71 research outputs found
3d-electron induced magnetic phase transition in half-metallic semi-Heusler alloys
We study the effect of the non-magnetic 3\textit{d} atoms on the magnetic
properties of the half-metallic (HM) semi-Heusler alloys CoCuMnSb
and NiCuMnSb () using first-principles
calculations. We determine the magnetic phase diagram of both systems at zero
temperature and obtain a phase transition from a ferromagnetic to an
antiferromagnetic state. For low Cu concentrations the ferromagnetic RKKY-like
exchange mechanism is dominating, while the antiferromagnetic superexchange
coupling becomes important for larger Cu content leading to the observed
magnetic phase transition. A strong dependence of the magnetism in both systems
on the position of the Fermi level within the HM gap is obtained. Obtained
results are in good agreement with the available experimental data
Effect of Local Electron-Electron Correlation in Hydrogen-like Impurities in Ge
We have studied the electronic and local magnetic structure of the hydrogen
interstitial impurity at the tetrahedral site in diamond-structure Ge, using an
empirical tight binding + dynamical mean field theory approach because within
the local density approximation (LDA) Ge has no gap. We first establish that
within LDA the 1s spectral density bifurcates due to entanglement with the four
neighboring sp3 antibonding orbitals, providing an unanticipated richness of
behavior in determining under what conditions a local moment hyperdeep donor or
Anderson impurity will result, or on the other hand a gap state might appear.
Using a supercell approach, we show that the spectrum, the occupation, and the
local moment of the impurity state displays a strong dependence on the strength
of the local on-site Coulomb interaction U, the H-Ge hopping amplitude, the
depth of the bare 1s energy level epsilon_H, and we address to some extent the
impurity concentration dependence. In the isolated impurity, strong interaction
regime a local moment emerges over most of the parameter ranges indicating
magnetic activity, and spectral density structure very near (or in) the gap
suggests possible electrical activity in this regime.Comment: 9 pages, 5 figure
Ab-initio determined electronic and magnetic properties of half-metallic NiCrSi and NiMnSi Heusler alloys; the role of interfaces and defects
Using state-of-the-art first-principles calculations we study the properties
of the ferromagnetic Heusler compounds NiYSi where Y stands for V, Cr or Mn.
NiCrSi and NiMnSi contrary to NiVSi are half-metallic at their equilibrium
lattice constant exhibiting integer values of the total spin magnetic moment
and thus we concentrate on these two alloys. The minority-spin gap has the same
characteristics as for the well-known NiMnSb alloy being around 1 eV.
Upon tetragonalization the gap is present in the density of states even for
expansion or contraction of the out-of-plane lattice parameter by 5%. The Cr-Cr
and Mn-Mn interactions make ferromagnetism extremely stable and the Curie
temperature exceeds 1000 K for NiMnSi. Surface and interfaces with GaP, ZnS and
Si semiconductors are not half-metallic but in the case of NiCrSi the Ni-based
contacts present spin-polarization at the Fermi level over 90%. Finally, we
show that there are two cases of defects and atomic-swaps. The first-ones which
involve the Cr(Mn) and Si atoms induce states at the edges of the gap which
persists for a moderate-concentration of defects. Defects involving Ni atoms
induce states localized within the gap completely destroying the
half-metallicity. Based on single-impurity calculations we associate these
states to the symmetry of the crystal
Exchange interactions and temperature dependence of the magnetization in half--metallic Heusler alloys
We study the exchange interactions in half-metallic Heusler alloys using
first-principles calculations in conjunction with the frozen-magnon
approximation. The Curie temperature is estimated within both mean-field (MF)
and random-phase-approximation (RPA) approaches. For the half-Heusler alloys
NiMnSb and CoMnSb the dominant interaction is between the nearest Mn atoms. In
this case the MF and RPA estimations differ strongly. The RPA approach provides
better agreement with experiment. The exchange interactions are more complex in
the case of full-Heusler alloys CoMnSi and CoCrAl where the dominant
effects are the inter-sublattice interactions between the Mn(Cr) and Co atoms
and between Co atoms at different sublattices. For these compounds we find that
both MF and RPA give very close values of the Curie temperature slightly
underestimating experimental quantities. We study the influence of the lattice
compression on the magnetic properties. The temperature dependence of the
magnetization is calculated using the RPA method within both quantum mechanical
and classical approaches.Comment: New figures and discussio
Wannier Function Approach to Realistic Coulomb Interactions in Layered Materials and Heterostructures
We introduce an approach to derive realistic Coulomb interaction terms in
free standing layered materials and vertical heterostructures from ab-initio
modelling of the corresponding bulk materials. To this end, we establish a
combination of calculations within the framework of the constrained random
phase approximation, Wannier function representation of Coulomb matrix elements
within some low energy Hilbert space and continuum medium electrostatics, which
we call Wannier function continuum electrostatics (WFCE). For monolayer and
bilayer graphene we reproduce full ab-initio calculations of the Coulomb matrix
elements within an accuracy of eV or better. We show that realistic
Coulomb interactions in bilayer graphene can be manipulated on the eV scale by
different dielectric and metallic environments. A comparison to electronic
phase diagrams derived in [M. M. Scherer et al., Phys. Rev. B 85, 235408
(2012)] suggests that the electronic ground state of bilayer graphene is a
layered antiferromagnet and remains surprisingly unaffected by these strong
changes in the Coulomb interaction.Comment: 12 pages, 8 figure
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
First principles design of Ohmic spin diodes based on quaternary Heusler compounds
The Ohmic spin diode (OSD) is a recent concept in spintronics, which is based
on half-metallic magnets (HMMs) and spin-gapless semiconductors (SGSs).
Quaternary Heusler compounds offer a unique platform to realize the OSD for
room temperature applications as these materials possess very high Curie
temperatures as well as half-metallic and spin-gapless semiconducting behavior
within the same family. Using state-of-the-art first-principles calculations
combined with the non-equilibrium Green's function method we design four
different OSDs based on half-metallic and spin-gapless semiconducting
quaternary Heusler compounds. All four OSDs exhibit linear current-voltage
() characteristics with zero threshold voltage . We show that these
OSDs possess a small leakage current, which stems from the overlap of the
conduction and valence band edges of opposite spin channels around the Fermi
level in the SGS electrodes. The obtained on/off current ratios vary between
and . Our results can pave the way for the experimental fabrication
of the OSDs within the family of ordered quaternary Heusler compounds.Comment: 7 pages, 5 figure
Study of Magnetic Tunnel Junctions Based on Half-Metallic and Spin-Gapless Semiconducting Heusler Compounds: Reconfigurable Diode and Inverse Tunnel-Magnetoresistance Effect
Magnetic tunnel junctions (MTJs) have attracted strong research interest
within the last decades due to their potential use as nonvolatile memory such
as MRAM as well as for magnetic logic applications. Half-metallic magnets
(HMMs) have been suggested as ideal electrode materials for MTJs to achieve an
extremely large tunnel-magnetoresistance (TMR) effect. Despite their high TMR
ratios, MTJs based on HMMs do not exhibit current rectification, i.e., a diode
effect, which was achieved in a magnetic tunnel junction concept based on HMMs
and type-II spin-gapless semiconductors (SGSs). The proposed concept has
recently been experimentally demonstrated using Heusler compounds. In the
present work, we investigate from first-principles MTJs based on type-II SGS
and HMM quaternary Heusler compounds FeVTaAl, FeVTiSi, MnVTiAl, and CoVTiSb.
Our quantum transport calculations based on a nonequilibrium
Green's function method have demonstrated that the MTJs under consideration
exhibit current rectification with relatively high on:off ratios. We show that,
in contrast to conventional semiconductor diodes, the rectification bias
voltage window (or breakdown voltage) of the MTJs is limited by the spin gap of
the HMM and SGS Heusler compounds. A unique feature of the present MTJs is that
the diode effect can be configured dynamically, i.e., depending on the relative
orientation of the magnetization of the electrodes, the MTJ allows the
electrical current to pass either in one or the other direction, which leads to
an inverse TMR effect. The combination of nonvolatility, reconfigurable diode
functionality, tunable rectification voltage window, and high Curie temperature
of the electrode materials makes the proposed MTJs very promising for
room-temperature spintronic applications and opens ways to magnetic memory and
logic concepts as well as logic-in-memory computing.Comment: 14+7 pages, 7+10 figure
Voids-driven breakdown of the local-symmetry and Slater-Pauling rule in half-metallic Heusler compounds
Slater-Pauling (SP) rules connect the magnetic and electronic properties of the half-metallic (HM) Heuslercompounds. Employing first-principles electronic structure calculations we explore the validity of the SP rulesin the case of transition from HM semi-Heusler compounds to various cases of HM full-Heusler compounds.We show that the coherent-potential approximation yields half-metallicity and thus a generalized version of theSP rules can be derived. On the contrary, supercell calculations, which are expected to describe the experimentalsituation more accurately, show that the energy gap considerably shrinks for the intermediate compounds and inseveral cases the half-metallicity is completely destroyed. The origin of this behavior is attributed to the voids,which influence the symmetry of the lattice
Role of the conduction electrons in mediating exchange interactions in Heusler alloys
Because of large spatial separation of the Mn atoms in Heusler alloys the Mn
3d states belonging to different atoms do not overlap considerably. Therefore
an indirect exchange interaction between Mn atoms should play a crucial role in
the ferromagnetism of the systems. To study the nature of the ferromagnetism of
various Mn-based semi- and full-Heusler alloys we perform a systematic
first-principles calculation of the exchange interactions in these materials.
The calculation of the exchange parameters is based on the frozen-magnon
approach. The calculations show that the magnetism of the Mn-based Heusler
alloys depends strongly on the number of conduction electrons, their spin
polarization and the position of the unoccupied Mn 3d states with respect to
the Fermi level. Various magnetic phases are obtained depending on the
combination of these characteristics. The Anderson's s-d model is used to
perform a qualitative analysis of the obtained results. The conditions leading
to diverse magnetic behavior are identified. If the spin polarization of the
conduction electrons at the Fermi energy is large and the unoccupied Mn 3d
states lie well above the Fermi level, an RKKY-type ferromagnetic interaction
is dominating. On the other hand, the contribution of the antiferromagnetic
superexchange becomes important if unoccupied Mn 3d states lie close to the
Fermi energy. The resulting magnetic behavior depends on the competition of
these two exchange mechanisms. The calculational results are in good
correlation with the conclusions made on the basis of the Anderson s-d model
which provides useful framework for the analysis of the results of
first-principles calculations and helps to formulate the conditions for high
Curie temperature.Comment: 16 pages, 9 figures, 2 table
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