313 research outputs found
Spin injection from Fe into Si(001): ab initio calculations and role of the Si complex band structure
We study the possibility of spin injection from Fe into Si(001), using the
Schottky barrier at the Fe/Si contact as tunneling barrier. Our calculations
are based on density-functional theory for the description of the electronic
structure and on a Landauer-Buttiker approach for the current. The
current-carrying states correspond to the six conduction band minima of Si,
which, when projected on the (001) surface Brillouin zone (SBZ), form five
conductance hot spots: one at the SBZ center and four symmetric satellites. The
satellites yield a current polarization of about 50%, while the SBZ center can,
under very low gate voltage, yield up to almost 100%, showing a zero-gate
anomaly. This extremely high polarization is traced back to the symmetry
mismatch of the minority-spin Fe wavefunctions to the conduction band
wavefunctions of Si at the SBZ center. The tunneling current is determined by
the complex band structure of Si in the [001] direction, which shows
qualitative differences compared to that of direct-gap semiconductors.
Depending on the Fermi level position and Schottky barrier thickness, the
complex band structure can cause the contribution of the satellites to be
orders of magnitude higher or lower than the central contribution. Thus, by
appropriate tuning of the interface properties, there is a possibility to cut
off the satellite contribution and to reach high injection efficiency. Also, we
find that a moderate strain of 0.5% along the [001] direction is sufficient to
lift the degeneracy of the pockets so that only states at the zone center can
carry current
Non-collinear Korringa-Kohn-Rostoker Green function method: Application to 3d nanostructures on Ni(001)
Magnetic nanostructures on non-magnetic or magnetic substrates have attracted
strong attention due to the development of new experimental methods with atomic
resolution. Motivated by this progress we have extended the full-potential
Korringa-Kohn-Rostoker (KKR) Green function method to treat non-collinear
magnetic nanostructures on surfaces. We focus on magnetic 3d impurity
nanoclusters, sitting as adatoms on or in the first surface layer on Ni(001),
and investigate the size and orientation of the local moments and moreover the
stabilization of non-collinear magnetic solutions. While clusters of Fe, Co, Ni
atoms are magnetically collinear, non-collinear magnetic coupling is expected
for Cr and Mn clusters on surfaces of elemental ferromagnets. The origin of
frustration is the competition of the antiferromagnetic exchange coupling among
the Cr or Mn atoms with the antiferromagnetic (for Cr) or ferromagnetic (for
Mn) exchange coupling between the impurities and the substrate. We find that Cr
and Mn first-neighbouring dimers and a Mn trimer on Ni(001) show non-collinear
behavior nearly degenerate with the most stable collinear configuration.
Increasing the distance between the dimer atoms leads to a collinear behavior,
similar to the one of the single impurities. Finally, we compare some of the
non-collinear {\it ab-initio} results to those obtained within a classical
Heisenberg model, where the exchange constants are fitted to total energies of
the collinear states; the agreement is surprisingly good.Comment: 11 page
First-principles calculations of exchange interactions, spin waves, and temperature dependence of magnetization in inverse-Heusler-based spin gapless semiconductors
Employing first principles electronic structure calculations in conjunction
with the frozen-magnon method we calculate exchange interactions, spin-wave
dispersion, and spin-wave stiffness constants in inverse-Heusler-based spin
gapless semiconductor (SGS) compounds MnCoAl, TiMnAl, CrZnSi,
TiCoSi and TiVAs. We find that their magnetic behavior is similar to
the half-metallic ferromagnetic full-Heusler alloys, i.e., the intersublattice
exchange interactions play an essential role in the formation of the magnetic
ground state and in determining the Curie temperature, . All
compounds, except TiCoSi possess a ferrimagnetic ground state. Due to the
finite energy gap in one spin channel, the exchange interactions decay sharply
with the distance, and hence magnetism of these SGSs can be described
considering only nearest and next-nearest neighbor exchange interactions. The
calculated spin-wave dispersion curves are typical for ferrimagnets and
ferromagnets. The spin-wave stiffness constants turn out to be larger than
those of the elementary 3-ferromagnets. Calculated exchange parameters are
used as input to determine the temperature dependence of the magnetization and
of the SGSs. We find that the of all compounds is
much above the room temperature. The calculated magnetization curve for
MnCoAl as well as the Curie temperature are in very good agreement with
available experimental data. The present study is expected to pave the way for
a deeper understanding of the magnetic properties of the inverse-Heusler-based
SGSs and enhance the interest in these materials for application in spintronic
and magnetoelectronic devices.Comment: Accepted for publ;ication in Physical Review
Lifetime reduction of surface states at Cu, Ag and Au(111) caused by impurity scattering
We present density-functional results on the lifetime of the (111) surface
state of the noble metals. We consider scattering on the Fermi surface caused
by impurity atoms belonging to the 3d and 4sp series. The results are analyzed
with respect to film thickness and with respect to separation of scattering
into bulk or into surface states. While for impurities in the surface layer the
overall trends are similar to the long-known bulk-state scattering, for
adatom-induced scattering we find a surprising behavior with respect to the
adatom atomic number. A plateau emerges in the scattering rate of the 3d
adatoms, instead of a peak characteristic of the d resonance. Additionally, the
scattering rate of 4sp adatoms changes in a zig-zag pattern, contrary to a
smooth parabolic increase following Linde's rule that is observed in bulk. We
interpret these results in terms of the weaker charge-screening and of
interference effects induced by the lowering of symmetry at the surface
Half-metallic ferromagnets for magnetic tunnel junctions
Using theoretical arguments, we show that, in order to exploit half-metallic
ferromagnets in tunneling magnetoresistance (TMR) junctions, it is crucial to
eliminate interface states at the Fermi level within the half-metallic gap;
contrary to this, no such problem arises in giant magnetoresistance elements.
Moreover, based on an a priori understanding of the electronic structure, we
propose an antiferromagnetically coupled TMR element, in which interface states
are eliminated, as a paradigm of materials design from first principles. Our
conclusions are supported by ab-initio calculations
Light curing time reduction: in vitro evaluation of new intensive light-emitting diode curing units
The aim of the present in vitro study was to establish the minimum necessary curing time to bond stainless steel brackets (Mini Diamond Twin™) using new, intensive, light-emitting diode (LED) curing units. Seventy-five bovine primary incisors were divided into five equal groups. A standard light curing adhesive (Transbond™ XT) was used to bond the stainless steel brackets using different lamps and curing times. Two groups were bonded using an intensive LED curing lamp (Ortholux™ LED) for 5 and 10 seconds. Two more groups were bonded using another intensive LED curing device (Ultra-Lume™ LED 5) also for 5 and 10 seconds. Finally, a high-output halogen lamp (Optilux™ 501) was used for 40 seconds to bond the final group, which served as a positive control. All teeth were fixed in hard acrylic and stored for 24 hours in water at 37°C. Shear bond strength (SBS) was measured using an Instron testing machine. Weibull distribution and analysis of variance were used to test for significant differences. The SBS values obtained were significantly different between groups (P < 0.001). When used for 10 seconds, the intensive LED curing units achieved sufficient SBS, comparable with the control. In contrast, 5 seconds resulted in significantly lower SBS. The adhesive remnant index (ARI) was not significantly affected. A curing time of 10 seconds was found to be sufficient to bond metallic brackets to incisors using intensive LED curing units. These new, comparatively inexpensive, curing lamps seem to be an advantageous alternative to conventional halogen lamps for bonding orthodontic bracket
Tuning the Curie temperature of FeCo compounds by tetragonal distortion
Combining density-functional theory calculations with a classical Monte Carlo
method, we show that for B2-type FeCo compounds tetragonal distortion gives
rise to a strong reduction of the Curie temperature . The
monotonically decreases from 1575 K (for ) to 940 K
(for c/a=\sqrtwo). We find that the nearest neighbor Fe-Co exchange
interaction is sufficient to explain the behavior of the
. Combination of high magnetocrystalline anisotropy energy with
a moderate value suggests tetragonal FeCo grown on the Rh
substrate with to be a promising material for heat-assisted magnetic
recording applications.Comment: 4 pages, 2 figure
Kondo decoherence: finding the right spin model for iron impurities in gold and silver
We exploit the decoherence of electrons due to magnetic impurities, studied
via weak localization, to resolve a longstanding question concerning the
classic Kondo systems of Fe impurities in the noble metals gold and silver:
which Kondo-type model yields a realistic description of the relevant multiple
bands, spin and orbital degrees of freedom? Previous studies suggest a fully
screened spin Kondo model, but the value of remained ambiguous. We
perform density functional theory calculations that suggest . We also
compare previous and new measurements of both the resistivity and decoherence
rate in quasi 1-dimensional wires to numerical renormalization group
predictions for and 3/2, finding excellent agreement for .Comment: 4 pages, 4 figures, shortened for PR
Magnetic tunneling junctions with the Heusler compound Co_2Cr_{0.6}Fe_{0.4}Al
The Heusler alloy is used as an electrode of magnetic tunneling junctions.
The junctions are deposited by magnetron dc sputtering using shadow mask
techniques with AlO_{x} as a barrier and cobalt as counter electrode.
Measurements of the magnetoresistive differential conductivity in a temperature
range between 4K and 300K are shown. An analysis of the barrier properties
applying the Simmons model to the bias dependent junction conductivity is
performed. VSM measurements were carried out to examine the magnetic properties
of the samples.Comment: 3 pages, 3 figures submitted to JMMM (proceedings of JEMS04
Half-metallic ferromagnetism induced by dynamic electron correlations in VAs
The electronic structure of the VAs compound in the zinc-blende structure is
investigated using a combined density-functional and dynamical mean-field
theory approach. Contrary to predictions of a ferromagnetic semiconducting
ground state obtained by density-functional calculations, dynamical
correlations induce a closing of the gap and produce a half-metallic
ferromagnetic state. These results emphasize the importance of dynamic
correlations in materials suitable for spintronics.Comment: Published in Phys. Rev. Lett. 96, 197203 (2006
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