294 research outputs found
Strong enhancement of direct magnetoelectric effect in strained ferroelectric-ferromagnetic thin-film heterostructures
The direct magnetoelectric (ME) effect resulting from the polarization
changes induced in a ferroelectric film by the application of a magnetic field
to a ferromagnetic substrate is described using the nonlinear thermodynamic
theory. It is shown that the ME response strongly depends on the initial strain
state of the film. The ME polarization coefficient of the heterostructures
involving Terfenol-D substrates and compressively strained lead zirconate
titanate (PZT) films, which stabilize in the out-of-plane polarization state,
is found to be comparable to that of bulk PZT/Terfenol-D laminate composites.
At the same time, the ME voltage coefficient reaches a giant value of 50 V/(cm
Oe), which greatly exceeds the maximum observed static ME coefficients of bulk
composites. This remarkable feature is explained by a favorable combination of
considerable strain sensitivity of polarization and a low electric permittivity
in compressively strained PZT films. The theory also predicts a further
dramatic increase of ME coefficients at the strain-induced transitions between
different ferroelectric phases.Comment: 7 pages, 3 figure
Thermodynamic theory of epitaxial ferroelectric thin films with dense domain structures
A Landau-Ginsburg-Devonshire-type nonlinear phenomenological theory is
presented, which enables the thermodynamic description of dense laminar
polydomain states in epitaxial ferroelectric thin films. The theory explicitly
takes into account the mechanical substrate effect on the polarizations and
lattice strains in dissimilar elastic domains (twins). Numerical calculations
are performed for PbTiO3 and BaTiO3 films grown on (001)-oriented cubic
substrates. The "misfit strain-temperature" phase diagrams are developed for
these films, showing stability ranges of various possible polydomain and
single-domain states. Three types of polarization instabilities are revealed
for polydomain epitaxial ferroelectric films, which may lead to the formation
of new polydomain states forbidden in bulk crystals. The total dielectric and
piezoelectric small-signal responses of polydomain films are calculated,
resulting from both the volume and domain-wall contributions. For BaTiO3 films,
strong dielectric anomalies are predicted at room temperature near special
values of the misfit strain.Comment: 19 pages, 8 figure
Soft modes of collective domain-wall vibrations in epitaxial ferroelectric thin films
Mechanical restoring forces acting on ferroelastic domain walls displaced
from the equilibrium positions in epitaxial films are calculated for various
modes of their cooperative translational oscillations. For vibrations of the
domain-wall superlattice with the wave vectors corresponding to the center and
boundaries of the first Brillouin zone, the soft modes are singled out that are
distinguished by a minimum magnitude of the restoring force. It is shown that,
in polydomain ferroelectric thin films, the soft modes of wall vibrations may
create enormously large contribution to the film permittivity.Comment: 6 pages, 3 figure
Creep motion of a domain wall in the two-dimensional random-field Ising model with a driving field
With Monte Carlo simulations, we study the creep motion of a domain wall in
the two-dimensional random-field Ising model with a driving field. We observe
the nonlinear fieldvelocity relation, and determine the creep exponent {\mu}.
To further investigate the universality class of the creep motion, we also
measure the roughness exponent {\zeta} and energy barrier exponent {\psi} from
the zero-field relaxation process. We find that all the exponents depend on the
strength of disorder.Comment: 5 pages, 4 figure
Polarization states of polydomain epitaxial Pb(Zr1-xTix)O3 thin films and their dielectric properties
Ferroelectric and dielectric properties of polydomain (twinned)
single-crystal Pb(Zr1-xTix)O3 thin films are described with the aid of a
nonlinear thermodynamic theory, which has been developed recently for epitaxial
ferroelectric films with dense laminar domain structures. For Pb(Zr1-xTix)O3
(PZT) films with compositions x = 0.9, 0.8, 0.7, 0.6, 0.5, and 0.4, the "misfit
strain-temperature" phase diagrams are calculated and compared with each other.
It is found that the equilibrium diagrams of PZT films with x > 0.7 are similar
to the diagram of PbTiO3 films. They consist of only four different stability
ranges, which correspond to the paraelectric phase, single-domain tetragonal
ferroelectric phase, and two pseudo-tetragonal domain patterns. In contrast, at
x = 0.4, 0.5, and 0.6, the equilibrium diagram displays a rich variety of
stable polarization states, involving at least one monoclinic polydomain state.
Using the developed phase diagrams, the mean out-of-plane polarization of a
poled PZT film is calculated as a function of the misfit strain and
composition. Theoretical results are compared with the measured remanent
polarizations of PZT films grown on SrTiO3. Dependence of the out-of-plane
dielectric response of PZT films on the misfit strain in the heterostructure is
also reported.Comment: 23 pages, 4 figure
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