609 research outputs found
Role of defects and disorder in the half-metallic full-Heusler compounds
Half-metallic ferromagnets and especially the full-Heusler alloys containing
Co are at the center of scientific research due to their potential applications
in spintronics. For realistic devices it is important to control accurately the
creation of defects in these alloys. We review some of our late results on the
role of defects and impurities in these compounds. More precisely we present
results for the following cases (i) doping and disorder in CoCr(Mn)Al(Si)
alloys, (ii) half-metallic ferrimagnetism appeared due to the creation of
Cr(Mn) antisites in these alloys, (iii) Co-doping in MnVAl(Si) alloys
leading to half-metallic antiferromagnetism, and finally (iv) the occurrence of
vacancies in the full-Heusler alloys containing Co and Mn. These results are
susceptible of encouraging further theoretical and experimental research in the
properties of these compounds.Comment: Chapter intended for a book with contributions of the invited
speakers of the International Conference on Nanoscale Magnetism 2007. Revised
version contains new figure
Nodal-antinodal dichotomy from pairing disorder in d-wave superconductors
We study the basic features of the local density of states (LDOS) observed in
STM experiments on high-T d-wave superconductors in the context of a
minimal model of a d-wave superconductor which has {\it weakly} modulated
off-diagonal disorder. We show that the low and high energy features of the
LDOS are consistent with the observed experimental patterns and in particular,
the anisotropic local domain features at high energies. At low energies, we
obtain not only the scattering peaks predicted by the octet model, but also
weak features that should be experimentally accessible. Finally, we show that
the emerging features of the LDOS lose their correspondence with the features
of the imposed disorder, as its complexity increases spatially
A first-principles DFT+GW study of spin-filter and spin-gapless semiconducting Heusler compounds
Among Heusler compounds, the ones being magnetic semiconductors (also known
as spin-filter materials) are widely studied as they offer novel
functionalities in spintronic/magnetoelectronic devices. The spin-gapless
semiconductors are a special case. They possess a zero or almost-zero energy
gap in one of the two spin channels. We employ the approximation, which
allows an elaborate treatment of the electronic correlations, to simulate the
electronic band structure of these materials. Our results suggest that in most
cases the use of self energy instead of the usual density functionals is
important to accurately determine the electronic properties of magnetic
semiconductors.Comment: Final version as publishe
Stability of ferromagnetism in the half-metallic pnictides and similar compounds: A first-principles study
Based on first-principles electron structure calculations and employing the
frozen-magnon approximation we study the exchange interactions in a series of
transition-metal binary alloys crystallizing in the zinc-blende structure and
calculate the Curie temperature within both the mean-field approximation (MFA)
and random-phase approximation (RPA). We study two Cr compounds, CrAs and CrSe,
and four Mn compounds: MnSi, MnGe, MnAs and MnC. MnC, MnSi and MnGe are
isovalent to CrAs and MnAs is isoelectronic with CrSe. Ferromagnetism is
particular stable for CrAs, MnSi and MnGe: All three compounds show Curie
temperatures around 1000 K. On the other hand, CrSe and MnAs show a tendency to
antiferromagnetism when compressing the lattice. In MnC the half-metallic gap
is located in the majority-spin channel contrary to the other five compounds.
The large half-metallic gaps, very high Curie temperatures, the stability of
the ferromagnetism with respect to the variation of the lattice parameter and a
coherent growth on semiconductors make MnSi and CrAs most promising candidates
for the use in spintronics devises.Comment: 17 pages, 6 figure
Broken-Bond Rule for the Surface Energies of Noble Metals
Using two different full-potential ab-initio techniques we introduce a
simple, universal rule based on the number of broken first-neighbor bonds to
determine the surface energies of the three noble metals Cu, Ag and Au. When a
bond is broken, the rearrangement of the electronic charge for these metals
does not lead to a change of the remaining bonds. Thus the energy needed to
break a bond is independent of the surface orientation. This novel finding can
lead to the development of simple models to describe the energetics of a
surface like step and kink formation, crystal growth, alloy formation,
equilibrium shape of mesoscopic crystallites and surface faceting.Comment: 4 pages, 2 figure
Quasiparticle band structure of the almost-gapless transition-metal-based Heusler semiconductors
Transition-metal-based Heusler semiconductors are promising materials for a
variety of applications ranging from spintronics to thermoelectricity.
Employing the approximation within the framework of the FLAPW method, we
study the quasi-particle band structure of a number of such compounds being
almost gapless semiconductors. We find that in contrast to the
\textit{sp}-electron based semiconductors such as Si and GaAs, in these systems
the many-body corrections have a minimal effect on the electronic band
structure and the energy band gap increases by less than 0.2~eV, which makes
the starting point density functional theory (DFT) a good approximation for the
description of electronic and optical properties of these materials.
Furthermore, the band gap can be tuned either by the variation of the lattice
parameter or by the substitution of the \emph{sp}-chemical element
Search for spin gapless semiconductors: The case of inverse Heusler compounds
We employ ab-initio electronic structure calculations to search for spin
gapless semiconductors, a recently identified new class of materials, among the
inverse Heusler compounds. The occurrence of this property is not accompanied
by a general rule and results are materials specific. The six compounds
identified show semiconducting behavior concerning the spin-down band structure
and in the spin-up band structure the valence and conduction bands touch each
other leading to 100% spin-polarized carriers. Moreover these six compounds
should exhibit also high Curie temperatures and thus are suitable for
spintronics applications.Comment: Submitted to Applied Physics Letter
Structural and magnetic properties of the (001) and (111) surfaces of the half-metal NiMnSb
Using the full potential linearised augmented planewave method we study the
electronic and magnetic properties of the (001) and (111) surfaces of the
half-metallic Heusler alloy NiMnSb from first-principles. We take into account
all possible surface terminations including relaxations of these surfaces.
Special attention is paid to the spin-polarization at the Fermi level which
governs the spin-injection from such a metal into a semiconductor. In general,
these surfaces lose the half-metallic character of the bulk NiMnSb, but for the
(111) surfaces this loss is more pronounced. Although structural optimization
does not change these features qualitatively, specifically for the (111)
surfaces relaxations can compensate much of the spin-polarization at the Fermi
surface that has been lost upon formation of the surface.Comment: 18 pages, 8 figure
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
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