137 research outputs found
Prediction of novel interface-driven spintronic effects
The recently-proposed coupling between the angular momentum density and
magnetic moment [A. Raeliarijaona et al, Phys. Rev. Lett. 110, 137205 (2013)]
is shown here to result in the prediction of (i) novel spin currents generated
by an electrical current and (ii) new electrical currents induced by a spin
current in systems possessing specific interfaces between two different
materials. Some of these spin (electrical) currents can be reversed near the
interface by reversing the applied electrical (spin) current. Similarities and
differences between these novel spintronic effects and the well-known spin Hall
and inverse spin Hall effects are also discussed.Comment: Accepted in J. Phys.::Condens. Matte
Dynamics of polar vortex crystallization
Vortex crystals are commonly observed in ultra-thin ferroelectrics. However,
a clear physical picture of origin of this topological state is currently
lacking. Here, we show that vortex crystallization in ultra-thin
Pb(Zr0.4,Ti0.6)O3 films stems from the softening of a phonon mode and can be
thus described as a SU(2) symmetry-breaking transition. This result sheds light
on the topology of the polar vortex patterns and bridges polar vortices with
smectic phases, spin spirals, and other modulated states. Finally, we predict
an ac-field driven resonant switching of the vortex tube orientation which
could enable new low-power electronic technologies.Comment: 3 figure
Engineering magnetic domain wall energies in multiferroic BiFeO via epitaxial strain
Epitaxial strain has emerged as a powerful tool to tune magnetic and
ferroelectric properties in functional materials such as in multiferroic
perovskite oxides. Here, we use first-principles calculations to explore the
evolution of magnetic interactions in the antiferromagnetic multiferroic
BiFeO (BFO), one of the most promising multiferroics for future technology.
The epitaxial strain in BFO(001) oriented film is varied between
. We find that both strengths of the
exchange interaction and Dzyaloshinskii-Moriya interaction (DMI) decrease
linearly from compressive to tensile strain whereas the uniaxial
magnetocrystalline anisotropy follows a parabolic behavior which lifts the
energy degeneracy of the (111) easy plane of bulk BFO. From the trends of the
magnetic interactions we can explain the destruction of cycloidal order in
compressive strain as observed in experiments due to the increasing anisotropy
energy. For tensile strain, we predict that the ground state remains unchanged
as a function of strain. By using the domain wall (DW) energy, we envision the
region where isolated chiral magnetic texture might occur as function of strain
i.e. where the DW and the spin spiral energy are equal. This transition between
and of strain should allow topologically stable magnetic
states such as antiferromagnetic skyrmions and merons to occur. Hence, our work
should trigger experimental and theoretical investigations in this range of
strain
Spin-current driven Dzyaloshinskii-Moriya interaction in the multiferroic BiFeO3 from first-principles
The electrical control of magnons opens up new ways to transport and process
information for logic devices. In magnetoelectrical multiferroics, the
Dzyaloshinskii-Moriya (DM) interaction directly allow for such a control and,
hence, is of major importance. We determine the origin and the strength of the
(converse) spin current DM interaction in the R3c bulk phase of the
multiferroic BiFeO3 based on density functional theory. Our data supports only
the existence of one DM interaction contribution originating from the spin
current model. By exploring then magnon dispersion in the full Brillouin Zone,
we show that the exchange is isotropic, but the DM interaction and anisotropy
prefer any propagation and any magnetization direction within the full (111)
plane. Our work emphasizes the significance of the asymmetric potential induced
by the spin current over the structural asymmetry induced by the anionic
octahedron in multiferroics such as BiFeO3
Ferroelectricity with Asymmetric Hysteresis in Metallic LiOsO3 Ultrathin Films
Bulk LiOsO3 was experimentally identified as a "ferroelectric" metal where
polar distortions coexist with metallicity [Shi et al., Nature Materials 12,
1024 (2013)]. It is generally believed that polar displacements in a
"ferroelectric" metal cannot be switched by an external electric field. Here,
via comprehensive density functional theory calculations, we demonstrate that a
two-unit-cell-thick LiOsO3 thin film exhibits a ferroelectric ground state
having an out-of-plane electric dipole moment that can be switched by an
external electric field. Moreover, its dipole moment-versus-electric-field
hysteresis loop is asymmetric because only surface Li ions'displacements are
reversed by external electric field whereas the field-induced force on inner Li
atoms are nearly fully screened by itinerant electrons. As a relevant
by-product of our study, we also extend the concept of "Born effective charge"
to finite metallic systems, and show its usefulness to rationalize the observed
effects.Comment: 13 pages, 4 figure
Energy storage properties of ferroelectric nanocomposites
An atomistic effective Hamiltonian technique is used to investigate the
finite-temperature energy storage properties of a ferroelectric nanocomposite
consisting of an array of BaTiO nanowires embedded in a SrTiO
matrix, for electric field applied along the long axis of the nanowires. We
find that the energy density \textit{versus} temperature curve adopts a
nonlinear, mostly temperature-independent response when the system exhibits
phases possessing an out-of-plane polarization and vortices while the energy
density more linearly increases with temperature when the nanocomposite either
only possesses vortices (and thus no spontaneous polarization) or is in a
paraelectric and paratoroidic phase for its equilibrium state. Ultrahigh energy
density up to 140 J/cm and an ideal 100% efficiency are also
predicted in this nanocomposite. A phenomenological model, involving a coupling
between polarization and toroidal moment, is further proposed to interpret
these energy density results.Comment: 7 pages, 5 figure
- …