370 research outputs found
Electronic structure and bond competition in the polar magnet PbVO
Density functional electronic structure studies of tetragonal PbVO are
reported. The results show a an important role for both Pb 6 - O 2 and V
- O bonding, with an interplay between these. This is discussed in
relation to the possibility of obtaining magnetoelectric behavior.Comment: 5 page
CaCu_3Ti_4O_12/CaTiO_3 Composite Dielectrics: A Ba/Pb-free Ceramics with High Dielectric Constants
We have measured dielectric properties of CaCuTiO
( = 0, 0.1, 0.5, 1, 1.5, 2, 2.9 and 3), and have found that
CaCuTiO (a composite of CaCuTiO and
CaTiO) exhibits a high dielectric constant of 1800 with a low dissipation
factor of 0.02 below 100 kHz from 220 to 300 K. These are comparable to (or
even better than) those of the Pb/Ba-based ceramics, which could be attributed
to a barrier layer of CaTiO on the surface of the CaCuTiO
grains. The composite dielectric ceramics reported here are environmentally
benign as they do not contain Ba/Pb.Comment: 4 pages, 4 figures, Appl. Phys. Lett. (scheduled on July 25, 2005
Magnetic control of large room-temperature polarization
Numerous authors have referred to room-temperature magnetic switching of
large electric polarizations as The Holy Grail of magnetoelectricity.We report
this long-sought effect using a new physical process of coupling between
magnetic and ferroelectric relaxor nano-regions. Here we report magnetic
switching between the normal ferroelectric state and the ferroelectric relaxor
state. This gives both a new room-temperature, single-phase, multiferroic
magnetoelectric, PbZr0.46Ti0.34Fe0.13W0.07O3, with polarization, loss (<4%),
and resistivity (typically 108 -109 ohm.cm) equal to or superior to BiFeO3, and
also a new and very large magnetoelectric effect: switching not from +Pr to
negative Pr with applied H, but from Pr to zero with applied H of less than a
Tesla. This switching of the polarization occurs not because of a conventional
magnetically induced phase transition, but because of dynamic effects:
Increasing H lengthens the relaxation time by x500 from 100 ?s, and
it couples strongly the polarization relaxation and spin relaxations. The
diverging polarization relaxation time accurately fits a modified Vogel-Fulcher
Equation in which the freezing temperature Tf is replaced by a critical
freezing field Hf that is 0.92 positive/negative 0.07 Tesla. This field
dependence and the critical field Hc are derived analytically from the
spherical random bond random field (SRBRF) model with no adjustable parameters
and an E2H2 coupling. This device permits 3-state logic (+Pr,0,negative Pr) and
a condenser with >5000% magnetic field change in its capacitance.Comment: 20 pages, 5 figure
Tetragonal tungsten bronze compounds: relaxor vs mixed ferroelectric - dipole glass behavior
We demonstrate that recent experimental data (E. Castel et al J.Phys. Cond.
Mat. {\bf 21} (2009), 452201) on tungsten bronze compound (TBC)
BaPrNdFeNbO can be well explained in our model
predicting a crossover from ferroelectric () to orientational (dipole)
glass (), rather then relaxor, behavior. We show, that since a "classical"
perovskite relaxor like Pb(Mn Nb)O is never a
ferroelectric, the presence of ferroelectric hysteresis loops in TBC shows that
this substance actually transits from ferroelectric to orientational glass
phase with growth. To describe the above crossover theoretically, we use
the simple replica-symmetric solution for disordered Ising model.Comment: 5 two-column pages, 4 figure
Quasi-long range order in glass states of impure liquid crystals, magnets, and superconductors
In this review we consider glass states of several disordered systems:
vortices in impure superconductors, amorphous magnets, and nematic liquid
crystals in random porous media. All these systems can be described by the
random-field or random-anisotropy O(N) model. Even arbitrarily weak disorder
destroys long range order in the O(N) model. We demonstrate that at weak
disorder and low temperatures quasi-long range order emerges. In
quasi-long-range-ordered phases the correlation length is infinite and
correlation functions obey power dependencies on the distance. In pure systems
quasi-long range order is possible only in the lower critical dimension and
only in the case of Abelian symmetry. In the presence of disorder this type of
ordering turns out to be more common. It exists in a range of dimensions and is
not prohibited by non-Abelian symmetries.Comment: 32 page
Landau Theory of Domain Wall Magnetoelectricity
We calculate the exact analytical solution to the domain wall properties in a
multiferroic system with two order parameters that are coupled
bi-quadratically. This is then adapted to the case of a magnetoelectric
multiferroic material such as BiFeO3, with a view to examine critically whether
the domain walls can account for the enhancement of magnetization reported for
thin films fo this material, in view of the correlation between increasing
magnetization and increasing volume fraction of domain walls as films become
thinner. The present analysis can be generalized to describe a class of
magnetoelectric devices based upon domain walls rather than bulk properties.Comment: 9 pages, 4 figure
Aging in the Relaxor Ferroelectric PMN/PT
The relaxor ferroelectric
(PbMnNbO)(PbTiO), ,
(PMN/PT(90/10)) is found to exhibit several regimes of complicated aging
behavior. Just below the susceptibility peak there is a regime exhibiting
rejuvenation but little memory. At lower temperature, there is a regime with
mainly cumulative aging, expected for simple domain-growth. At still lower
temperature, there is a regime with both rejuvenation and memory, reminiscent
of spin glasses. PMN/PT (88/12) is also found to exhibit some of these aging
regimes. This qualitative aging behavior is reminiscent of that seen in
reentrant ferromagnets, which exhibit a crossover from a domain-growth
ferromagnetic regime into a reentrant spin glass regime at lower temperatures.
These striking parallels suggest a picture of competition in PMN/PT (90/10)
between ferroelectric correlations formed in the domain-growth regime with
glassy correlations formed in the spin glass regime. PMN/PT (90/10) is also
found to exhibit frequency-aging time scaling of the time-dependent part of the
out-of-phase susceptibility for temperatures 260 K and below. The stability of
aging effects to thermal cycles and field perturbations is also reported.Comment: 8 pages RevTeX4, 11 figures; submitted to Phys. Rev.
Low Symmetry Phase in (001) BiFeO Epitaxial Constrained Thin Films
The lattice of (001)-oriented BiFeO epitaxial thin film has been
identified by synchrotron x-ray diffraction. By choosing proper scattering
zones containing the fixed (001) reflection, we have shown that low-symmetry
phases similar to a phase exist in the thin film at room temperature.
These results demonstrate a change in phase stability from rhombohedral in bulk
single crystals, to a modified monoclinic structure in epitaxial thin films
Monte Carlo Study of Relaxor Systems: A Minimum Model for Pb(InNb)O}
We examine a simple model for Pb(InNb)O (PIN), which
includes both long-range dipole-dipole interaction and random local anisotropy.
A improved algorithm optimized for long-range interaction has been applied for
efficient large-scale Monte Carlo simulation. We demonstrate that the phase
diagram of PIN is qualitatively reproduced by this minimum model. Some
properties characteristic of relaxors such as nano-scale domain formation, slow
dynamics and dispersive dielectric responses are also examined.Comment: 5 pages, 4 figure
Finite size and intrinsic field effect on the polar-active properties of the ferroelectric-semiconductor heterostructures
Using Landau-Ginzburg-Devonshire approach we calculated the equilibrium
distributions of electric field, polarization and space charge in the
ferroelectric-semiconductor heterostructures containing proper or incipient
ferroelectric thin films. The role of the polarization gradient and intrinsic
surface energy, interface dipoles and free charges on polarization dynamics are
specifically explored. The intrinsic field effects, which originated at the
ferroelectric-semiconductor interface, lead to the surface band bending and
result into the formation of depletion space-charge layer near the
semiconductor surface. During the local polarization reversal (caused by the
inhomogeneous electric field induced by the nanosized tip of the Scanning Probe
Microscope (SPM) probe) the thickness and charge of the interface layer
drastically changes, it particular the sign of the screening carriers is
determined by the polarization direction. Obtained analytical solutions could
be extended to analyze polarization-mediated electronic transport.Comment: 35 pages, 12 figures, 1 table, 2 appendices, to be submitted to Phys.
Rev.
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