2,926 research outputs found
Ferroelectric Rashba Semiconductors as a novel class of multifunctional materials
The discovery of novel properties, effects or microscopic mechanisms in
modern materials science is often driven by the quest for combining, into a
single compound, several functionalities: not only the juxtaposition of the
latter functionalities, but especially their coupling, can open new horizons in
basic condensed matter physics as well as in technology. Semiconductor
spintronics makes no exception. In this context, we have discovered by means of
density-functional simulations that, when a sizeable spin-orbit coupling is
combined with ferroelectricity, such as in GeTe, one obtains novel
multifunctional materials - called Ferro-Electric Rashba Semi-Conductors
(FERSC) - where, thanks to a giant Rashba spin-splitting, the spin texture is
controllable and switchable via an electric field. This peculiar spin-electric
coupling can find a natural playground in small-gap insulators, such as
chalcogenides, and can bring brand new assets into the field of
electrically-controlled semiconductor spintronicsComment: 9 pages, 2 figures, in press on "Frontiers in Condensed Matter
Physics
Interplay between charge-order, ferroelectricity and ferroelasticity: tungsten bronze structures as a playground for multiferroicity
Large electron-electron Coulomb-interactions in correlated systems can lead
to a periodic arrangement of localized electrons, the so called "charge-order".
The latter is here proposed as a driving force behind ferroelectricity in iron
fluoride K0.6FeF3. By means of density functional theory, we propose different
non-centrosymmetric d5/d6 charge-ordering patterns, each giving rise to
polarization along different crystallographic axes and with different
magnitudes. Accordingly, we introduce the concept of "ferroelectric anistropy"
(peculiar to improper ferroelectrics with polarization induced by electronic
degrees of freedom), denoting the small energy difference between competing
charge-ordered states that might be stabilized upon electrical field-cooling.
Moreover, we suggest a novel type of charge-order-induced ferroelasticity:
first-principles simulations predict a monoclinic distortion to be driven by a
specific charge-ordering pattern, which, in turn, unambiguously determines the
direction of ferroelectric polarization. K0.6FeF3 therefore emerges as a
prototypical compound, in which the intimately coupled electronic and
structural degrees of freedom result in a manifest and peculiar
multiferroicity.Comment: 4 pages, 4 figures, Submitted for publicatio
Mechanisms and origin of multiferroicity
Motivated by the potential applications of their intrinsic cross-coupling
properties, the interest in multiferroic materials has constantly increased
recently, leading to significant experimental and theoretical advancements.
From the theoretical point of view, recent progresses have allowed to identify
different mechanisms responsible for the appearence of ferroelectric
polarization coexisting with -- and coupled to -- magnetic properties. This
chapter aims at reviewing the fundamental mechanisms devised so far, mainly in
transition-metal oxides, which lie at the origin of multiferroicity
Energetic stability and magnetic properties of Mn dimers in silicon
We present an accurate first-principles study of magnetism and energetics of single Mn impurities and Mn dimers in Si. Our results, in general agreement with available experiments, show that (i) Mn atoms tend to aggregate, the formation energy of dimers being lower than the sum of the separate constituents, (ii) ferromagnetic coupling is favored between the Mn atoms constituting the dimers in p-type Si, switching to an antiferromagnetic coupling in n-type Si, (iii) Mn atoms show donors (acceptor) properties in p-type (n-type) Si, therefore they tend to compensate doping, while dimers being neutral or acceptors allow for Si to be doped p-type. (C) 2004 American Institute of Physics
Spontaneous skyrmionic lattice from anisotropic symmetric exchange in a Ni-halide monolayer
Topological spin structures, such as magnetic skyrmions, hold great promises
for data storage applications, thanks to their inherent stability. In most
cases, skyrmions are stabilized by magnetic fields in non-centrosymmetric
systems displaying the chiral Dzyaloshinskii-Moriya exchange interaction, while
spontaneous skyrmion lattices have been reported in centrosymmetric itinerant
magnets with long-range interactions. Here, a spontaneous anti-biskyrmion
lattice with unique topology and chirality is predicted in the monolayer of a
semiconducting and centrosymmetric metal halide, NiI. Our first-principles
and Monte Carlo simulations reveal that the anisotropies of the short-range
symmetric exchange, when combined with magnetic frustration, can lead to an
emergent chiral interaction that is responsible for the predicted topological
spin structures. The proposed mechanism finds a prototypical manifestation in
two-dimensional magnets, thus broadening the class of materials that can host
spontaneous skyrmionic states.Comment: submitte
Interface effects at a half-metal/ferroelectric junction
Magnetoelectric effects are investigated ab-initio at the interface between
half-metallic and ferroelectric prototypes: Heusler CoMnSi and perovskite
BaTiO. For the Co-termination ferroelectricity develops in BaTiO down
to nanometer thicknesses, whereas for the MnSi-termination a paraelectric and a
ferroelectric state energetically compete, calling for a full experimental
control over the junction atomic configuration whenever a ferroelectric barrier
is needed. Switch of the electric polarization largely affects magnetism in
CoMnSi, with magnetoelectric coupling due to electronic hybridization at
the MnSi termination and to structural effects at the Co-termination.
Half-metallicity is lost at the interface, but recovered already in the
subsurface layer.Comment: 4 pages, 3figures, accepted for publication in Appl. Phys. Let
Magnetically induced ferroelectricity in Cu2MnSnS4 and Cu2MnSnSe4
We investigate magnetically-induced ferroelectricity in Cu2MnSnS4 by means of
Landau theory of phase transitions and of ab initio density functional theory.
As expected from the Landau approach, ab initio calculations show that a
non-zero ferroelectric polarization P along the y direction is induced by the
peculiar antiferromagnetic configuration of Mn spins occurring in Cu2MnSnS4.
The comparison between P, calculated either via density-functional-theory or
according to Landau approach, clearly shows that ferroelectricity is mainly
driven by Heisenberg-exchange terms and only to a minor extent by relativistic
terms. At variance with previous examples of collinear antiferromagnets with
magnetically-induced ferroelectricity (such as AFM-E HoMnO3), the ionic
displacements occurring upon magnetic ordering are very small, so that the
exchange-striction mechanism (i.e. displacement of ions so as to minimize the
magnetic coupling energy) is not effective here. Rather, the microscopic
mechanism at the basis of polarization has mostly an electronic origin. In this
framework, we propose the small magnetic moment at Cu sites induced by
neighboring Mn magnetic moments to play a relevant role in inducing P. Finally,
we investigate the effect of the anion by comparing Cu2MnSnSe4 and Cu2MnSnS4:
Se-4p states, more delocalized compared to S-3p states, are able to better
mediate the Mn-Mn interaction, in turn leading to a higher ferroelectric
polarization in the Se-based compound
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