Defect‐Driven Magnetization Configuration of Isolated Linear Assemblies of Iron Oxide Nanoparticles

International audienceThe magnetization state of one-dimensional magnetic nanoparticle chains plays a key role for a wide range of applications ranging from diagnosis and therapy in medicine to actuators, sensors and quantum recording media. The interplay between the exact particle orientation and the magnetic anisotropy is in turn crucial for controlling the overall magnetization state with high precision. Here, we report on a three-dimensional description of the magnetic structure of one-NP-wide chains. In this aim, we combined two complementary experimental techniques, magnetic force microscopy (MFM) and electronic holography (EH) which are sensitive to out-of-plane and in-plane magnetization components, respectively. We extended our approach to micromagnetic simulations which provided results in good agreement with MFM and EH. The findings are at variance with the known results on unidirectional nanoparticle assemblies, and show that magnetization is rarely strictly collinear to the chain axis. The magnetic structure of one-NP-wide chains can be interpreted as head-to-head magnetic domain structures with off-axis magnetization components, which is very sensitive to morphological defects in the chain structure such as minute size variation of NPs, tiny misalignment of NPs and/or crystal orientation with respect to easy magnetization axis

Two dimensional dipolar coupling in monolayers of silver and gold nanoparticles on a dielectric substrate

The dimensionality of assembled nanoparticles plays an important role in their optical and magnetic properties, via dipolar effects and the interaction with their environment. In this work we develop a methodology for distinguishing between two (2D) and three (3D) dimensional collective interactions on the surface plasmon resonance of assembled metal nanoparticles. Towards that goal, we elaborate different sets of Au and Ag nanoparticles as suspensions, random 3D arrangements and well organized 2D arrays. Then we model their scattering cross-section using effective field methods in dimension n, including interparticle as well as particle-substrate dipolar interactions. For this modelling, two effective field medium approaches are employed, taking into account the filling factors of the assemblies. Our results are important for realizing photonic amplifier devices

Assemblages de nanoparticules de Co sur des surfaces et leurs caractérisations locales par microscopie à force magnétique sous champ variable et spectroscopie tunnel

Résumé français : notice = 7Ko MAXIMUM : résumé trop long empêche la validation : longueur = 1700 caractéresRésumé anglais : idemSTRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

Chiromagnetoptics of Au and Ag Nanoparticulate Systems

International audiencePhotoelectron excitations into unoccupied energy states are fundamental in solid-state physics and many modern applications. The operation of such internal electronic transitions relies not only on photon absorption efficiency, which is pivotal in sensing, photovoltaics, active plasmonics, etc., but also on light-matter interactions and near/far-field spectroscopic techniques. Here, we report on a spectroscopic absorption fingerprint in magnetic circular dichroism (MCD) responses of various nanoparticulate systems. MCD spectra acquired at low temperatures on assemblies of uncoupled/coupled Ag and Au nanoparticles reveal asymmetric MCD spectral line shapes strongly depending on the nature and shape of nanoparticles and on their interparticular interactions. Assembling for instance the NPs into 2D networks with interparticle distances allowing dipolar interactions or transforming the spherical nanoparticles into disks was found to significantly alter the magneto-optic response in the spectral line shape. The findings are interpreted within the framework of chiral magneto-plasmonic effects by considering the k-dependent electronic states involved in both inter-and intraband electronic transitions

Nonlinear Phase Imaging of Gold Nanoparticles Embedded in Organic Thin Films

International audienceThe phase detection in the dynamic mode of the atomic force microscopes is a known technique for mapping nanoscale surface heterogeneities. We present here an additional functionality of this technique, which allows high-resolution imaging of embedded inorganic nanoparticles with diameter and interparticle distances of a few nanometers. The method is based on a highly nonlinear tip–sample interaction occurring markedly above the nanoparticles, giving thus a high phase contrast between zones with and without nanoparticles. A relationship between the tip–sample interaction strength and the phase signal is established in experiments and from calculations conducted with the model developed by Haviland et al. [ Soft Matter 2016, 12, 619], which is based on solving a combined equation of motion for both the cantilever and surface while taking into account the time-varying interaction forces. The nonlinear phase behavior at the origin of the subnanometer spatial resolution is found by numerical analyses to be the result of a local mechanical stiffening of the zone containing nanoparticles, which is enhanced by 2 orders of magnitude or mor

Thickness Dependence and Strain Effects in Ferroelectric Bi 2 FeCrO 6 Thin Films

International audienceWe report on the ferroelectric properties of epitaxially grown Bi 2 FeCrO 6 (BFCO) films with thicknesses of 7.5, 49 and 98 nm obtained by pulsed laser deposition. Because of the strains induced by the Nb-doped SrTiO 3 (001) substrate, the films exhibit a variable Fe-Cr order along the growth axis, with a disordered phase located near the interface and an increased order at the top of the films. This is first evidenced by X-ray diffraction and UV-visible-NIR absorption measurements as the ordered / disordered phases show different lattice parameters and band gaps. The strain effects which depend on the film thickness, are found to strongly impact on the ferroelectric properties. For the 49 nm thick film, piezoresponse force microscopy (PFM) reveals an out of plane intrinsic polarization orientation, effect which is absent for 98 nm thick films. The polarization anisotropy increases when reducing the thickness to 7.5 nm. The intrinsic polarization of the as-deposited 49 nm films induces a significant shift 1 of the current-voltage characteristics, demonstrating an I(V) hysteresis loop strongly weighted towards positive voltage values. The strain effects impact therefore on fer-roelectric domains polarization. Conductive atomic force microscopy (C-AFM) carried out on poled areas, demonstrates a significant asymmetric current-voltage characteristics and an open circuit voltage up to 4.3 V for the 49 nm-thick sample which decreases to 1.2 V for the 98 nm thick one. These values are larger or at least comparable with the band gap of the ordered phase Bi 2 FeCrO 6 phase (1.5 eV). Local current densities up to 20 A/cm 2 were measured by C-AFM under weak illumination, thus confirming the potential of Bi 2 FeCrO 6 for ferroelectric solar cells

Photonic Excitation of a Micromechanical Cantilever in Electrostatic Fields

International audienceWe present a specific near-field configuration where an electrostatic force gradient is found to strongly enhance the optomechanical driving of an atomic force microscope cantilever sensor. It is shown that incident photons generate a photothermal effect which couples with electrostatic fields even at tip-surface separations as large as several wavelengths, dominating the cantilever dynamics. The effect is the result of resonant phenomena where the photothermal-induced parametric driving acts conjointly (or against, depending on electric field direction) with a photovoltage generation in the cantilever. The results are achieved experimentally in an atomic force microscope operating in vacuum and explained theoretically through numerical simulations of the equation of motion of the cantilever. Intrinsic electrostatic effects arising from electronic work-function difference of tip and surface are also highlighted. The findings are readily relevant for other opto-micromechanical systems where electrostatic force gradients can be implemented

Role of lattice defects on the magnetism of gold nanoparticles irradiated with neutrons

International audienceThe origin of the unexpected magnetic properties of nanoparticles made of gold, silver, or other diamagnetic metals remains obscure despite a large body of experimental and theoretical studies. Whereas many studies have endeavoured at finding correlations between magnetism and nanoparticle size or ligand coating, none so far has attempted at investigating the role of core crystallinity. We have irradiated gold nanoparticles with energetic neutrons or protons with the aim of nucleating lattice defects in the crystalline cores. Comparison of the magnetic behaviours before and after irradiation demonstrates clearly that the presence of defects in the crystalline lattice of the nanoparticles cores contributes efficiently to their magnetism

Structural properties and polarization switching of epitaxial Bi2FeCrO6 thin films grown on La2/3Sr1/3MnO3/SrTiO3 (111) substrates

Ferroelectric Bi 2 FeCrO 6 (BFCO) double perovskite thin films were grown by pulsed laser deposition on SrTiO 3 (111) covered with a 20 nm thick La 2/3 Sr 1/3 MnO 3 buffer layer. X-ray diffraction measurements performed in θ-2θ and φ scans modes reveal an epitaxial growth of the BFCO with a compressive inplane strained structure and no spurious phases. The presence of superstructure diffraction peaks indicates an Fe-Cr cationic ordering along the growth axis. Local measurements realized by piezoresponse force microscopy show an asymmetric polarization switching with respect to the external bias voltage suggesting the presence of a bias field. This agrees with a preferential ferroelectric polarization observed upon local poling of the BFCO film in opposite directions. The findings can be of importance for understanding the relationship between the atomic structure and polarization dynamics in ferroelectric thin-films.Quantum Science and NanomaterialsNanostructures en Interaction avec leur Environnemen