Riad Nechache

Bifunctional catalytic/magnetic Ni@Ru core–shell nanoparticles

Core-shell structured Ni@Ru bimetallic nanoparticles are demonstrated as a bifunctional nanoplatform system for the hydrolysis reaction of ammonia-borane and also for magnetic separation.

Growth, structure, and properties of epitaxial thin films of first-principles predicted multiferroic Bi2FeCrO6

The authors report the structural and physical properties of epitaxial Bi2FeCrO6 thin films on epitaxial SrRuO3 grown on (100)-oriented SrTiO3 substrates by pulsed laser ablation. The 300nm thick films exhibit both ferroelectricity and magnetism at room temperature with a maximum dielectric polarization of 2.8μC∕cm2 at Emax=82kV∕cm and a saturated magnetization of 20emu∕cm3 (corresponding to ∼0.26μB per rhombohedral unit cell), with coercive fields below 100Oe. The results confirm the predictions made using ab initio calculations about the existence of multiferroic properties in Bi2FeCrO6.

Epitaxial thin films of the multiferroic double perovskite Bi2FeCrO6 grown on (100)-oriented SrTiO3 substrates: Growth, characterization, and optimization

The influence of the deposition pressure (PO2) and substrate temperature (TS) during the growth of Bi2FeCrO6 thin films grown by pulsed laser deposition has been investigated. It is found that the high volatility of Bi makes the deposition very difficult and that the growth of pure Bi2FeCrO6 thin films on SrTiO3 substrates is possible only in a narrow deposition parameter window. We find that the pure Bi2FeCrO6 phase is formed within a narrow window around an oxygen pressure PO2=1.2×10−2 mbar and around a substrate temperature TS=680 °C. At lower temperature or higher pressure, Bi7.38Cr0.62O12+x (also called b∗Bi2O3) and Bi2Fe4O9/Bi2(Fe,Cr)4O9+x phases are detected, while at lower pressure or higher temperature a (Fe,Cr)3O4 phase forms. Some of these secondary phases are not well known and have not been previously studied. We previously reported Fe/Cr cation ordering as the probable origin of the tenfold improvement in magnetization at saturation of our Bi2FeCrO6 film, compared to BiFeO3. Here, we address...

Epitaxial Patterning of Bi2FeCrO6 Double Perovskite Nanostructures: Multiferroic at Room Temperature

There is an increasing interest in developing and characterizing multifunctional materials, as they exhibit rich physical and chemical properties and offer exciting opportunities, for example, to miniaturize integrated devices and extend the potential of establishing nanoarchitecture concepts. [ 1 , 2 ] In this context, multiferroic materials [ 3 ] which combine two or more ferroic functionalities are promising for applications in fi elds such as spintronics and non-volatile data storage. [ 4 ] As recently demonstrated [ 5 ] ferromagnetic–ferroelectric multiferroics can be advantageously used to encode the information in electric polarization and magnetization giving rise to a four logic state memory. The coupling between magnetic and electrical properties leads to additional versatility for related devices, such as electric fi eld-controlled magnetic data storage. [ 6 ] The recent emergence of Bi-based double perovskite thin fi lms, such as Bi 2 FeCrO 6 (BFCO) [ 7– 9 ] and Bi 2 CoMnO 6 , [ 10 ] with strong magnetic behavior at room temperature, create opportunities to practically apply multiferroics. In competition with rival technologies, downscaling the feature size of multifunctional materials is an important step to achieve very high-density memory devices. [ 11 , 12 ]

Enhanced magnetism in epitaxial BiFeO3∕BiCrO3 multiferroic heterostructures

Multifunctional BiFeO3 (BFO), BiCrO3 (BCO), and BFO/BCO heteroepitaxial films with good ferroelectric properties were grown by pulsed laser deposition on epitaxial CaRuO3-coated (100)-oriented LaAlO3 and (LaAlO3)0.3(Sr2LaTaO6)0.7. The in-plane compressive strain induces a tetragonal crystal structure in both BFO and BCO films in the BFO/BCO bilayer, in contrast to their bulk counterparts. The small saturation magnetization observed at room temperature for the individual layers of BiFeO3 and BiCrO3 is significantly enhanced (twofold) in the bilayer heterostructures. We attribute this improvement to the superexchange magnetic interaction between Fe3+ and Cr3+, occurring at the interface, combined with the compressive strain.

Epitaxial thin films of multiferroic Bi2FeCrO6 with B-site cationic order

Epitaxial thin films of Bi2FeCrO6 have been synthesized by pulsed laser deposition on SrRuO3 on (100)- and (111)-oriented SrTiO3 substrates. Detailed X-ray diffraction and cross-section transmission electron microscopy analysis revealed a double perovskite crystal structure of the Bi2FeCrO6 epitaxial films very similar to that of BiFeO3 along with a particularly noteworthy Fe3+/Cr3+ cation ordering along the [111] direction. The films contain no detectable magnetic iron oxide impurities and have the correct cationic average stoichiometry throughout their thickness. They however exhibit a slight modulation in the Fe and Cr compositions forming complementary stripe patterns, suggesting minor local excess or depletion of Fe and Cr. The epitaxial BFCO films exhibit good ferroelectric and piezoelectric properties, in addition to magnetic properties at room temperature, as well as an unexpected crystallographic orientation dependence of their room temperature magnetic properties. Our results qualitatively confirm the predictions made using the ab-initio calculations: the double-perovskite structure of Bi2FeCrO6 films exhibit a Fe3+/Cr3+ cation ordering and good multiferroic properties, along with the unpredicted existence of magnetic ordering at room temperature.

Imaging, Core-Loss, and Low-Loss Electron-Energy-Loss Spectroscopy Mapping in Aberration-Corrected STEM

High-angle annular dark-field and annular bright-field imaging experiments were carried out on an aberration-corrected transmission electron microscope. These techniques have been demonstrated on thin films of complex oxides Ba3.25La0.75Ti3O12 and on LaB6. The results show good agreement between theory and experiments, and for the case of LaB6 they demonstrate the detection of contrast from the B atoms in the annular bright-field images. Elemental mapping with electron-energy-loss spectroscopy has been used to deduce the distribution of Cr and Fe in a thin film of the complex oxide Bi2(Fe1/2Cr3/2)O6 at the unit cell level and the changes in the near-edge structure within the inequivalent regions in the crystalline unit cell. Energy-filtered images in the low-loss region of the energy-loss spectrum show contrast and resolution consistent with the modulation of the signals from elastic scattering. High-resolution contrast, mediated by phonon scattering, is observed for interband transitions. The limitations in terms of detection and signal are discussed.

Single-crystalline BiFeO3 nanowires and their ferroelectric behavior

We report the ferroelectric properties of single-crystalline BiFeO3 nanowires using piezoresponse force microscopy (PFM). The nanowires, synthesized by a hydrothermal approach, have a rhombohedral perovskite structure and a preferential growth of the (211) crystallographic plane perpendicular to the wire axis, as revealed by x-ray and electron diffraction investigations. PFM measurements reveal that the as-synthesized BiFeO3 nanowires, down to 40 nm in diameter, have components of spontaneous polarization along both the axial and radial directions, thereby demonstrating the ferroelectric nature of the wires. The results indicate that such ferroelectric BiFeO3 nanowires should provide promising opportunity for nanoscale nonvolatile memory devices.

Epitaxial Bi2FeCrO6 multiferroic thin films

We present experimental results on Bi2FeCrO6 (BFCO) epitaxial films deposited by laser ablation directly on SrTiO3 substrates. It has been theoretically predicted by Baettig and Spaldin [Appl. Phys. Lett. 86 012505 (2005)], using first-principles density functional theory, that BFCO is ferrimagnetic (with a magnetic moment of 2 µB per formula unit) and ferroelectric (with a polarization of ∼80 µC/cm2 at 0 K). 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 on deposition conditions 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 plus 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 demonstrated 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.

Towards ferroelectric and multiferroic nanostructures and their characterisation

We summarise here our efforts toward the fabrication and characterisation of ferroelectric and multiferroic films and structures. First, we discuss the challenges related with the fabrication and characterisation of nanostructures of functional complex oxides. In particular, to demonstrate the functionality of our films and especially of our structures, we briefly describe atomic force microscopy techniques tailored for local electrical or magnetic characterisation. Piezoresponse Force Microscopy and Magnetic Force Microscopy enable the characterisation of piezoelectric, ferroelectric and magnetic properties at the nanoscale. We then report the fabrication of various functional oxide films by Pulsed Laser Deposition (PLD), in particular the deposition of the conducting oxide electrode SrRuO3 at room temperature. We also describe the fabrication of ferroelectric BaTiO3 and BiFeO3, both in the form of film and mesoscopic (sub-micron size) islands. The formation of ferroelectric structures of arbitrary shape at controlled location was also achieved by nanostencilling, i.e., using a shadow-mask with nanoscale features. Finally, the successful synthesis of Bi2FeCrO6 films by pulsed laser deposition is then detailed; this is a new multiferroic material predicted by ab-initio calculations. The Bi2FeCrO6 films have the correct cationic stoichiometry throughout their thickness and their crystal structure is found to be very similar to that of BiFeO3. Bi2FeCrO6 films exhibit good piezoelectric and ferroelectric properties at room temperature, a property that was not predicted. Magnetic Force Microscopy reveals the presence of magnetic domains and confirms the macroscopic magnetic measurements showing that the Bi2FeCrO6 films do exhibit a saturation magnetisation about one order of magnitude higher than that of BiFeO3 films having the same thickness.