67 research outputs found
Growth, Structure and Properties of BiFeO3-BiCrO3 Films obtained by Dual Cross Beam PLD
The properties of epitaxial Bi2FeCrO6 thin films, recently synthesized by
pulsed laser deposition, have partially confirmed the theoretical predictions
(i.e. a magnetic moment of 2 muB per formula unit and a polarization of ~80
microC/cm2 at 0K). The existence of magnetic ordering at room temperature for
this material is an unexpected but very promising result that needs to be
further investigated. Since magnetism is assumed to arise from the exchange
interaction between the Fe and Cr cations, the magnetic behaviour is strongly
dependent on both their ordering and the distance between them. We present here
the successful synthesis of epitaxial Bi2FexCryO6 (BFCO x/y) films grown on
SrTiO3 substrates using dual crossed beam pulsed laser deposition. The crystal
structure of the films has different types of (111)-oriented superstructures
depending on the deposition conditions. The multiferroic character of BFCO
(x/y) films is proven by the presence of both ferroelectric and magnetic
hysteresis at room temperature. The oxidation state of Fe and Cr ions in the
films is shown to be 3+ only and the difference in macroscopic magnetization
with Fe/Cr ratio composition could only be due to ordering of the Cr3+ and Fe3+
cations therefore to the modification of the exchange interaction between them.Comment: Manuscript accepted for publication in IEEE-UFF
Infrared and magnetic characterization of the multiferroic Bi2FeCrO6 thin films in a broad temperature range
Infrared reflectance spectra of an epitaxial Bi2FeCrO6 thin film prepared by
pulsed laser deposition on LaAlO3 substrate were recorded between 10 and 900 K.
No evidence for a phase transition to the paraelectric phase was observed, but
some phonon anomalies were revealed near 600 K. Most of the polar modes exhibit
only a gradual softening, which results in a continuous increase of the static
permittivity on heating. It indicates that the ferroelectric phase transition
should occur somewhere above 900 K. Magnetic measurements performed up to 1000
K, revealed a possible magnetic phase transition between 600 and 800 K, but the
exact critical temperature cannot be determined due to a strong diamagnetic
signal from the substrate. Nevertheless, our experimental data show that the
B-site ordered Bi2FeCrO6 is one of the rare high-temperature multiferroics.Comment: subm. to PR
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
Ferroelectricity induced by interatomic magnetic exchange interaction
Multiferroics, where two or more ferroic order parameters coexist, is one of
the hottest fields in condensed matter physics and materials science[1-9].
However, the coexistence of magnetism and conventional ferroelectricity is
physically unfavoured[10]. Recently several remedies have been proposed, e.g.,
improper ferroelectricity induced by specific magnetic[6] or charge orders[2].
Guiding by these theories, currently most research is focused on frustrated
magnets, which usually have complicated magnetic structure and low magnetic
ordering temperature, consequently far from the practical application. Simple
collinear magnets, which can have high magnetic transition temperature, have
never been considered seriously as the candidates for multiferroics. Here, we
argue that actually simple interatomic magnetic exchange interaction already
contains a driving force for ferroelectricity, thus providing a new microscopic
mechanism for the coexistence and strong coupling between ferroelectricity and
magnetism. We demonstrate this mechanism by showing that even the simplest
antiferromagnetic (AFM) insulator MnO, can display a magnetically induced
ferroelectricity under a biaxial strain
Ordered arrays of multiferroic epitaxial nanostructures
Epitaxial heterostructures combining ferroelectric (FE) and ferromagnetic (FiM) oxides are a possible route to explore coupling mechanisms between the two independent order parameters, polarization and magnetization of the component phases. We report on the fabrication and properties of arrays of hybrid epitaxial nanostructures of FiM NiFe2O4 (NFO) and FE PbZr0.52Ti0.48O3 or PbZr0.2Ti0.8O3, with large range order and lateral dimensions from 200 nm to 1 micron
Photovoltaic properties of Bi2FeCrO6 films epitaxially grown on (100)-oriented silicon substrates
We demonstrate the promising potential of using perovskite Bi2FeCrO6 (BFCO) for niche applications in photovoltaics (PV) (e.g. self-powered sensors that simultaneously exploit PV conversion and multiferroic properties) or as a complement to mature PV technologies like silicon. BFCO thin films were epitaxially grown on silicon substrates using an MgO buffer layer. Piezoresponse force microscopy measurements revealed that the tensile strained BFCO phase exhibits a polarization predominantly oriented through the in-plane direction. The semiconducting bandgap of the ordered BFCO phase combined with ferroelectric properties, opens the possibility of a ferroelectric PV efficiency above 2% in a thin film device and the use of ferroelectric materials simultaneously as solar absorber layers and carrier separators in PV devices. A large short circuit photocurrent density of 13.8 mA cm-2 and a photovoltage output of 0.5 V are typically obtained at FF of 38% for BFCO devices fabricated on silicon. We believe that the reduced photovoltage is due to the low diffusion length of photogenerated charge carriers in the BFCO material where the ferroelectric domains are predominately oriented in-plane and thus do not contribute efficiently to the photocharge separation process
Improved photovoltaic performance from inorganic perovskite oxide thin films with mixed crystal phases
Inorganic ferroelectric perovskites are attracting attention for the realization of highly stable photovoltaic cells with large open-circuit voltages. However, the power conversion efficiencies of devices have been limited so far. Here, we report a power conversion efficiency of ~4.20% under 1 sun illumination from Bi-Mn-O composite thin films with mixed BiMnO3 and BiMn2O5 crystal phases. We show that the photocurrent density and photovoltage mainly develop across grain boundaries and interfaces rather than within the grains. We also experimentally demonstrate that the open-circuit voltage and short-circuit photocurrent measured in the films are tunable by varying the electrical resistance of the device, which in turn is controlled by externally applying voltage pulses. The exploitation of multifunctional properties of composite oxides provides an alternative route towards achieving highly stable, high-efficiency photovoltaic solar energy conversion
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