Emergence of a Dynamic Super-Structural Order Integrating Antiferroelectric and Antiferrodistortive Competing Instabilities in EuTiO3

Microscopic structural instabilities of EuTiO3 single crystal were investigated by synchrotron x-ray diffraction. Antiferrodistortive (AFD) oxygen octahedral rotational order was observed alongside Ti derived antiferroelectric (AFE) distortions. The competition between the two instabilities is reconciled through a cooperatively modulated structure allowing both to coexist. The electric and magnetic field effect on the modulated AFD order shows that the origin of large magnetoelectric coupling is based upon the dynamic equilibrium between the AFD - antiferromagnetic interactions versus the electric polarization - ferromagnetic interactions

Remanence plots as a probe of spin disorder in magnetic nanoparticles

Remanence magnetization plots (e.g., Henkel or δM plots) have been extensively used as a straightforward way to determine the presence and intensity of dipolar and exchange interactions in assemblies of magnetic nanoparticles or single domain grains. Their evaluation is particularly important in functional materials whose performance is strongly affected by the intensity of interparticle interactions, such as patterned recording media and nanostructured permanent magnets, as well as in applications such as hyperthermia and magnetic resonance imaging. Here, we demonstrate that δM plots may be misleading when the nanoparticles do not have a homogeneous internal magnetic configuration. Substantial dips in the M plots of γ-FeO nanoparticles isolated by thick SiO shells indicate the presence of demagnetizing interactions, usually identified as dipolar interactions. Our results, however, demonstrate that it is the inhomogeneous spin structure of the nanoparticles, as most clearly evidenced by Mössbauer measurements, that has a pronounced effect on the δM plots, leading to features remarkably similar to those produced by dipolar interactions. X-ray diffraction results combined with magnetic characterization indicate that this inhomogeneity is due to the presence of surface structural (and spin) disorder. Monte Carlo simulations unambiguously corroborate the critical role of the internal magnetic structure in the δM plots. Our findings constitute a cautionary tale on the widespread use of remanence plots to assess interparticle interactions as well as offer new perspectives in the use of Henkel and δM plots to quantify the rather elusive inhomogeneous magnetization states in nanoparticles

Core Size and Interface Impact on the Exchange Bias of Cobalt/Cobalt Oxide Nanostructures

Two series of Co/Co-oxide nanostructures have been synthesized by the co-precipitation method followed by different reduction and oxidation processes in an attempt to optimize their exchange bias (EB) properties. The samples are characterized by X-ray diffraction, scanning and transmission electron microscopy, and SQUID (superconducting quantum interference device) magnetometry. The two series differ with respect to their average Co core grain sizes: in one (the l-series), the size is ≈100 nm, and in the other (the s-series, obtained using lower synthesis temperatures than the l-series), it is ≈10 nm. In the l-series, progressive oxidation yields an increase in the EB field together with a reduction in Co core size. In contrast, progressive oxidation in the s-series results in growth of the Co-oxide fraction at the expense of the Co core upon oxidation, which is accompanied by a decrease in the EB effect that is attributed to an ordering of the ferromagnetic–antiferromagnetic interface and therefore a reduction of uncompensated spins density. These results illustrate how the interface details become relevant only for small enough ferromagnetic cores

Gas Phase Synthesis of Multi-Element Nanoparticles

The advantages of gas-phase synthesis of nanoparticles in terms of size control and flexibility in choice of materials is well known. There is increasing interest in synthesizing multi-element nanoparticles in order to optimize their performance in specific applications, and here, the flexibility of material choice is a key advantage. Mixtures of almost any solid materials can be manufactured and in the case of core–shell particles, there is independent control over core size and shell thickness. This review presents different methods of producing multi-element nanoparticles, including the use of multiple targets, alloy targets and in-line deposition methods to coat pre-formed cores. It also discusses the factors that produce alloy, core–shell or Janus morphologies and what is possible or not to synthesize. Some applications of multi-element nanoparticles in medicine will be described

Reconfigurable Mechanical Anisotropy in Self-Assembled Magnetic Superstructures

Enhancement of mechanical properties in self-assembled superstructures of magnetic nanoparticles is a new emerging aspect of their remarkable collective behavior. However, how magnetic interactions modulate mechanical properties is, to date, not fully understood. Through a comprehensive Monte Carlo investigation, we demonstrate how the mechanical properties of self-assembled magnetic nanocubes can be controlled intrinsically by the nanoparticle magnetocrystalline anisotropy (MA), as well as by the superstructure shape anisotropy, without any need for changes in structural design (i.e. nanoparticle size, shape and packing arrangement). A low MA-to-dipolar energy ratio, as found in iron oxide and permalloy systems, favors isotropic mechanical superstructure stabilization, whereas a high ratio yields magnetically blocked nanoparticle macrospins which can give rise to metastable superferromagnetism, as expected in cobalt ferrite simple cubic supercrystals. Such full parallel alignment of the particle moments is shown to induce mechanical anisotropy, where the superior high-strength axis can be remotely reconfigured by means of an applied magnetic field. Our results pave the way for the experimental realization of smart magneto-micromechanical systems (based, e.g., on the permanent supermagnetostriction effect illustrated here) and inspire new design rules for applied functional materials

Magnetically Enhanced Mechanical Stability and Super-Size Effects in Self-Assembled Superstructures of Nanocubes

Artificial materials from the self‐assembly of magnetic nanoparticles exhibit extraordinary collective properties; however, to date, the contribution of nanoscale magnetism to the mechanical properties of this class of materials is overlooked. Here, through a combination of Monte Carlo simulations and experimental magnetic measurements, this contribution is shown to be important in self‐assembled superstructures of magnetite nanocubes. By simulating the relaxation of interacting macrospins in the superstructure systems, the relationship between nanoscale magnetism, nanoparticle arrangement, superstructure size, and mechanical stability is established. For all considered systems, a significant enhancement in cohesive energy per nanocube (up to 45%), and thus in mechanical stability, is uncovered from the consideration of magnetism. Magnetic measurements fully support the simulations and confirm the strongly interacting character of the nanocube assembly. The studies also reveal a novel super‐size effect, whereby mechanically destabilization occurs through a decrease in cohesive energy per nanocube as the overall size (number of particles) of the system decreases. The discovery of this effect opens up new possibilities in size‐controlled tuning of superstructure properties, thus contributing to the design of next‐generation self‐assembled materials with simultaneous enhancement of magnetic and mechanical properties

Nanoribbon arrays of palladium nanoparticles on flexible substrates for transparent conduction and hydrogen detection

CMD2020GEFES, August 31st - September 4t

A High-Pressure Structure in Curium Linked to Magnetism

Curium lies at the center of the actinide series and has a half-filled shell with seven 5f electrons spatially residing inside its radon core. As a function of pressure, curium ibits five different crystallographic phases up to 100 gigapascals, of which all but one are so found in the preceding element, americium. We describe here a structure in curium, Cm III, with monoclinic symmetry, space group C2/c, found at intermediate pressures (between 37 and 56 gigapascals). Ab initio electronic structure calculations agree with the observed sequence of structures and establish that it is the spin polarization of curium's 5f electrons that stabilizes Cm III. The results reveal that curium is one of a few elements that has a lattice structure stabilized by magnetism.JRC.E.6-Actinides researc

Maximizing Exchange Bias in Co/CoO Core/Shell Nanoparticles by Lattice Matching between the Shell and the Embedding Matrix

The exchange bias properties of 5 nm Co/CoO ferromagnetic/antiferromagnetic core/shell nanoparticles, highly dispersed in a CuO matrix, have been optimized by matching the lattice parameter of the matrix with that of the CoO shell. Exchange bias and coercivity fields as large as H = 7780 Oe and H = 6950 Oe are linked to the presence of a CuO matrix (0.3% lattice mismatch with respect to the shells). The small mismatch between CuO and CoO plays a dual role: it (i) structurally stabilizes the CoO and (ii) favors the existence of a large amount of uncompensated moments in the shell that enhance the exchange bias effects. The results indicate that lattice matching may be a very efficient way to improve the exchange bias properties of core/shell nanoparticles, paving the way to novel approaches to tune their magnetic properties

Effectiveness of Silver Nanoparticles Deposited in Facemask Material for Neutralising Viruses

Cloth used for facemask material has been coated with silver nanoparticles using an aerosol method that passes pure uncoated nanoparticles through the cloth and deposits them throughout the volume. The particles have been characterized by electron microscopy and have a typical diameter of 4 nm with the atomic structure of pure metallic silver presented as an assortment of single crystals and polycrystals. The particles adhere well to the cloth fibers, and the coating consists of individual nanoparticles at low deposition times, evolving to fully agglomerated assemblies in heavy coatings. The cloth was exposed to Usutu virus and murine norovirus particles in suspension and allowed to dry, following which, the infectious virus particles were rescued by soaking the cloth in culture media. It was found that up to 98% of the virus particles were neutralized by this contact with the silver nanoparticles for optimum deposition conditions. The best performance was obtained with agglomerated films and with polycrystalline nanoparticles. The work indicates that silver nanoparticles embedded in masks can neutralize the majority of virus particles that enter the mask and thus increase the opacity of masks to infectious viruses by up to a factor of 50. In addition, the majority of the virus particles released from the mask after use are non-infectious