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Core-shell structures of oxide nanoparticles having a high dielectric constant, and organic shells with large breakdown field are attractive candidates for large electrical energy storage applications. A high growth temperature, however, is required to obtain the dielectric oxide nanoparticles, which affects the process of core-shell formation and also leads to poor control of size, shape, and size-distribution. In this communication, we report a new synthetic process to grow core-shell nanoparticles by means of an experimental method that can be easily adapted to synthesize core-shell structures from a variety of inorganic-organic or inorganic-inorganic materials. Monodisperse and spherical TiO2 nanoparticles were produced at room temperature as a collimated cluster beam in the gas phase using a cluster-deposition source and subsequently coated with uniform paraffin nanoshells using in situ thermal evaporation, prior to deposition on substrates for further characterization and device processing. The paraffin nanoshells prevent the TiO2 nanoparticles from contacting each other and also act as a matrix in which the volume fraction of TiO2 nanoparticles was varied by controlling the thickness of the nanoshells. Parallel-plate capacitors were fabricated using dielectric core-shell nanoparticles having different shell thicknesses. With respect to the bulk paraffin, the effective dielectric constant of TiO2-paraffin core-shell nanoparticles is greatly enhanced with a decrease in the shell thickness. The capacitors show a minimum dielectric dispersion and low dielectric losses in the frequency range of 100 Hz-1 MHz, which are highly desirable for exploiting these core-shell nanoparticles for potential applications.

FePt nanoparticles with a size of 3 nm and thermally stable room-temperature ferromagnetism are investigated. The monodisperse nanoparticles were prepared by chemical synthesis and a salt-matrix annealing technique. Structural and magnetic characterizations confirmed the phase transition from the disordered face-centered cubic structure to the L10 structure with the chemical ordering parameter of 0.62±0.05. Analysis in blocking temperature and fitting of temperature dependence of switching field reveals that the transformed 3 nm nanoparticles have a magnetic anisotropy constant of (2.8±0.2)×106 J/m3, smaller than those for the bigger particles and the fully ordered L10 bulk phase. [ABSTRACT FROM AUTHOR]

The permittivity and permeability of the Tb-doped and undoped Fe core-shell nanoparticles were investigated for frequencies from 2 to 18 GHz. The particles were synthesized by arc discharge and contain some oxygen, probably in the form of Fe2O3, Fe3O4, and Tb2O3, whereas the core material is Fe. Both the electromagnetic materials constants and the morphology of the Fe nanoparticles are changed by Tb addition, which gives rise to the shifts to higher frequencies and thinner thicknesses of the maximum microwave absorption in the Tb-doped Fe nanoparticles. [ABSTRACT FROM AUTHOR]

The magnetic ground state and the magnetization reversal of aspherical nanoparticles are investigated by model calculations and micromagnetic simulations. Essential deviations from Kittel’s domain theory occur for very flat and very elongated particles, for particles whose size is comparable to the domain-wall width, and when the particle shape is strongly nonellipsoidal. For example, the single-domain state of square rod is much less stable than that of comparable elongated ellipsoids, because most of the surface charge of long ellipsoids is located on the sides. Another specific feature of flat particles is suppression of the curling mode. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The synthesis of hard-magnetic CoPt nanoparticle films by hydrogen reduction of Co nitride and Pt chloride mixture is reported. Thin porous alumina films are adopted as the carrier of the initial aqueous solution and of the final reducing products of CoPt nanoparticles. It is found that chemical ordering of L1[SUB0]-CoPt occurs above 400 °C. Partial phase transformation occurs in the alumina substrate when the treatment temperature is higher than 600 °C. The film coercivity increases with increasing annealing temperature and reaches a maximum value of 24.2 kOe when the reduction is carried out at 700 °C for 2 h. [ABSTRACT FROM AUTHOR]

Magnetization reversal in arrays of magnetic nanoparticles with perpendicular anisotropy is investigated. Aside from domain-wall propagation effects, the reversal involves two main aspects: the nucleation behavior of individual particles and interparticle interactions. Due to magnetostatic self-interaction effects, the interparticle interaction cannot be reduced to a stray-field correction to the external field. Exact nucleation fields and explicit stray-field and self-interaction contributions are obtained for rings of equidistant dots. An exact treatment of self-interactions in various structurally inhomogeneous but rotationally symmetric wire, sphere, and thin-film nanostructures leads to renormalization of the uniaxial anisotropy. Finally, an approximate method to calculate nucleation fields is discussed. [ABSTRACT FROM AUTHOR]

Cooperative and noncooperative magnetization processes in magnetic nanostructures are investigated. Using model calculations it is shown that the Preisach model and related approaches, such as Henkel, ΔM, and ΔH plots, describe magnetism on a mean-field level and cannot account for intra- and inter-granular cooperative effects. For example, the ΔM plot of a nucleation-controlled two-domain particle gives the false impression of a positive intergranular interaction. A simple but nontrivial cooperative model, consisting of two interacting but nonequivalent particles, is used to show that cooperative effects are most pronounced for narrow switching-field distributions, i.e., for nearly rectangular loops. This is unfavorable from the point of magnetic recording, where one aims at combining narrow loops with a noncooperative local switching behavior. A general rule is that the neglect of cooperative effects leads to an overestimation of the coercivity. &cop! y; 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The magnetization of noninteracting metallic nanoparticles is investigated by comparing the particle-size and temperature dependences of the magnetization for several mechanisms. The nanoparticle magnetization deviates from that of the underlying bulk materials due to zero-temperature and thermal effects, and on a mean-field level, the corresponding surface-core interaction is described by a Landau–Ginzburg approach. A major factor is the reduced atomic coordination at the surface, which has often, but not always, opposite effects on the zero-temperature magnetization and Curie temperatures. The coordination effect is particularly pronounced for very weak itinerant ferromagnets and for strongly exchange-enhanced Pauli paramagnets. With regard to external magnetic fields, the nanoparticle magnetization involves several “superparamagnetic” phenomena, namely, Néel relaxation, Brownian relaxation, and Langevin macrospin paramagnetism. [ABSTRACT FROM AUTHOR]

Intergranular interactions in nanostructured FePt:Ag thin films and their effect on magnetic hysteresis are investigated. The films, produced by multilayer deposition plus rapid thermal annealing, consist of FePt nanoparticles embedded in a silver matrix. They are investigated by magnetization mea- surements and magnetic force microscopy. Analytical model - calculations, supported by micromagnetic simulations, are used to elucidate the relation between coercivity, hysteresis-loop slope, and spatial correlations during magnetization reversal. The analytical calculations yield simple expressions for the loop slope and the coercivity as a function of the intergranular exchange. Small intergranular exchange enhances the coercivity, but for strong exchange there is a self-energy cutoff, associated with the onset of cooperative reversal and preventing alpha from becoming negative. [ABSTRACT FROM AUTHOR]

The atomic origin of micromagnetic damping is investigated by developing and solving a quantum-mechanical relaxation model. A projection-operator technique is used to derive an analytical expression for the relaxation time as a function of the heat-bath and interaction parameters. The present findings are consistent with earlier research beyond the Landau-Lifshitz-Gilbert (LLG) equation and show that the underlying relaxation mechanism is very general. Zermelo's recurrence paradox means that there is no true irreversibility in non-interacting nanoparticles, but the corresponding recurrence times are very long and can be ignored in many cases. [ABSTRACT FROM AUTHOR]

Sm-Co bulk alloys have shown superior permanent-magnet properties, but research on Sm-Co nanoparticles is challenging because of the need to control particle size, size-distribution, crystalline ordering, and phase purity. In the present study, a cluster-deposition method was used to produce Sm-Co nanoparticles having desired crystal structures without the requirement of subsequent high-temperature thermal annealing. Poorly crystallized SmCo5 nanoparticles exhibit a low room-temperature coercivity of only 100 Oe, whereas crystalline SmCo5 and Sm2Co17 nanoparticles show room-temperature coercivities of 2000 and 750 Oe, respectively. The direct synthesis of Sm-Co nanoparticles having sizes of less than 10 nm and a high degree of atomic ordering is an important step toward creating nanoparticle building blocks for permanent-magnets and other significant applications. [ABSTRACT FROM AUTHOR]

Nanoparticles of YCo2, YCo3, and YCo5 are produced by cluster-deposition and investigated bothstructurally and magnetically. The nanoparticles have sizes of less than 10 nm and aresuperparamagnetic at 300 K, irrespective of stoichiometry. As-produced nanoparticles exhibitdisordered structures with magnetic properties differing from those of the bulk particles.The temperature-dependent magnetization curves of the nanoparticles reveal blocking temperatures from 110 to 250 K, depending on stoichiometry. The magnetic anisotropy constant K1 of disordered YCo5 nanoparticles of 7.8 nm in size is 3.5 × 106 ergs/cm3, higher than those of thedisordered YCo2 (8.9 × 105 ergs/cm3) and YCo3 (1.0 × 106 ergs/cm3) nanoparticles. [ABSTRACT FROM AUTHOR]

The effect of magnetostatic and exchange interactions on the spin structure of interacting nanoparticles and granular nanomagnets is investigated by model calculations. Effective exchange stiffnesses for inhomogeneous media are defined and determined for some geometries and interactions, and it is argued that typical ensembles of interacting small nanoparticles are micromagnetic systems rather than superspin glasses or superferromagnets. The spin structures of granular magnets often have the character of interaction domains, with far-reaching implications for magnetic phenomena such as hysteresis-loop overskewing. [ABSTRACT FROM AUTHOR]

Rock-salt NiO and Ni0.7Zn0.3O nanoparticles were investigated by x-ray diffraction, atomic-force microscopy, and magnetic measurements. Nanoparticle diameters varied from 8 to 30 nm depending on reaction conditions. There are two main magnetization contributions, the field-induced spin canting of the antiferromagnetic sublattices and the magnetization rotation caused by uncompensated spins interacting with the magnetic field. The former is a bulk effect, modified by the presence of Zn, whereas the latter is a nanoscale effect that increases with decreasing particle size. The relative contributions of the two effects depend on particle size with a critical size of about 18 nm resulting in bulklike behavior. [ABSTRACT FROM AUTHOR]

We report HfCo7 nanoparticles with appreciable permanent-magnet properties (magnetocrystalline anisotropy K1 ≈ 10 Mergs/cm3, coercivity Hc ≈ 4.4 kOe, and magnetic polarization Js ≈ 10.9 kG at 300 K) deposited by a single-step cluster-deposition method. The direct crystalline-ordering of nanoparticles during the gas-aggregation process, without the requirement of a high-temperature thermal annealing, provides an unique opportunity to align their easy axes uniaxially by applying a magnetic field of about 5 kOe prior to deposition, and subsequently to fabricate exchange-coupled nanocomposites having Js as high as 16.6 kG by co-depositing soft magnetic Fe-Co. This study suggests HfCo7 as a promising rare-earth-free permanent-magnet alloy, which is important for mitigating the critical-materials aspects of rare-earth elements. [ABSTRACT FROM AUTHOR]

Magnetic investigations on a nanocomposite material obtained by spinning solutions of styrene–butadiene–styrene block copolymer containing barium ferrite nanoparticles onto Si wafers are reported. The effect of the spinning frequency on the magnetic features is discussed. It is observed that the magnetization at saturation is decreased as the spinning frequency is increased as the centrifuge force removes the magnetic nanoparticles from the solution. This is supported by the derivative of the hysteresis loops, which show two components, one with a high coercive field and another with a small coercive field. Increasing the spinning frequency increases the weight of the low coercive field component. The anisotropy in the distribution of magnetic nanoparticles, triggered eventually by the self-assembly capabilities of the matrix, is revealed by the difference between the coercive field in parallel and perpendicular configuration. It is noticed that increasing the spinning frequency enhances this difference. The effect of annealing the nanocomposite films is discussed. [Copyright &y& Elsevier]