24 research outputs found
Pulsed laser deposition growth of heteroepitaxial YBa2Cu3O7/La0.67Ca0.33MnO3 superlattices on NdGaO3 and Sr0.7La0.3Al0.65Ta0.35O3 substrates
Heteroepitaxial superlattices of [YBa2Cu3O7(n)/ La0.67Ca0.33MnO3(m)]x, where
n and m are the number of YBCO and LCMO monolayers and x the number of bilayer
repetitions, have been grown with pulsed laser deposition on NdGaO3 (110) and
Sr0.7La0.3Al0.65Ta0.35O3 (LSAT) (001). These substrates are well lattice
matched with YBCO and LCMO and, unlike the commonly used SrTiO3, they do not
give rise to complex and uncontrolled strain effects due to structural
transitions at low temperature. The growth dynamics and the structure have been
studied in-situ with reflection high energy electron diffraction (RHEED) and
ex-situ with scanning transmission electron microscopy (STEM), x-ray
diffraction, and neutron reflectometry. The individual layers are found to be
flat and continuous over long lateral distances with sharp and coherent
interfaces and with a well-defined thickness of the individual layer. The only
visible defects are antiphase boundaries in the YBCO layers that originate from
perovskite unit cell height steps at the interfaces with the LCMO layers. We
also find that the first YBCO monolayer at the interface with LCMO has an
unusual growth dynamics and is lacking the CuO chain layer while the subsequent
YBCO layers have the regular Y-123 structure. Accordingly, the CuO2 bilayers at
both the LCMO/YBCO and the YBCO/LCMO interfaces are lacking one of their
neighboring CuO chain layers and thus half of their hole doping reservoir.
Nevertheless, from electric transport measurements on asuperlattice with n=2 we
obtain evidence that the interfacial CuO2 bilayers remain conducting and even
exhibit the onset of a superconducting transition at very low temperature.
Finally, we show from dc magnetization and neutron reflectometry measurements
that the LCMO layers are strongly ferromagnetic
Directed evolution of artificial repeat proteins as habit modifiers for the morphosynthesis of (111)-terminated gold nanocrystals
Natural biocomposites are shaped by proteins that have evolved to interact with inorganic materials. Protein directed evolution methods which mimic Darwinian evolution have proven highly successful to generate improved enzymes or therapeutic antibodies but have rarely been used to evolve proteinâmaterial interactions. Indeed, most reported studies have focused on short peptides and a wide range of oligopeptides with chemical binding affinity for inorganic materials have been uncovered by phage display methods. However, their small size and flexible unfolded structure prevent them from dictating the shape and crystallinity of the growing material. In the present work, a specific set of artificial repeat proteins (αRep), which exhibit highly stable 3D folding with a well-defined hypervariable interacting surface, is selected by directed evolution of a very efficient home-built protein library for their high and selective affinity for the Au(111) surface. The proteins are built from the extendable concatenation of self-compatible repeated motifs idealized from natural HEAT proteins. The high-yield synthesis of Au(111)-faceted nanostructures mediated by these αRep proteins demonstrates their chemical affinity and structural selectivity that endow them with high crystal habit modification performances. Importantly, we further exploit the protein shell spontaneously assembled on the nanocrystal facets to drive protein-mediated colloidal self-assembly and on-surface enzymatic catalysis. Our method constitutes a generic tool for producing nanocrystals with determined faceting, superior biocompatibility and versatile bio-functionalization towards plasmon-based devices and (bio)molecular sensors
Chemical Ordering in Bimetallic FeCo Nanoparticles: From a Direct Chemical Synthesis to Application As Efficient High-Frequency Magnetic Material
Single-crystalline FeCo nanoparticles with tunable size and shape were prepared by co-decomposing two metal-amide precursors under mild conditions. The nature of the ligands introduced in this organometallic synthesis drastically affects the reactivity of the precursors and, thus, the chemical distribution within the nanoparticles. The presence of the B2 short-range order was evidenced in FeCo nanoparticles prepared in the presence of HDAHCl ligands, combining 57 Fe Mössbauer, zero-field 59 Co ferromagnetic nuclear resonance (FNR), and X-ray diffraction studies. This is the first time that the B2 structure is directly formed during synthesis without the need of any annealing step. The as-prepared nanoparticles exhibit magnetic properties comparable with the ones for the bulk (M s = 226 Am 2 ·kg -1 ). Composite magnetic materials prepared from these FeCo nanoparticles led to a successful proof-of-concept of the integration on inductor-based filters (27% enhancement of the inductance value at 100 MHz)
Magnetic Proximity Effect in YBaâCuâOâ/La<sub>2/3</sub>Ca<sub>1/3</sub>MnOâ and YBaâCuâOâ/LaMnOââ Superlattices
Using neutron reflectometry and resonant x-ray techniques we studied the magnetic proximity effect (MPE) in superlattices composed of superconducting YBaâCuâOâ and ferromagnetic-metallic La0.67Ca0.33MnOâ or ferromagnetic-insulating LaMnOââ. We find that the MPE strongly depends on the electronic state of the manganite layers, being pronounced for the ferromagnetic-metallic La0.67Ca0.33MnOâ and almost absent for ferromagnetic-insulating LaMnOââ. We also detail the change of the magnetic depth profile due to the MPE and provide evidence for its intrinsic nature
Le volcanisme de l'ßle d'Erromango (République de Vanuatu) : calendrier de l'activité (données 40K-40Ar)
Le volcanisme de l'ßle d'Erromango (République de Vanuatu) : calendrier de l'activité (données 40K-40Ar)
Switching field distribution of ultradense arrays of single-crystalline magnetic nanowires
International audienceUltradense arrays of magnetic nanoelements present considerable interest for extending areal densities in magnetic recording media, provided that they display high switching fields and corresponding low standard deviations. Here, we report the switching field distribution of bottomâup synthesized single-crystalline vertical Co nanowires self-organized in 2D hexagonal superlattices. The combined shape and Co hexagonal compact magnetocrystalline anisotropies in individual nanowires of diameter as small as 6ânm define a robust perpendicular magnetic anisotropy despite important interactions in superlattices of 10âĂâ1012 NWs/in2. Using quantitative analysis of temperature-dependent first-order reversal curves, we capture the switching field distribution in this dipolar-coupled perpendicularly magnetized nanomagnets. First, the interwire dipolar interactions are treated separately and show a dominant mean field character with temperature independent amplitudes that scale with the nanowire packing fraction. Then, the intrinsic switching field distribution, namely, independent of interwire interactions, is determined as a function of temperature in the 5â300âK range. The mean value and deviation are both found to be driven by the intrawire dipolar interaction and the temperature-dependent uniaxial magnetocrystalline anisotropy, but of smaller amplitudes than those expected from bulk behavior. With coercive fields ranging between 0.3 and 0.8âT, the switching field deviations relative to coercivity reach 20%, which is a moderate value regarding pitch arrays as small as 8ânm
Directed evolution of artificial repeat proteins as habit modifiers for the morphosynthesis of (111)-terminated gold nanocrystals
International audienceNatural biocomposites are shaped by proteins that have evolved to interact with inorganic materials. Protein directed evolution methods which mimic Darwinian evolution have proven highly successful to generate improved enzymes or therapeutic antibodies but have rarely been used to evolve protein-material interactions. Indeed, most reported studies have focused on short peptides and a wide range of oligopeptides with chemical binding affinity for inorganic materials have been uncovered by phage display methods. However, their small size and flexible unfolded structure prevent them from dictating the shape and crystallinity of the growing material. In the present work, a specific set of artificial repeat proteins (αRep), which exhibit highly stable 3D folding with a well-defined hypervariable interacting surface, is selected by directed evolution of a very efficient home-built protein library for their high and selective affinity for the Au(111) surface. The proteins are built from the extendable concatenation of self-compatible repeated motifs idealized from natural HEAT proteins. The high-yield synthesis of Au(111)-faceted nanostructures mediated by these αRep proteins demonstrates their chemical affinity and structural selectivity that endow them with high crystal habit modification performances. Importantly, we further exploit the protein shell spontaneously assembled on the nanocrystal facets to drive protein-mediated colloidal self-assembly and on-surface enzymatic catalysis. Our method constitutes a generic tool for producing nanocrystals with determined faceting, superior biocompatibility and versatile bio-functionalization towards plasmon-based devices and (bio)molecular sensors
On the advantages of spring magnets compared to pure FePt: Strategy for rare-earth free permanent magnets following a bottom-up approach
International audienceNanostructured magnets benefiting from efficient exchange-coupling between hard and soft grains represent an appealing approach for integrated miniaturized magnetic power sources. Using a bottom-up approach, nanostructured materials were prepared from binary assemblies of bcc FeCo and fcc FePt nanoparticles and compared with pure L10-FePt materials. The use of a bifunctional mercapto benzoic acid yields homogeneous assemblies of the two types of particles while reducing the organic matter amount. The 650 °C thermal annealing, mandatory to allow the L10-FePt phase transition, led to an important interdiffusion and thus decreased drastically the amount of soft phase present in the final composites. The analysis of recoil curves however evidenced the presence of an efficient interphase exchange coupling, which allows obtaining better magnetic performances than pure L10 FePt materials, energy product above 100 kJ mâ3 being estimated for a Pt content of only 33%. These results clearly evidenced the interest of chemically grown nanoparticles for the preparation of performant spring-magnets, opening promising perspective for integrated subcentimetric magnets with optimized properties