191 research outputs found
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
Simulation of the enhanced Curie temperature in Mn_5Ge_3C_x compounds
Mn_5Ge_3C_x films with x>0.5 were experimentally shown to exhibit a strongly
enhanced Curie temperature T_C compared to Mn_5Ge_3. In this letter we present
the results of our first principles calculations within Green's function
approach, focusing on the effect of carbon doping on the electronic and
magnetic properties of the Mn_5Ge_3. The calculated exchange coupling constants
revealed an enhancement of the ferromagnetic Mn-Mn interactions mediated by
carbon. The essentially increased T_C in Mn_5Ge_3C is well reproduced in our
Monte Carlo simulations and together with the decrease of the total
magnetisation is found to be predominantly of an electronic nature
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
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
Changing the Magnetic Configurations of Nanoclusters Atom-by-Atom
The Korringa-Kohn-Rostoker Green (KKR) function method for non-collinear
magnetic structures was applied on Mn and Cr ad-clusters deposited on the
Ni(111) surface. By considering various dimers, trimers and tetramers, a large
amount of collinear and non-collinear magnetic structures is obtained.
Typically all compact clusters have very small total moments, while the more
open structures exhibit sizeable total moments, which is a result of the
complex frustration mechanism in these systems. Thus, as the motion of a single
adatom changes the cluster structure from compact to open and vice versa, this
can be considered as a magnetic switch, which via the local exchange field of
the adatom allows to switch the cluster moment on and off, and which might be
useful for future nanosize information storage.Comment: 7 page
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
Structural and magnetic properties of Fe/ZnSe(001) interfaces
We have performed first principles electronic structure calculations to
investigate the structural and magnetic properties of Fe/ZnSe(001) interfaces.
Calculations involving full geometry optimizations have been carried out for a
broad range of thickness of Fe layers(0.5 monolayer to 10 monolayers) on top of
a ZnSe(001) substrate. Both Zn and Se terminated interfaces have been explored.
Total energy calculations show that Se segregates at the surface which is in
agreement with recent experiments.
For both Zn and Se terminations, the interface Fe magnetic moments are higher
than the bulk bcc Fe moment.
We have also investigated the effect of adding Fe atoms on top of a
reconstructed ZnSe surface to explore the role of reconstruction of
semiconductor surfaces in determining properties of metal-semiconductor
interfaces. Fe breaks the Se dimer bond formed for a Se-rich (2x1)
reconstructed surface. Finally, we looked at the reverse growth i.e. growth of
Zn and Se atoms on a bcc Fe(001) substrate to investigate the properties of the
second interface of a magnetotunnel junction. The results are in good agreement
with the theoretical and experimental results, wherever available.Comment: 7 pages, 8 figures, accepted for publication in PR
Electron spin operation by electric fields: spin dynamics and spin injection
Spin-orbit interaction couples electron spins to electric fields and allows
electrical monitoring of electron spins and electrical detection of spin
dynamics. Competing mechanisms of spin-orbit interaction are compared, and
optimal conditions for the electric operation of electrons spins in a quantum
well by a gate voltage are established. Electric spin injection into
semiconductors is discussed with a special emphasis on the injection into
ballistic microstructures. Dramatic effect of a long range Coulomb interaction
on transport phenomena in space-quantized low-dimensional conductors is
discussed in conclusion.Comment: A plenary paper at the 11th Intern. Conf. on Narrow Gap
Semiconductors (Buffalo, NY, June 2003). To be published in Physica
A Hierarchical Profiler of Intermediate Representation Code based on LLVM
Profiling based techniques have gained much attention on computer architecture and software analysis communities. The target is to rely on one or more profiling tools in order to identify specific code pieces of interest e.g., code pieces that slowdown a given application. The extracted code pieces can be further modified and optimized. In general, the profiling tools can be classified as deterministic, statistical-based, or rely on hardware performance counters. A common characteristic of the available profiling tools is typically based on analyzing or even manipulating (in case of binary instrumentation tools) machine-level code. This approach come with two main drawbacks. First, a lot of information (even GBytes of data) needs to be gathered, stored, post-processed, and visualized. Second, the performed analysis of the gathered data is platform-specific and it is not straightforward to categorize the given applications/program phases/kernels into distinct categories that have the same or almost the same behavior (e.g., the same percentage of computational vs. control instructions). The latter stems from the fact even small changes in the source code of the applications might lead to significantly different machine code implementations. Therefore, even two specific program kernels exhibit the same behavior (e.g., they have the same number of instructions, but with a different ordering), it is very difficult for a machine-code level profiling tool to assess their similarity, simply because the generated machine level code might have significant differences resulting in many missing opportunities for the available profiling tools. To address this issue, in this paper, we present a new profiling tool that is able to operate on the machine independent intermediate representation (IR) level. The profiler (still in development phase) relies on the LLVM API and it is able to hierarchically (at various levels of the call stack) and recursively parse the IR code and extract various useful statistics. We showcase the practicality of our profiler by analyzing a subset of the PolyBench benchmarks assuming (as pointed out by a recent study) that there is a strong correlation of LLVM IR code
Nonmonotonic inelastic tunneling spectra due to surface spin excitations in ferromagnetic junctions
The paper addresses inelastic spin-flip tunneling accompanied by surface spin
excitations (magnons) in ferromagnetic junctions. The inelastic tunneling
current is proportional to the magnon density of states which is
energy-independent for the surface waves and, for this reason, cannot account
for the bias-voltage dependence of the observed inelastic tunneling spectra.
This paper shows that the bias-voltage dependence of the tunneling spectra can
arise from the tunneling matrix elements of the electron-magnon interaction.
These matrix elements are derived from the Coulomb exchange interaction using
the itinerant-electron model of magnon-assisted tunneling. The results for the
inelastic tunneling spectra, based on the nonequilibrium Green's function
calculations, are presented for both parallel and antiparallel magnetizations
in the ferromagnetic leads.Comment: 9 pages, 4 figures, version as publishe
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