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A new application of a high sensitivity magnetoelastic resonator able to measure period and damping constant of low frequency torsional oscillation is described and validated by experimental tests. The sensitive parameter is the amplitude of resonant magnetoelastic waves in the soft ferromagnetic core (Fe 62.5 Co 6 Ni 7.5 Zr 6 Cu 1 Nb 2 B 15 amorphous ribbon). The theoretical model of the device has been developed, correlating torsional oscillations to the friction force applied by the fluid in which they occur. Thus, an accurate indirect evaluation of fluid viscosity has been demonstrated. The main prerogative of the proposed sensor is to work without contact with the oscillating mechanism. As experimental validation, viscosity of UHT milk was measured versus different fat content. The experimental comparison with a standard rheometer demonstrates the new device competitiveness in the measure of low viscosity fluids at low share rate. Moreover, the detected behaviors at increasing temperature are in agreement with previous literature. In perspective, the new magnetoelastic resonators application can be very ductile and effective in on-line monitoring of viscosity change with time to control composition, degradation or contamination of liquids.

Magnetic field assisted electrospinning is supplemented by conductive sheets that envelop the magnets and have the same potential as the deposition electrode. By optimizing the experimental conditions, it is possible to produce wires made of polymer nanofibers that incorporate longitudinally oriented magnetic particles. The morphological and magnetic characterizations confirm the preferential orientations. The new nanofiber wires are easily aligned along the maximum intensity line of the applied magnetic field. Moreover, the nanostructured wires have high longitudinal elastomagnetic strain and high transversal deflection as a response to magnetic field stimuli. Therefore, these new threadlike aggregates of magnetic nanofibers could be very appealing for application in all microelectronic or biomedical devices whose functionality requires orientation or deformation.

An innovative experimental apparatus for the direct measurement of yield stress was conceived and realized. It is based on a torsion pendulum equipped with a magnetic dipole and a rotating cylinder immersed in the material to be investigated. The pendulum equilibrium state depends on the mechanical torque applied due to an external magnetic induction field, elastic reaction of the suspension wire, and shear yield stress. Experimental results are reported showing that the behavior of the pendulum rotation angle, in different equilibrium conditions, provides evidence of the yield stress presence and enables its evaluation by equilibrium equations. The dependence on time of the equilibrium approach was also studied, contributing to shed light on the relaxation effect in the transition from a fluid-like to solid-like behavior, as well as on the eventual thixotropic effects in non-Newtonian fluids. The validity of the proposed technique and related experimental apparatus was tested in aqueous Carbopol solutions, with different weight percentages. The linear procedure, combined with the effectiveness and reliability of the proposed experimental method, candidates it to be used for the study of peculiar behaviors of other yield stress complex fluid such as blood, crude waxy oils, ice slurries, and coating layer used in the food industry and also for fault sliding in geodynamics.

The topic of magnetic elastomers and their properties was reorganized to clarify the host of elastomagnetic effects useful to explain the basic principles of magnetopiezoresistivity in micro-structured materials composed of magnetic particles homogeneously dispersed in an elastomer matrix. The working mechanism of the magnetopiezoresistive sensors is clarified and new results depending on the particle shape and preorientation are reported. A 20% improvement in sensor sensitivity for detecting magnetic field direction is experimentally demonstrated and qualitatively explained. The performances and limits of an optimized prototype sensor, to detect a random sequence of magnetic poles, have been investigated as a function of both the polar magnetization intensity and the poles sequence velocity. Concrete perspectives for application in MEMS devices are deduced.

Small polymeric ducts incorporating a ribbon-shaped mat of densely packed magnetic nanofibers have been manufactured via electrospinning by using a cylindrical manifold, alternately under rotation or static. The magnetic nanofibers mat is located on the side of the tube and aligned to the longitudinal axis using the assistance of a magnetic field. The designed methodology ensures that the magnetic particles are completely wrapped into a protective polymer shell. Experimental results demonstrate that the innovative confinement of magnetic nanofibers, forming a longitudinal ribbon on a tube side, confers a high and reversible transverse strain under a moderate magnetic field stimulus: a magnetic field gradient

Electrospun nanofibres were prepared starting from a dispersion of Nickel magnetic nanoparticles (MNPs) in Polydimethylsiloxane (PDMS) and Tetrahydrofurane (THF). Coaxial electrospinning was used to realize a temporary structure which wraps PDMS fibers into a thin shell of PolyvynilPirollidone (PVP), in which the crosslinking reaction of the inner fiber core is completed. After the removal of the PVP shell, the fiber skeins exhibit remarkable elastomagnetic deflection induced by transversal magnetization. The proposed methodology could potentially open up really intriguing perspectives in the fabrication of devices - deformable by magnetic forces - for bio-electronics and bio-medical applications.

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.

Ni Nanoparticle assembled thin films were prepared using an unconventional approach based on the use of a secondary nanosecond (ns) ultraviolet (UV) laser irradiation interacting with the plume of ablated nanoparticles (NPs) during the femtosecond pulsed laser deposition (fs-PLD). The secondary laser beam determines the reduction of the NPs size and their dispersion by partial vaporization of the NPs during their flight from the target to the substrate. The proper selection of the time delay between fs and ns laser pulses allows the latter to interact selectively with different parts of the NPs plume, controlling, to a certain degree, the reduction of the NPs size and dispersion. Another original effect of the UV laser irradiation is the change in the deposited films topology, due to a reduction of the NP-aggregates density and size, fostering non-uniform dense assemblies of NPs with concentration well above the percolation threshold, with the consequent reduction of the influence of the exchange interactions on the macroscopic magnetic properties. The magnetic behavior of the films prepared using two laser beams with respect to that obtained in the case of fs-PLD only is characterized by higher H(c) values (up to approximately 70%) and a good compromise between the hysteresis loops squareness and moderate exchange interactions, strongly correlated with the NPs topology.

Deposition temperature effect on morphological, topological and magnetic characteristics of nanoparticle-assembled Co50Fe50 films produced by femtosecond pulsed laser deposition (fs-PLD) on Kapton substrate was investigated. For substrate temperature Ts ≥ 550 K, a decrement of the nanoparticle's aggregates and an increment of the nanoparticle's density were observed with respect to room temperature deposition; this in association with a strong increase of the magnetoelastic anisotropy energy lead to a reduction of the remanence ratio, a significant rise of the saturation and coercive fields and an enhancement of the saturation magnetization. The results are discussed focusing on: i) the correlation between films structure and their magnetic behavior; ii) the role of the different anisotropy energies in determining the harder in-plane magnetic behavior for Ts ≥ 550 K. The thermal strain of Kapton substrate holds an important role in generating morphological and topological characteristics of the fs-PLD films and corresponding magnetic properties.

A novel elastomagnetic attenuator was developed and tested. The innovation lies in the use of permanent magnets instead of non-magnetic parts in the standard attenuator devices made of alternating elastomeric and rigid metallic elements. In this way, the standard mechanism of passive energy dissipation is reinforced by the action of magnetic forces which can be non-linear and therefore introduce asymmetric strain dependent forces. The performed experimental tests reveal the advantages and the limits of the new attenuator device in contrasting vibrations of a simple structure. The elastomagnetic attenuator shows good vibration control, in terms of amplitude damping and resonance frequency shift towards higher values, for longitudinal oscillations propagating parallel to the main magnetic forces axis and transmitted from the outside through the base of the structure to be protected.

The competitive performance of an optimized magneto-elastic resonator as sensor of a local magnetic field is reported. The resonator core is constituted by an amorphous ferromagnetic ribbon in which stationary waves are established by means of magneto-elastic coupling. The appropriate choice of the ribbon composition and the use of a special heat treatment are determinant on the sensor response. In proper conditions, the sensitive parameter, namely the electromotive force (root mean square value) induced in a miniaturized pick-up coil, has relative changes of 10 4 for a magnetic field increment of 10 A/m. Magnetic induction fields as small as 10 −8 T can be measured with a technology simpler than in other used devices. A good linearity of the sensor response at low field (from 0 A/m up to 50 A/m) is demonstrated too.

The use of ultrashort laser pulses enables the deposition of films composed of mono-component nanoparticles exhibiting similar shape and size. Films made of nickel (Ni) and silicon (Si) nanoparticles have been produced and investigated in view of the expected interesting properties, resulting from the uniform distribution of metallic, magnetic Ni particles among semiconductive, non-magnetic Si particles. The morphology of the deposited nanoparticles and the related magnetic and magneto-transport characteristics of the films have been studied for different Ni contents, evidencing properties deriving from the peculiar deposition technique and in particular the important role of the free volume inclusions and the particles tendency to not coalesce.

The direct elastomagnetic effect in composite materials, made of ferromagnetic micro-magnets inside an elastomeric matrix, consists in their relative elastic deformation (10-4-10-3) under the action of an external magnetic field, dependent on the equilibrium between the magneto-mechanical moments and the internal elastic reaction, but independent of the standard magnetostriction. This investigation is focused on the evaluation of the strain linearity with the excitation field in view of the application of these materials for MEMS devices. The studied composite is made of permanently magnetized Sm2Co7 microparticles uniformly dispersed inside a silicone matrix. The field-induced strain is measured using a fiber Bragg grating sensor and, at the same time, theoretically evaluated from the model of the direct elastomagnetic effect. The linearity and reversibility of the material response are experimentally verified up to a threshold value of the magnetic field corresponding to the break in the coupling between the particles' magnetic moments and their body and to the passage from the linear magnetization to the non-linear regime accompanied by irreversible magnetization processes.

Ultrashort pulsed laser ablation in vacuum of different targets was performed in order to investigate the possibility of producing nanoparticles with controlled size and shape. A systematic morphology characterization of deposited products was performed for nickel and silicon as a function of laser pulse intensity and wavelength, at a fixed pulse repetition rate. The nanoparticles were investigated by atomic force microscopy, and clear trends for their size and shape anisotropy were evidenced. The best conditions to obtain nanosized particles of oblate ellipsoidal shape, with the minor axis below 10 nm, were determined in the case of nickel targets. Our results show that ultrashort pulse laser deposition can be considered as an interesting technique for the tailoring of nanogranular films with the desired particles dimension and shape, according to the peculiar properties required in specific applications. Moreover, the preliminary features are very promising from the point of view of the production of magnetoresistive films with specific anisotropy. (c) 2005 Elsevier B.V. All rights reserved.

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)Fe-x 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.

The relation structural and magnetic properties of Co-Ni-Cr-Al-Y-N thin films deposited by reactive r.f. magnetron sputtering was investigated. A marked change in the magnetic behaviour of the films with the different nitrogen partial pressure in the Ar/N2 deposition atmosphere was observed and qualitatively explained in correlation with the phase composition. The nanocrystalline metal solid-solution obtained at low N2 content and the nanocrystalline nitride/amorphous composite obtained at high N2 content are not magnetic, whereas the amorphous phase produced for intermediate N2 pressures behaves like a ferromagnetic semi-permanent material. The results demonstrate the possibility of modulating the magnetic properties of r.f. magnetron sputtered Co-Ni-Cr-Al-Y-N thin films, thus opening a new route for magnetic multilayer deposition.

An elastic magnet in bar shape, consisting of permanently magnetized Sm2Co7 micro-particles, was produced. The longitudinal strain and/or force amounts given by this novel material under the action of a longitudinal magnetic field are here investigated in order to evaluate the potential applications in very ductile actuator systems.

The dependence of the elastomagnetic effect on the orientation of Ni ellipsoidal microparticles dispersed in a silicone matrix is investigated. When a volume strain is produced on the composite material in a fixed magnetizing field, the effects of mechanical and magnetic torque are simply modeled and the induced magnetization change can be predicted as a function of the particles' pre-orientation. The theoretical predictions are experimentally verified. As a consequence, it is possible to govern the elastomagnetic effect by proper microparticle orientation.

The microstructure morphology of Nd90-x FexAl10 (x = 35 - 50) ribbons, obtained by melt spinning at two surface wheel velocities, was investigated by atomic force microscopy. A direct dependence of microstructure homogeneity on both production parameters and Nd content was evidenced. In particular high surface wheel velocity promotes an amorphous structure while low Nd content promotes grain formation. At the same time, the temperature dependence of magnetization, in field cooled and zero field cooled samples, shows the coexistence of short-range spin glass-like interactions and long-range ferromagnetic order, which are also coherent with the morphologic characteristics. (C) 2003 Elsevier Science B.V. All rights reserved.

A magnetoelastic amorphous ribbon enveloped in silicone sheath has attenuated longitudinal magnetization intensity when is magnetized. At fixed magnetizing field, a longitudinal deformation applied to the silicone determines an ulterior decrease of magnetization intensity due to the induced friction at the interface between elastic sheath and ferromagnetic ribbon. When a threshold value of the silicone deformation is reached, an abrupt increase of magnetization occurs, corresponding to the exceeding of the maximum internal friction by the induced deformation. This effect can be used to design strain, stress, and/or magnetic field threshold indicators, which can be calibrated for different levels.

A traditional TG apparatus was modified by placing two permanent magnets producing a controlled magnetic field (TG(M): Magneto Thermogravimetry). This technique proved to be useful to study both structural relaxation and crystallisation of ferromagnetic metallic glasses. Results obtained for the amorphous alloys Fe40Ni40P14B6 and Fe62.5Co6Ni7.5Zr6Nb2Cu1B15, are reported in this paper. Structural relaxation can be evaluated by measuring changes in Curie temperature induced by thermal treatments. Crystallisation in TG(M) is detected through a change in the measured apparent mass (difference between the sample mass and magnetic force driving it upward). These results were confirmed by DSC analysis. Whether the obtained crystalline phase is ferromagnetic, it can be identified through its Curie temperature, measured by TG(M). In fact the value of 770°C measured as Curie temperature of crystallised Fe62.5Co6Ni7.5Zr6Nb2Cu1B15led to conclude that the only ferromagnetic crystalline phase is a-Fe. These hypothesis was confirmed by XRD analysis, showing that the first crystallisation yields to a-Fe nanocrystals.

When a constant magnetizing field is applied to a Terfenol sample at a fixed temperature, a small fraction (

The magnetic, magnetoelastic, and structural properties of the Fe62.5Co6Ni7.5Zr6Cu1Nb2B15 at.- alloy, recently produced by rapid quenching from the melt, have been observed and explained with regards to the proper choice of components and their percentages. In particular, it was shown that a giant magnetoelastic wave amplitude, obtained with a very low magnetising field, is a consequence of simultaneous very high values of permeability, magnetomechanical coupling factor, and Youngs modulus. The importance of the new amorphous alloy, both for applications and basic research, is founded on its structural development by means of isothermal heat treatments. In particular, the tendency to maintain structural disorder at temperatures above where the first crystallisation occurrs is fundamental. X-ray analysis demonstrates that this property determines the presence of nanocrystalline phases after heating at a temperature well above that of the first crystallisation.

A composite material of a polyethylene matrix filled by a fine silver powder was prepared with different Ag contents and physical behaviours ranging from insulator to conductor. Ac differential magnetic susceptibility chi measurements show the samples are paramagnetic up to an Ag concentration of similar to 65%. At low temperatures the composite is spin-glass type, whereas the transition from insulator to conductor corresponds to an abatement of chi at zero magnetizing field. Magneto-conductivity effects have been observed in resistivity measurements at low temperatures. They can be explained in terms of an effective exchange electronic scattering mechanism between the conduction electrons and the diluted magnetic moments arising from unpaired electron spins of boundary silver particles. Moreover, the presence of a broad minimum in the resistivity curve at T =similar to 20 K, observed in samples with an Ag concentration just above the percolation threshold, addresses to possible interference effects similar to those reported in disordered materials.

Strips of Fe62.5Co6Ni7.5Zr6Cu1Nb2B15, prepared by rapid quenching from the melt, were annealed at different temperatures and studied performing Mossbauer and magnetoelastic measurements. By increasing annealing temperature, the long range order increases strengthening the exchange interactions. The maximum of magnetoelastic response corresponds to the maximum of the out of plane magnetization. A possible explanation for the onset of the perpendicular magnetic anisotropy is proposed.

On the basis of recent results a new composition for easy rapid quenching production of good magnetoelastic amorphous ribbons was selected. The production was successful and the experimental results in agreements with the predictions. The structural relaxation induced by annealing produces a very high improvement of the magnetoelastic response so that the new alloy results the most promising for magnetoelastic wave application in sensor devices. In particular the amplitude of resonant magnetoelastic wave results the highest still now obtained in an amorphous ribbon.

Magnetic field dependences of the piezomagnetic dynamics Ζd and moduli of elasticity in the Fe73.5Cu1Ta3Si15.5B7 alloy strips in as-quenched state and after annealing for 1 h in vacuum in the temperature range from 400°C to 600°C were studied. The elasticity moduli were changed from 75 to 210 GPa depending on magnetic bias field and annealing temperature. A crystalline structure having nanometer sized crystals was observed after annealing at 500°C.

It is shown that in appropriate experimental conditions a direct relation between the magnetoelastic coupling coefficient k and the amplitude A o of resonant magnetoelastic longitudinal waves can be demonstrated in ribbons of soft ferromagnetic metal. The model conclusions on the behaviour of A versus k were experimentally verified in Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 amorphous alloy; k optimization were produced by both heat induced structural relaxation and nanocrystallization. Since k is directly dependent on Young modulus, a comparison, both experimental and theoretical, between Young modulus behaviour and magnetoelastic waves amplitude in resonance conditions is consequent.

Composite material constituted by Fe micro-particles homogeneously dispersed in a silicone matrix, at a volume concentration slightly above the percolation threshold but separated by a thin silicone layer, was produced. The particle magnetic softness and their average size, have been properly improved with respect to previous investigations in order to maximize the piezo-resistive and the piezo-magnetic effects. The optimal combination of magneto-elasticity and piezo-resistivity enables to achieve a record value of magneto-piezo-resistivity sensitivity. An analytical model is proposed to simulate the theoretically expected behavior of electric resistance vs. the applied induction field gradient, so to predict the magneto-piezoresistive response and explain the obtained material tailoring. The experimental results have been in good agreement with the theoretically predicted behaviors, so validating the employed model and the interpretation of the phenomenon. A simple basic application in position sensing is also reported. The analytical model presented in this paper has demonstrated its potentiality to project further improvements, while the experimental results allow for different innovative applications.

A study is reported on the devitrification behaviour of the amorphous alloy Fe73.5Si13.5B9Nb3Cu1. Samples of the studied glass underwent isothermal and non-isothermal heat treatments in a thermal analysis apparatus. In addition some samples were very rapidly heated to very high temperatures by means of a laser beam. In this way a large temperature range was explored and information was obtained on the overall thermal evolution of the studied amorphous alloy. The experimental results suggest that, as recently proposed in the literature, nanocrystallization can be linked to a rate limiting Nb diffusion stage in the crystal growth process. However topological short range ordering (TSRO) and chemical short range ordering (CSRO) also affect the devitrification behaviour. In the case of the studied alloy, three temperature ranges can be defined. At low temperature only TSRO occurs. Above a temperature that lies approximately in the range 450–500°C, glass in glass phase separation occurs up to a temperature that lies approximately in the range 700–750°C, above which it appears to be very limited. The occurrence of glass in glass phase separation appears to be necessary to obtain a fine microstructure, because Nb concentrates in the boron depleted, iron rich glassy phase. The occurrence of TSRO prior to demixing should be avoided if nanocrystallization is desired. All this appears to be satisfied by isothermally treating the sample at a temperature of 555°C.

Fe nanoparticle (NP)-assembled thin films with different thickness were prepared by femtosecond-pulsed laser deposition using different deposition times. The proper selection of the deposition time allows to control, to a certain degree, the morphology and topology of the deposited Fe nanoparticles (NPs) assembly, fostering non-uniform dense assemblies of NPs, with the consequent reduction of the influence of the exchange interactions on the macroscopic magnetic properties with decreasing thickness. The magnetic behavior of the Fe NP-assembled films with decreasing thickness is characterized by higher coercive field (H-c) values (a factor approximate to 4.5) and a good compromise between the hysteresis loops squareness and moderate exchange interactions, strongly correlated with the NPs topology.

The effect of different heat treatments on the piezomagnetic properties and magnetostriction of Fe73.5Cu1Nb3Si13.5B9 metallic glass were investigated as a function of the bias magnetic field. The maximum values of the magnetomechanical coupling coefficient were equal to 0.15 for the as-quenched state, and 0.4 and 0.6 after annealing at 510 °C for 1/4 and 1 h, respectively. The magnetostrictive hysteresis loops were measured using the strain gauge method. The values of magnetostriction (A) at a magnetic field equal to about 3 kA/m in all cases were equal to about 22 × 10−6.

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.

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.

Samples of composite material made of Ni ellipsoidal particles (with a fixed eccentricity but different major axis dimensions) uniformly dispersed in a silicone matrix were produced and characterized by both magnetization cycles and strain sensitivity of magnetization tests. It was theoretically justified and experimentally shown that at constant Ni percentage, in the case of nanometric size particles, higher coercive and saturation fields and a higher strain sensitivity of the magnetization intensity in comparison with the case of micrometric particles are obtained. (C) 2004 Elsevier B.V. All rights reserved.

An elastic and permanently magnetized material (Sm2Co7 particles uniformly dispersed in a silicone matrix) was produced. Bars of this material were put together to form samples with magnetic moment directions distributed as a fishbone. Results on the potentiality of this kind of material as both actuator and sensor, when a deformation is induced by an external magnetic field and when a magnetization change is induced by ail external dynamic deformation, respectively, are reported. (C) 2004 Elsevier B.V. All rights reserved.

Elastomagnetic samples made of quasi-ellipsoidal nickel microparticles separated by a thin silicone layer were produced. A static magnetic field was applied during the process of sample solidification, obtaining a preferred orientation of the particles major axis through the coupling of the magnetic moment with the easy magnetization axis. The influence of the particle pre-orientation on the elasticity, piezoresistivity and magneto-piezoresistivity were investigated and discussed in order to maximize the resistance change due to a moderate magnetic field gradient, in view of conceiving an innovative core material for sensor devices. It was shown as the relative change of resistance induced by a low magnetic field gradient can be improved of over 15% in comparison with previous results, approaching an absolute value of the magneto-piezoresistive ratio equal to 80%. Since a local magnetic field change can be also produced by a displacement of a magnetic body, in perspective the effect can be applied for sensing position or mechanical vibration too.

It is shown as magnetoelastic sensor may be able to measure point displacement from some centimeters down to 10 -7 m. In some examples of basic applications the good agreement among the theoretical predictions and the experimental results is reported. The proposed sensors are both sensitive as those founded on optical techniques and strength as the mechanical similar devices.

Nanosized crystalline materials prepared by devitrification from the amorphous state are an interesting subject of physical research thanks to their attractive magnetic properties related to very low value of magnetostrition. This work is a comparison study of some devitrification tecniques to individuate both their advantages and disvantages in obtaining excellent nanocrystalline magnetic alloys. In particular we investigate differences between laser annealing, furnace heating both in an amorphous metal obtained by rapid quenching from the melt and in a Cop multilayer produced by electrochemical deposition.

Summary The relative vibrations of two small frames with different stiffness, subjected to the same oscillating excitation coming from their base platform, were investigated. The effects of the innovative coupling of standard passive attenuation and time variation of magnetic forces (self-produced by the vibration itself) were investigated. The first experimental tests demonstrate that the amplitude of relative vibration can be easily decreased up to 75% without requiring external energy. In perspective, by varying the attenuator shape and power, it is possible to use it in all the cases in which it is necessary to reduce the vibrations of a breakable or unstable structure inside a building accidentally exposed to vibration, such as a rotor inside a frame or a shelf in a museum room. Copyright © 2016 John Wiley & Sons, Ltd.

The role of interparticle interactions and the morphology in the magnetic behavior of dense assemblies of Fe nanoparticles with concentration well above the percolation threshold has been studied using the Monte Carlo simulations technique. The initial and temperature-dependent magnetization curves have been calculated for different conditions of the assembly morphology and the interparticle interaction strengths. Our simulations showed that the strong competition between the anisotropy and exchange energies in nonuniform dense assemblies results in a frustration of the nanoparticles moments coupling and creates plateaus and abrupt steps, which indicate a sudden, collective spin reversal, for low and intermediate dipolar strengths. In the case of strong dipolar interactions, the stepwise behavior becomes smoother and gradually disappears.

Ultrafast pulsed laser deposition is an interesting technique for the synthesis of nanoparticles (NP) and NP-assembled films (NPA). The NPA films show a structure with agglomerates of NPs sticking to one another, with a significant shape and orientation anisotropy and negligible coalescence. In this communication, we report preliminary results concerning the effect of the substrate temperature on the magnetic response of the NPA films in the case of Fe NPs deposited on Si (1 0 0). The experimental data evidence a clear role of the substrate temperature on the hysteresis loop of the deposited NPA films due to the aggregation and oxidation degree of the NPs.

The dependence of magnetoelastic wave amplitude on structure-sensitive parameters, in addition to the possibility of producing and detecting these waves in different parts of a sample, has been used to study the effects induced by laser heating in ferromagnetic materials, during and after heat treatment. Here we report experimental results and theoretical considerations that explain the correlations between the magnetoelastic wave amplitude and the magnetic permeability, the stress sensitivity and the elastic coefficient. In particular, it is shown that structural changes can be seen, and a simple model of thermomagnetic coupling is developed.

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.

The magnetic properties of Fe nanoparticles films were investigated by magnetization measurements and Monte Carlo simulations. The films were produced by femtosecond pulsed laser deposition assisted by irradiation of nanoparticles with a nanosecond UV laser pulse, appropriately delayed, during their flight from the target to the substrate. The films consist of nanoparticles with a disklike shape. The initial magnetization curve exhibits a stepwise behavior, characterized by a reversible plateau followed by rapid irreversible increments. The observed behavior, induced by the peculiar nanoparticle film morphology, is the result of the competition between particle anisotropy, dipole-dipole interactions, and interparticle exchange coupling. This picture is supported by Monte Carlo simulations, satisfactorily reproducing the experimental observations.

Femtosecond pulsed laser deposition, carried out in high vacuum, leads the ablation of any target material, producing a plume of nano-drops which are deposited on a substrate as particles, preserving the parent material composition. The deposited nanoparticles (NPs) exhibit a characteristic disk-shape with a major diameter ranging from 5 nm up to 60 nm and a thickness ranging from 1 nm up to 10 nm. Also when the particles form a thick layer, they remain separated by an interface of free volume. This characteristic morphology of the deposited films gives an interplay of interparticle and intraparticle physical correlations which deeply differentiates their properties from those of similar nanogranular films deposited by other techniques. This investigation is focused on the influence of the novel structural conditions on the coupling between strain and magnetization, and/or strain and resistivity, in Co(50)Fe(50) NP films deposited on Kapton substrate. Due to the NP nature of the films, these couplings are substantially different from standard magnetostrictive and piezoresistive effects. Magnetization and resistivity versus applied strain were studied, thus evidencing novel elastoresistive and elastomagnetic functionalities and evaluating their relative sensitivities. The limits and potentiality for the application of the new NP magnetic films in microelectromechanical systems (MEMS) will be briefly discussed.

Exchange-coupled monocomponent magnetic films constituted of disk-shaped Ni and Fe nanoparticles were produced by ultrafast pulsed laser deposition, in vacuum. These films show a peculiar cauliflower-like structure, made of granular agglomerates of nanoparticles sticking to one another with a significant shape and orientation anisotropy. Both as-deposited Ni and Fe films present hysteresis loops with a high in-plane remanence ratio (0.61 and 0.81 at 250 K, respectively), relatively low values of the saturation and coercive fields and a steep slope near coercivity. At temperature of 10 K and 250 K, the magnetization curves confirm the strong influence of the production technique on the topologic structure of these films, and consequently on their magnetic properties. In perspective, the striking and intriguing properties of these nanogranular films appear very promising for potential application as permanent magnets and in data storage technology.

The problem of coupling between mechanical shear waves and electromagnetic waves propagating in magnetostrictive materials has been stressed. The system of the acoustic field equations and the Maxwell ones has been combined with the piezomagnetic constitutive relations to obtain z‐propagating x‐polarized transversal standing wave characterized both by particle displacement, U x (z,t), and magnetic field,H x (z,t). Attention has been paid on the separation of the purely acoustic wave from the hybrid one to the scope to evaluate their difference in velocity D. Generally, the latter is very small (1/105) but it cannot be neglected in new technological materials exhibiting low magnetoelastic coupling coefficients. The difference D has been theoretically obtained for magnetoelasticwaves generated in transversally isotropic samples versus the permeability and the magnetoelastic coupling coefficient. Moreover, it has been possible to check the electromagnetic disturbance behavior as a function of the bias field by using our experimental results.