V. Iannotti

Generation of silicon nanoparticles via femtosecond laser ablation in vacuum

We demonstrate that femtosecond laser ablation of silicon targets in vacuum is a viable route to the generation and deposition of nanoparticles with radii of ≈5–10 nm. The nanoparticles dynamics during expansion has been analyzed through their structureless continuum optical emission, while atoms and ions, also present in the plume, have been identified by their characteristic emission lines. Atomic force microscopy analysis of the material deposited at room temperature has allowed the characterization of the nanoparticles size distribution. Taking into account the emissivity of small particles we show that the continuum emission is a blackbody-like radiation from the nanoparticles. Our results suggest that nanoclusters are generated as a result of relaxation processes of the extreme material state reached by the irradiated target surface, in agreement with recently published theoretical studies.

Magnetic and morphological characteristics of nickel nanoparticles films produced by femtosecond laser ablation

We have used the technique of femtosecond (fs) laser ablation in a vacuum to produce films of nickel nanoparticles. A peculiarity of this fs laser deposition is the significant shape and orientation anisotropy of the nanoparticles, which are oblate ellipsoids with the major axis parallel to the deposition substrate. The deposited films present unique magnetic properties, and, in specific conditions, very high remanence ratios (up to 0.7) accompanied with relatively low values of saturation and coercive fields can be obtained. We have interpreted these results in terms of the mentioned anisotropies, and of the occurrence of a thermally induced in-plane tensile stress, which is a function of the nanoparticles size.

The potentiality of composite elastic magnets as novel materials for sensors and actuators

Abstract In the last decade the progress of standard magneto-elastic materials is going toward its physical limits. New horizons are recently opened by the development of artificial composites having both elastic and magnetic properties. The working principle of these elastomagnetic materials is not depending on intrinsic magnetostriction, but on the coupling between magnetic moments of the particles and particles themselves. The possibility to study the physical mechanism that relates the elastic and magnetic properties in the new scenery furnished by composites of magnetic particles in an elastic matrix is very promising from the basic knowledge point of view. On the other side, the potential competitiveness in several applications justifies the increasing efforts to improve the production and to perform technical characterization of these magnetoelastic composites in different experimental conditions.

Direct magnetostriction and magnetoelastic wave amplitude to measure a linear displacement

The change of the magnetization intensity induced by a tensile stress in magnetoelastic ribbons can be useful to evaluate a point displacement of an elastic body under deformation. The same measurement can be performed by means of the change in the amplitude of resonant magneteolastic waves, when a longitudinal stress is produced in the ribbons. A comparison between the two techniques is reported in this paper, with the aim to separate the sensitivity limits and the optimum method in different experimental conditions. It is shown how a large stress sensitivity of magnetization is related to a high stress sensitivity of the magnetoelastic waves amplitude but in a different range of applied stress. Generally, the first technique is most useful to investigate a large range of displacement (from 1 μm up to 1 cm) with a good linearity but a decreasing precision if the displacement increases, while the second technique is the most sensitive and also precise (0.01 μm) if little displacement ranges are to be investigated.

Nanometric crystallization of Fe73.5Cu1Nb3Si13.5B9 by laser annealing

The possibility of producing nanocrystallites by laser heat treatment of a metallic glass is investigated. Structural and magnetic properties, after various annealing conditions, are reported. Laser annealing is demonstrated to be an effective, alternative technique for control of nanometric crystallization. This technique also shows a number of advantages with respect to conventional heat treatments.

Morphology, structure and magnetic properties of (Tb0.3Dy0.7Fe2)100−xFex nanogranular films produced by ultrashort pulsed laser deposition

Magnetic films were produced by ultrashort pulsed laser deposition (uPLD) using a rotating multitarget of terfenol-D (nominal composition: Tb0.3Dy0.7Fe2) and iron. The composite films obtained have a nanoparticle morphology typical of material produced by the uPLD technique, where each particle retains the stoichiometry of the parent target material. The co-deposition allows the production of (Tb0.3Dy0.7Fe2)100−x Fex films, where x can range from 0% to 100%. Unlike films obtained by standard nanosecond PLD, pure terfenol-D layers (x = 0) are amorphous, while the addition of iron induces the formation of Fe crystalline nanoparticles inside an amorphous nanogranular matrix of terfenol-D. The magnetic properties depend on the nanoparticle morphology and more strictly on the fraction of iron particles. In particular, it was demonstrated that the exchange interaction between hard magnetic terfenol-D nanoparticles and iron nanoparticles is active in the uPLD films, giving a cumulative magnetic response resulting from an averaging of the properties of the two component phases.

Magneto-piezoresistance in elastomagnetic composites

Ni microparticles were homogeneously dispersed into a silicone matrix preventing their direct contact even at volume fractions near the percolation threshold. In this condition, owing to the co-presence of elastomagnetic and piezoresistive effects, a moderate gradient of an external magnetizing field induces an electron conduction increment higher than 60% in Ni-silicone elastomagnetic composites. This demonstrates a peculiar kind of magnetoresistance mechanism that we define as “magneto-piezoresistance.” Theoretical predictions and validating experiments of this effect are reported. Owing to its innovative nature and potential improvements, the magneto-piezoresistance opens new perspectives for the elastomagnetic composites application in microdevices such as atomic force microscope tips and magnetic lecture heads.

Coexistence of very soft magnetism and good magnetoelastic coupling in the amorphous alloy Fe62.5Co6Ni7.5Zr6Cu1Nb2B15

Amorphous ferromagnetic ribbons in a new composition were prepared by melt spinning. The alloy components were calibrated in order to obtain a very soft magnetic material that, after proper annealing, represents giant amplitude of the resonant magnetoacoustic waves, at zero magnetising field. The main characteristic of the new material are reported with regard to the hysteresis cycle, the annealing effect on the structure and the dynamic magnetoelastic coupling.

Fe-Doping-Induced Magnetism in Nano-Hydroxyapatites

Doping of biocompatible nanomaterials with magnetic phases is currently one of the most promising strategies for the development of advanced magnetic biomaterials. However, especially in the case of iron-doped magnetic hydroxyapatites, it is not clear if the magnetic features come merely from the magnetic phases/ions used as dopants or from complex mechanisms involving interactions at the nanoscale. Here, we report an extensive chemical-physical and magnetic investigation of three hydroxyapatite nanocrystals doped with different iron species and containing small or no amounts of maghemite as a secondary phase. The association of several investigation techniques such as X-ray absorption spectroscopy, Mössbauer, magnetometry, and TEM allowed us to determine that the unusual magnetic properties of Fe2+/3+-doped hydroxyapatites (FeHA) occur by a synergy of two different phenomena: i.e., (i) interacting superparamagnetism due to the interplay between iron-doped apatite and iron oxide nanoparticles as well as to the occurrence of dipolar interactions and (ii) interacting paramagnetism due to Fe3+ ions present in the superficial hydrated layer of the apatite nanophase and, to a lesser extent, paramagnetism due to isolated Fe3+ ions in the apatite lattice. We also show that a major player in the activation of the above phenomena is the oxidation of Fe2+ into Fe3+, as induced by the synthesis process, and their consequent specific positioning in the FeHA structure.

Zero-dimensional superconducting fluctuations and fluctuating diamagnetism in lead nanoparticles

High resolution SQUID magnetization measurements in lead nanoparticles are used to study the fluctuating diamagnetism in zero-dimensional condition, namely for particle size d lesser than the coherence length. The diamagnetic magnetization Mdia (H, T= const) as a function of the field H at constant temperature is reported in the critical region and compared with the behaviour in the temperature range where the first-order fluctuation correction is expected to hold. The magnetization curves are analysed in the framework of exact fluctuation theories based on the Ginzburg-Landau functional for the coherence length much greater than d. The role of the upturn field Hup where Mdia reverses the field dependence is discussed and its relevance for the study of the fluctuating diamagnetism, particularly in the critical region where the first-order fluctuation correction breaks down, is pointed out. The size and temperature dependence of Hup is theoretically derived and compared to the experimental data. The relevance and the magnetization curves for non-evanescent field and of the upturn field for the study of the fluctuating diamagnetism above the superconducting transition temperature is emphasized.