121 research outputs found
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
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
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
Phase transition close to room temperature in BiFeO3 thin films
BiFeO3 (BFO) multiferroic oxide has a complex phase diagram that can be
mapped by appropriately substrate-induced strain in epitaxial films. By using
Raman spectroscopy, we conclusively show that films of the so-called
supertetragonal T-BFO phase, stabilized under compressive strain, displays a
reversible temperature-induced phase transition at about 100\circ, thus close
to room temperature.Comment: accepted in J. Phys.: Condens. Matter (Fast Track Communication
Irradiation-induced Ag nanocluster nucleation in silicate glasses: analogy with photography
The synthesis of Ag nanoclusters in sodalime silicate glasses and silica was
studied by optical absorption (OA) and electron spin resonance (ESR)
experiments under both low (gamma-ray) and high (MeV ion) deposited energy
density irradiation conditions. Both types of irradiation create electrons and
holes whose density and thermal evolution - notably via their interaction with
defects - are shown to determine the clustering and growth rates of Ag
nanocrystals. We thus establish the influence of redox interactions of defects
and silver (poly)ions. The mechanisms are similar to the latent image formation
in photography: irradiation-induced photoelectrons are trapped within the glass
matrix, notably on dissolved noble metal ions and defects, which are thus
neutralized (reverse oxidation reactions are also shown to exist). Annealing
promotes metal atom diffusion, which in turn leads to cluster nuclei formation.
The cluster density depends not only on the irradiation fluence, but also - and
primarily - on the density of deposited energy and the redox properties of the
glass. Ion irradiation (i.e., large deposited energy density) is far more
effective in cluster formation, despite its lower neutralization efficiency
(from Ag+ to Ag0) as compared to gamma photon irradiation.Comment: 48 pages, 18 figures, revised version publ. in Phys. Rev. B, pdf fil
Tunnel magnetoresistance and robust room temperature exchange bias with multiferroic BiFeO3 epitaxial thin films
We report on the functionalization of multiferroic BiFeO3 epitaxial films for
spintronics. A first example is provided by the use of ultrathin layers of
BiFeO3 as tunnel barriers in magnetic tunnel junctions with La2/3Sr1/3MnO3 and
Co electrodes. In such structures, a positive tunnel magnetoresistance up to
30% is obtained at low temperature. A second example is the exploitation of the
antiferromagnetic spin structure of a BiFeO3 film to induce a sizeable (~60 Oe)
exchange bias on a ferromagnetic film of CoFeB, at room temperature.
Remarkably, the exchange bias effect is robust upon magnetic field cycling,
with no indications of training.Comment: 15 pages, 4 figure
Electric-field control of spin waves at room temperature in multiferroic BiFeO3
To face the challenges lying beyond current CMOS-based technology, new
paradigms for information processing are required. Magnonics proposes to use
spin waves to carry and process information, in analogy with photonics that
relies on light waves, with several advantageous features such as potential
operation in the THz range and excellent coupling to spintronics. Several
magnonic analog and digital logic devices have been proposed, and some
demonstrated. Just as for spintronics, a key issue for magnonics is the large
power required to control/write information (conventionally achieved through
magnetic fields applied by strip lines, or by spin transfer from large
spin-polarized currents). Here we show that in BiFeO3, a room-temperature
magnetoelectric material, the spin wave frequency (>600 GHz) can be tuned
electrically by over 30%, in a non-volatile way and with virtually no power
dissipation. Theoretical calculations indicate that this effect originates from
a linear magnetoelectric effect related to spin-orbit coupling induced by the
applied electric field. We argue that these properties make BiFeO3 a promising
medium for spin wave generation, conversion and control in future magnonics
architectures.Comment: 3 figure
Influence of parasitic phases on the properties of BiFeO3 epitaxial thin films
We have explored the influence of deposition pressure and temperature on the
growth of BiFeO3 thin films by pulsed laser deposition onto (001)-oriented
SrTiO3 substrates. Single-phase BiFeO3 films are obtained in a region close to
10-2 mbar and 580C. In non-optimal conditions, X-ray diffraction reveals the
presence of Fe oxides or of Bi2O3. We address the influence of these parasitic
phases on the magnetic and electrical properties of the films and show that
films with Fe2O3 systematically exhibit a ferromagnetic behaviour, while
single-phase films have a low bulk-like magnetic moment. Conductive-tip atomic
force microscopy mappings also indicate that Bi2O3 conductive outgrowths create
shortcuts through the BiFeO3 films, thus preventing their practical use as
ferroelectric elements in functional heterostructures.Comment: sumbitted to Appl. Phys. Let
Epitaxial Bi2FeCrO6 Multiferroic Thin Films
We present here experimental results obtained on Bi2FeCrO6 (BFCO) epitaxial
films deposited by laser ablation directly on SrTiO3 substrates. It has been
theoretically predicted, by Baettig and Spaldin, using first-principles density
functional theory that BFCO is ferrimagnetic (with a magnetic moment of 2 Bohr
magneton per formula unit) and ferroelectric (with a polarization of ~80
microC/cm2 at 0K). The crystal structure has been investigated using X-ray
diffraction which shows that the films are epitaxial with a high crystallinity
and have a degree of orientation depending of the deposition conditions and
that is determined by the substrate crystal structure. Chemical analysis
carried out by X-ray Microanalysis and X-ray Photoelectron Spectroscopy (XPS)
indicates the correct cationic stoichiometry in the BFCO layer, namely
(Bi:Fe:Cr = 2:1:1). XPS depth profiling revealed that the oxidation state of Fe
and Cr ions in the film remains 3+ throughout the film thickness and that both
Fe and Cr ions are homogeneously distributed throughout the depth.
Cross-section high-resolution transmission electron microscopy images together
with selected area electron diffraction confirm the crystalline quality of the
epitaxial BFCO films with no identifiable foreign phase or inclusion. The
multiferroic character of BFCO is proven by ferroelectric and magnetic
measurements showing that the films exhibit ferroelectric and magnetic
hysteresis at room temperature. In addition, local piezoelectric measurements
carried out using piezoresponse force microscopy (PFM) show the presence of
ferroelectric domains and their switching at the sub-micron scale.Comment: Accepted for publication in Philosophical Magazine Letter
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