98 research outputs found
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
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
Ferroelectricity due to orbital ordering in E-type undoped rare-earth manganites
Aiming at understanding the origin of the electronic contribution to
ferroelectric polarization in undoped manganites, we evaluate the Berry phase
of orbital-polarizable Bloch electrons as an orbital ordering (OO) establishes
in the background of an antiferromagnetic E-type configuration. The onset of OO
is tuned by the Jahn-Teller (JT) interaction in a tight-binding model for
interacting electrons moving along zigzag chains. A finite polarization is
found as soon as the JT coupling is strong enough to induce OO, supporting the
large electronic contribution predicted from first principles.Comment: 4 pages, 2 figures, figure and text substantially improved. Title
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Ferroelectricity in multiferroic magnetite Fe3O4 driven by noncentrosymmetric Fe2+/Fe3+ charge-ordering: First-principles study
By means of first-principles simulations, we unambiguously show that improper
ferroelectricity in magnetite in the low-temperature insulating phase is driven
by charge-ordering. An accurate comparison between monoclinic ferroelectric Cc
and paraelectric P2/c structures shows that the polarization arises because of
"shifts" of electronic charge between octahedral Fe sites, leading to a
non-centrosymmetric Fe2+/Fe3+ charge-ordered pattern. Our predicted values for
polarization, in good agreement with available experimental values, are
discussed in terms of point-charge dipoles located on selected Fe tetrahedra,
pointing to a manifest example of electronic ferroelectricity driven by charge
rearrangement.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev.
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