India), and Admira (Voco, Germany). Methods. 30 samples were fabricated for evaluation of color change using CIELAB color system and Gonioreflectometer (GK 311/M, ZEISS). 45 disc-shaped specimens were made for evaluation of surface topography after bleaching (Nupro White Gold; Dentsply) using SEM. Statistical analysis. One way ANOVA and Multiple comparison tests were used to analyze the data. Statistical significance was declared if the P value was .05 or less. Results and conclusion. All the specimens showed significant discoloration (ΔE > 3.3) after their immersion in solutions representing food and beverages. The total color change after bleaching as compared to baseline color was significant in Filtek Z350 (P = .000) and Esthet X (P = .002), while it was insignificant for Admira (P = .18). Esthet X showed maximum surface roughness followed by Admira and Filtek Z350. Bleaching was effective in reducing the discoloration to a clinically acceptable value in all the three groups (ΔE < 3.3)
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