The fundamental question as to the relative importance of interparticle superexchange versus dipolar interaction between oxide magnetic particles in direct physical contact is addressed by examining the magnetic properties of a series of compacted samples comprising identical maghemite particles (8 nm in diameter) coated by nonmagnetic shells (oleic acid or silica) of varying thickness that control the distance between the magnetic cores and hence the packing density (particle volume fraction). A remarkably narrow maghemite particle size distribution is established by electron microscopy and small-angle X-ray scattering. The series includes a sample made up of bare particles in a random-close-packed configuration (therefore in direct contact) that exhibits ideal superspin-glass behavior with a relatively high freezing transition temperature. It is shown that interparticle superexchange interactions between the nanoparticles in this sample play a minor role compared to classical dipolar interactions in establishing the collective, superspin-glass state. This follows from the freezing temperature of the most concentrated samples in the series (those with 0 ≤ shell thickness < 3 nm), which are found to vary in direct proportionality with the volume fraction of the maghemite cores and therefore with the strength of dipolar interactions.
A simple single-phase material, a random close-packed (volume fraction 67%) ensemble of highly monodisperse bare maghemite (γ-Fe2O3) nanoparticles, is shown to exhibit ideal superspin-glass behavior (mimicking that of model spin-glasses), namely, an unprecedentedly sharp onset of the absorption component of the ac susceptibility, narrow memory dips in the zero-field-cooled magnetization and a spin-glass characteristic field-dependence of the magnetic susceptibility. This ideal behavior is attributed to the remarkably narrow dispersion in particle size and to the highly dense and spatially homogeneous configuration ensured by the random close-packed arrangement. This material is argued to constitute the closest nanoparticle analogue to a conventional (atomic) magnetic state found to date.
A numerical investigation of an Immersed Boundary (IB) model of an effectively inextensible, finite swimmer in a Stokesian Oldroyd-B flow is presented. The swimmer model is a two-dimensional sheet of finite extent and its gait is generated by an elastic force which penalizes deviations from a target shape. A non-stiff IB method is employed to remove the impeding time step limitation induced by strong tangential forces on the swimmer. It is found that for a swimmer with a prescribed gait its mean propulsion speed decreases with increasing Deborah number De toward an apparent asymptotic minimal value. However, as the swimmer is allowed to deviate more from the target shape, the monotonic locomotion behavior with De is broken. For a sufficiently flexible swimmer, viscoelasticity can enhance locomotion but the swimmer in the viscoelastic fluid always remains slower than when it is propelling in a Newtonian fluid. Remarkably, the addition of viscoelastic stress diffusion dramatically alters the swimmer propulsion and can lead to a speed-up over the swimmer in the Newtonian fluid.
This paper describes the open Novamag database that has been developed for the design of novel Rare-Earth free/lean permanent magnets. The database software technologies, its friendly graphical user interface, advanced search tools and available data are explained in detail. Following the philosophy and standards of Materials Genome Initiative, it contains significant results of novel magnetic phases with high magnetocrystalline anisotropy obtained by three computational high-throughput screening approaches based on a crystal structure prediction method using an Adaptive Genetic Algorithm, tetragonally distortion of cubic phases and tuning known phases by doping. Additionally, it also includes theoretical and experimental data about fundamental magnetic material properties such as magnetic moments, magnetocrystalline anisotropy energy, exchange parameters, Curie temperature, domain wall width, exchange stiffness, coercivity and maximum energy product, that can be used in the study and design of new promising high-performance Rare-Earth free/lean permanent magnets. The results therein contained might provide some insights into the ongoing debate about the theoretical performance limits beyond Rare-Earth based magnets. Finally, some general strategies are discussed to design possible experimental routes for exploring most promising theoretical novel materials found in the database. NOVAMAG H c > M r /2 [5]. Extrinsic properties also depend on temperature, in fact they typically decrease as temperature increases, reducing the magnet's performance, especially close to the Curie temperature T C (i.e. ferromagnetic-paramagenetic transition). The macroscopic behavior is tightly linked to the microscopic properties called intrinsic. Main magnetic intrinsic properties are atomic magnetic moment µ at (the magnetic moment per volume gives the maximum theoretical M s ), exchange interactions J ij (which determine the magnetic order and T C )and magnetocrystalline anisotropy K 1 (that can enhance H c and it is indispensable in modern magnets to get H c > M r /2) [6]. PMs should have high atomic mangetic moments per volume (> 0.1µ B /Å 3 ), strong ferromangetic exchange interactions (able to give T C > 600 K) and high easy axis magnetocrystalline anisotropy (K 1 > 1 MJ/m 3 ) in order to exhibit good extrinsic properties suitable for PM applcations. In particular, magnetic materials with hardness parameter κ = K 1 /(µ 0 M 2 s ) > 1 (called "hard" magnets) are very valuable since can be used to make efficient magnets of any shape [6]. At mesoscopic scale, intergranular structure between the grains and crystallographic defects can strongly affect the performance of a magnet [7]. Therefore, the optimization of the material's microstructure is also very important in the design and devel-
A multistep superelastic behavior, with up to a 12% strain, is reported in a 〈001〉P-oriented Ni49Mn28Ga23 single crystal. The observed behavior is produced by intermartensitic transformations during the tensile stress–strain measurements at temperatures between −140 °C and +60 °C. The tensile stress–temperature phase diagram and the stress dependence of the intermartensitic transformation entropies have been obtained. These results provide important input for theoretical modeling of the phase transformations in these alloys and show promising mechanical properties of the classical Ni-Mn-Ga ferromagnetic shape memory alloys.
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