284 research outputs found
Scanning tunneling microscopy of surfaces of half-metals: an ab-initio study on NiMnSb(001)
We present a first-principles study of the unreconstructed (001) surfaces of
the half-metallic ferromagnet NiMnSb. Both terminations (MnSb and Ni) are
considered. We find that half-metallicity is lost at the surfaces. After a
discussion of the geometric relaxations and the spin-polarized surface band
structure, we focus on topography images which are expected to be found with
spin-polarized scanning tunneling microscopy. For the MnSb-terminated surface
we find that only the Sb atoms are visible, reflecting a geometric buckling
caused by relaxations. For the Ni-terminated surface we find a strong contrast
between the images of forward and reverse tip-sample-bias of 0.5 eV, as well as
a stripe-like image for reverse bias. We interpret these findings in terms of
highly directional surface states which are formed in the spin-down gap region.Comment: Submitted to J. Phys. D: Appl. Phy
Oxygen-enabled control of Dzyaloshinskii-Moriya Interaction in ultra-thin magnetic films
The search for chiral magnetic textures in systems lacking spatial inversion
symmetry has attracted a massive amount of interest in the recent years with
the real space observation of novel exotic magnetic phases such as skyrmions
lattices, but also domain walls and spin spirals with a defined chirality. The
electrical control of these textures offers thrilling perspectives in terms of
fast and robust ultrahigh density data manipulation. A powerful ingredient
commonly used to stabilize chiral magnetic states is the so-called
Dzyaloshinskii-Moriya interaction (DMI) arising from spin-orbit coupling in
inversion asymmetric magnets. Such a large antisymmetric exchange has been
obtained at interfaces between heavy metals and transition metal ferromagnets,
resulting in spin spirals and nanoskyrmion lattices. Here, using relativistic
first-principles calculations, we demonstrate that the magnitude and sign of
DMI can be entirely controlled by tuning the oxygen coverage of the magnetic
film, therefore enabling the smart design of chiral magnetism in ultra-thin
films. We anticipate that these results extend to other electronegative ions
and suggest the possibility of electrical tuning of exotic magnetic phases
First-principles analysis of a homo-chiral cycloidal magnetic structure in a monolayer Cr on W(110)
The magnetic structure of a Cr monolayer on a W(110) substrate is
investigated by means of first-principles calculations based on the
noncollinear spin density functional theory (DFT). As magnetic ground state we
find a long-period homochiral left-rotating spin spiral on-top of an
atomic-scale anti-ferromagnetic order of nearest neighbor atoms. The rotation
angle of the magnetic moment changes inhomogeneously from atom to atom across
the spiral. We predict a propagation direction along the crystallographic [001]
direction with a period length of 14.3 nm, which is in excellent agreement with
a modulation of the local anti-ferromagnetic contrast observed in
spin-polarized scanning tunneling microscope experiments by Santos et al. [New
J. Phys. 10, 013005 (2008)]. We identify the Dzyaloshinskii-Moriya interaction
(DMI) as origin of the homochiral magnetic structure, competing with the
Heisenberg-type exchange interaction and magneto-crystalline anisotropy energy.
From DFT calculations we extract parameters for a micromagnetic model and
thereby determine a considerable inhomogeneity of the spin spiral, increasing
the period length by 6% compared to homogeneous spin spirals. The results are
compared to the behavior of a Mn and Fe monolayer and Fe doublelayer on a
W(110) substrate
Role of Dzyaloshinskii-Moriya interaction for magnetism in transition-metal chains at Pt step-edges
We explore the emergence of chiral magnetism in one-dimensional monatomic Mn,
Fe, and Co chains deposited at the Pt(664) step-edge carrying out an ab-initio
study based on density functional theory (DFT). The results are analyzed
employing several models: (i) a micromagnetic model, which takes into account
the Dzyaloshinskii-Moriya interaction (DMI) besides the spin stiffness and the
magnetic anisotropy energy, and (ii) the Fert-Levy model of the DMI for diluted
magnetic impurities in metals. Due to the step-edge geometry, the direction of
the Dzyaloshinskii vector (D-vector) is not predetermined by symmetry and
points in an off-symmetry direction. For the Mn chain we predict a long-period
cycloidal spin-spiral ground state of unique rotational sense on top of an
otherwise atomic-scale antiferromagnetic phase. The spins rotate in a plane
that is tilted relative to the Pt surface by towards the upper step
of the surface. The Fe and Co chains show a ferromagnetic ground state since
the DMI is too weak to overcome their respective magnetic anisotropy barriers.
Beyond the discussion of the monatomic chains we provide general expressions
relating ab-initio results to realistic model parameters that occur in a
spin-lattice or in a micromagnetic model. We prove that a planar homogeneous
spiral of classical spins with a given wave vector rotating in a plane whose
normal is parallel to the D-vector is an exact stationary state solution of a
spin-lattice model for a periodic solid that includes Heisenberg exchange and
DMI. The validity of the Fert-Levy model for the evaluation of micromagnetic
DMI parameters and for the analysis of ab-initio calculations is explored for
chains. The results suggest that some care has to be taken when applying the
model to infinite periodic one-dimensional systems.Comment: 21 pages, 9 figure
Topological phases of Bi(111) bilayer in an external exchange field
Using first principles methods, we investigate topological phase transitions
as a function of exchange field in a Bi(111) bilayer. Evaluation of the spin
Chern number for different magnitudes of the exchange field reveals that when
the time reversal symmetry is broken by a small exchange field, the system
enters the time-reversal broken topological insulator phase, introduced by Yang
{\it et al.} in Phys. Rev. Lett. 107, 066602 (2011). After a metallic phase in
the intermediate region, the quantum anomalous Hall phase with non-zero Chern
number emerges at a sufficiently large exchange field. We analyze the phase
diagram from the viewpoint of the evolution of the electronic structure, edge
states and transport properties, and demonstrate that different topological
phases can be distinguished by the spin-polarization of the edge states as well
as spin or charge transverse conductivity.Comment: 7 pages, 3+1 figures; accepted versio
Tuning of the Rashba effect in Pb quantum well states via a variable Schottky barrier
Spin-orbit interaction (SOI) in low-dimensional systems results in the
fascinating property of spin-momentum locking. In a Rashba system the inversion
symmetry normal to the plane of a two-dimensional (2D) electron gas is broken,
generating a Fermi surface spin texture reminiscent of spin vortices of
different radii. This can be exploited in a spin-based field-effect transistor
(spin- FET), where the Rashba system forms a 2D channel between ferromagnetic
(FM) source and drain electrodes. The electron spin precesses when propagating
through the Rashba channel and spin orientations (anti)parallel to the drain
give (low) high conductivity. Crucial is the possibility to tune the momentum
splitting, and consequently the precession angle, through an external
parameter. Here we show that this can be achieved in Pb quantum well states
through the doping dependence of the Schottky barrier, opening up the
possibility of a terahertz spin-FET.Comment: 8 pages, 7 figure
Engineering quantum anomalous Hall phases with orbital and spin degrees of freedom
Combining tight-binding models and first principles calculations, we
investigate the quantum anomalous Hall (QAH) effect induced by intrinsic
spin-orbit coupling (SOC) in buckled honeycomb lattice with sp orbitals in an
external exchange field. Detailed analysis reveals that nontrivial topological
properties can arise utilizing not only spin but also orbital degrees of
freedom in the strong SOC limit, when the bands acquire non-zero Chern numbers
upon undergoing the so-called orbital purification. As a prototype of a buckled
honeycomb lattice with strong SOC we choose the Bi(111) bilayer, analyzing its
topological properties in detail. In particular, we show the emergence of
several QAH phases upon spin exchange of the Chern numbers as a function of SOC
strength and magnitude of the exchange field. Interestingly, we observe that in
one of such phases, namely, in the quantum spin Chern insulator phase, the
quantized charge and spin Hall conductivities co-exist. We consider the
possibility of tuning the SOC strength in Bi bilayer via alloying with
isoelectronic Sb, and speculate that exotic properties could be expected in
such an alloyed system owing to the competition of the topological properties
of its constituents. Finally, we demonstrate that 3d dopants can be used to
induce a sizeable exchange field in Bi(111) bilayer, resulting in non-trivial
Chern insulator properties
Functionalized Bismuth Films: Giant Gap Quantum Spin Hall and Valley-Polarized Quantum Anomalous Hall States
The search for new large band gap quantum spin Hall (QSH) and quantum
anomalous Hall (QAH) insulators is critical for their realistic applications at
room temperature. Here we predict, based on first principles calculations, that
the band gap of QSH and QAH states can be as large as 1.01 eV and 0.35 eV in an
H-decorated Bi(111) film. The origin of this giant band gap lies both in the
large spin-orbit interaction of Bi and the H-mediated exceptional electronic
and structural properties. Moreover, we find that the QAH state also possesses
the properties of quantum valley Hall state, thus intrinsically realising the
so-called valley-polarized QAH effect. We further investigate the realization
of large gap QSH and QAH states in an H-decorated Bi(\={1}10) film and
X-decorated (X=F, Cl, Br, and I) Bi(111) films.Comment: submitted to Physical Reveiw Letters on 25th of September 201
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