155 research outputs found
Nanoscale studies of domain wall motion in epitaxial ferroelectric thin films
Atomic force microscopy was used to investigate ferroelectric switching and
nanoscale domain dynamics in epitaxial PbZr0.2Ti0.8O3 thin films. Measurements
of the writing time dependence of domain size reveal a two-step process in
which nucleation is followed by radial domain growth. During this growth, the
domain wall velocity exhibits a v ~ exp[-(1/E)^mu] dependence on the electric
field, characteristic of a creep process. The domain wall motion was analyzed
both in the context of stochastic nucleation in a periodic potential as well as
the canonical creep motion of an elastic manifold in a disorder potential. The
dimensionality of the films suggests that disorder is at the origin of the
observed domain wall creep. To investigate the effects of changing the disorder
in the films, defects were introduced during crystal growth (a-axis inclusions)
or by heavy ion irradiation, producing films with planar and columnar defects,
respectively. The presence of these defects was found to significantly decrease
the creep exponent mu, from 0.62 - 0.69 to 0.38 - 0.5 in the irradiated films
and 0.19 - 0.31 in the films containing a-axis inclusions.Comment: 13 pages, 15 figures, to be published in J. App. Phys. special issue
on ferroelectric
Lateral piezoelectric response across ferroelectric domain walls in thin films
In purely c-axis oriented PbZrTiO ferroelectric thin
films, a lateral piezoresponse force microscopy signal is observed at the
position of 180{\deg}domain walls, where the out-of-plane oriented polarization
is reversed. Using electric force microscopy measurements we exclude
electrostatic effects as the origin of this signal. Moreover, our mechanical
simulations of the tip/cantilever system show that the small tilt of the
surface at the domain wall below the tip does not satisfactorily explain the
observed signal either. We thus attribute this lateral piezoresponse at domain
walls to their sideways motion (shear) under the applied electric field. From
simple elastic considerations and the conservation of volume of the unit cell,
we would expect a similar lateral signal more generally in other ferroelectric
materials, and for all types of domain walls in which the out-of-plane
component of the polarization is reversed through the domain wall. We show that
in BiFeO thin films, with 180, 109 and 71{\deg}domain walls, this is indeed
the case.Comment: 31 pages, 10 figures. to appear in J. Appl. Phys. Special topic:
invited papers from the international symposium on piezoresponse force
microscopy and nanoscale phenomena in polar materials. Aveiro - portugal 200
Thermal quench effects on ferroelectric domain walls
Using piezoresponse force microscopy on epitaxial ferroelectric thin films,
we have measured the evolution of domain wall roughening as a result of
heat-quench cycles up to 735C, with the effective roughness exponent \zeta\
changing from 0.25 to 0.5. We discuss two possible mechanisms for the observed
\zeta\ increase: a quench from a thermal 1-dimensional configuration, and from
a locally-equilibrated pinned configuration with a crossover from a 2- to
1-dimensional regime. We find that the post-quench spatial structure of the
metastable states, qualitatively consistent with the existence of a growing
dynamical length scale whose ultra slow evolution is primarily controlled by
the defect configuration and heating process parameters, makes the second
scenario more plausible. This interpretation suggests that pinning is relevant
in a wide range of temperatures, and in particular, that purely thermal domain
wall configurations might not be observable in this glassy system. We also
demonstrate the crucial effects of oxygen vacancies in stabilizing domain
structures.Comment: 17 pages (preprint), 4 figure
Thermal Effects in the dynamics of disordered elastic systems
Many seemingly different macroscopic systems (magnets, ferroelectrics, CDW,
vortices,..) can be described as generic disordered elastic systems.
Understanding their static and dynamics thus poses challenging problems both
from the point of view of fundamental physics and of practical applications.
Despite important progress many questions remain open. In particular the
temperature has drastic effects on the way these systems respond to an external
force. We address here the important question of the thermal effect close to
depinning, and whether these effects can be understood in the analogy with
standard critical phenomena, analogy so useful to understand the zero
temperature case. We show that close to the depinning force temperature leads
to a rounding of the depinning transition and compute the corresponding
exponent. In addition, using a novel algorithm it is possible to study
precisely the behavior close to depinning, and to show that the commonly
accepted analogy of the depinning with a critical phenomenon does not fully
hold, since no divergent lengthscale exists in the steady state properties of
the line below the depinning threshold.Comment: Proceedings of the International Workshop on Electronic Crystals,
Cargese(2008
Multiscaling analysis of ferroelectric domain wall roughness
Using multiscaling analysis, we compare the characteristic roughening of
ferroelectric domain walls in PZT thin films with numerical simulations of
weakly pinned one-dimensional interfaces. Although at length scales up to a
length scale greater or equal to 5 microns the ferroelectric domain walls
behave similarly to the numerical interfaces, showing a simple mono-affine
scaling (with a well-defined roughness exponent), we demonstrate more complex
scaling at higher length scales, making the walls globally multi-affine
(varying roughness exponent at different observation length scales). The
dominant contributions to this multi-affine scaling appear to be very localized
variations in the disorder potential, possibly related to dislocation defects
present in the substrate.Comment: 5 pages, 4 figure
Fractal dimension and size scaling of domains in thin films of multiferroic BiFeO3
We have analyzed the morphology of ferroelectric domains in very thin films
of multiferroic BiFeO3. Unlike the more common stripe domains observed in
thicker films BiFeO3 or in other ferroics, the domains tend not to be straight,
but irregular in shape, with significant domain wall roughening leading to a
fractal dimensionality. Also contrary to what is usually observed in other
ferroics, the domain size appears not to scale as the square root of the film
thickness. A model is proposed in which the observed domain size as a function
of film thickness can be directly linked to the fractal dimension of the
domains.Comment: 4 pages, 3 figure
Shear effects in lateral piezoresponse force microscopy at 180 ferroelectric domain walls
In studies using piezoresponse force microscopy, we observe a non-zero
lateral piezoresponse at 180 domain walls in out-of-plane polarized,
c-axis-oriented tetragonal ferroelectric Pb(ZrTi)O
epitaxial thin films. We attribute these observations to a shear strain effect
linked to the sign change of the piezoelectric coefficient through the
domain wall, in agreement with theoretical predictions. We show that in
monoclinically distorted tetragonal BiFeO films, this effect is
superimposed on the lateral piezoresponse due to actual in-plane polarization,
and has to be taken into account in order to correctly interpret the
ferroelectric domain configuration.Comment: 4 pages, 3 figure
Optimal ferromagnetically-coated carbon nanotube tips for ultra-high resolution magnetic force microscopy
Combining half-metals and multiferroics into epitaxial heterostructures for spintronics
We report on the growth of epitaxial bilayers of the La2/3Sr1/3MnO3 (LSMO)
half-metallic ferromagnet and the BiFeO3 (BFO) multiferroic, on SrTiO3(001) by
pulsed laser deposition. The growth mode of both layers is two-dimensional,
which results in unit-cell smooth surfaces. We show that both materials keep
their properties inside the heterostructures, i.e. the LSMO layer (11 nm thick)
is ferromagnetic with a Curie temperature of ~330K, while the BFO films shows
ferroelectricity down to very low thicknesses (5 nm). Conductive-tip atomic
force microscope mappings of BFO/LSMO bilayers for different BFO thicknesses
reveal a high and homogeneous resistive state for the BFO film that can thus be
used as a ferroelectric tunnel barrier in tunnel junctions based on a
half-metal
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