24 research outputs found
High coercitivity carbon embedded L10-FePt ferromagnetic nanoparticles
Stoichiometric FePt nanoparticles in the tetragonal L10 phase, (Ku = 6.6?107 erg/cm3) are one of the leading candidates for next generation high-density recording media, allowing theoretical grain stability down to 3nm [1]. As-synthesized FePt nanoparticles produced by the conventional soft chemical route (polyol process) [2,3] shows disordered face centered cubic (fcc) structure with low Ku and superparamagnetic behavior at RT. The ordered L10 tetragonal structure is usually obtained by post-annealing in a reducing environment [4,5] giving rise to particle aggregation produced by sintering that affects significantly both the final particle size and the polidispersity. A preliminary work we performed pointed out that a direct synthesis of ferromagnetic particles, based on the decomposition of Fe(acac)3 and Pt(acac)2 in reducing solvent and inert atmosphere, is made possible by the increase of the reaction temperature at 290-330?C obtained by the use of Triton X-100 as solvent and polyvinylpyrrolidone (PVP) as protective agent. The resulting nanoparticles are ferromagnetic at RT with coercitive field (Hc) ranging from 0.4 to 1.0 KOe depending on the synthesis temperature. However, as evidenced by TEM analyses, they are magnetically aggregate and, for synthesis temperatures above 300?C, embedded in an amorphous matrix produced by partial decomposition of the solvent. These observations suggested us a novel approach to the synthesis of non-aggregate ferromagnetic nanoparticles. The basic idea is to block the nanoparticles in a rigid matrix, during the synthesis, before they become ferromagnetic, to prevent magnetic aggregation. Using PEG-600 as solvent and quickly raising the temperature above 300?C cause the polyol to condense in flakes. The rapid heating, joined to the increased viscosity, limits the diffusion of the nutrient phase to the growing nuclei, resulting in monodisperse nanoparticles, with a typical size ranging around 5nm (determined by XRD and TEM), randomly dispersed in the condensed matrix. In agreement with the XRD analysis, pointing out a disordered fcc structure, the magnetic measurements show at RT a superparamagnetic behaviour of the as-grown particles, with a blocking temperature TB of 60K and large distribution of energy barriers. The phase transformation to the ferromagnetic ordered tetragonal L10 structure is achieved by thermal annealing in dynamic high vacuum; the annealing transforms the organic matrix into amorphous carbon that preserves the original nanoparticle size and prevents the aggregation up to 1000?C, where it transforms into pyrolitic graphite. XRD shows the appearing of the L10 diffraction peaks after a 1 hour treatment at 650? and an almost complete phase transition after 4hours at the same temperature, where a coercitive field (Hc) of 2,5kOe at RT and 13kOe at 5K is detected. Annealing at higher temperatures, even if results in a further enhancement of the structural properties, gives rise to complex behaviour of the hysteresis, whose origin is still under investigation
Thermally activated magnetization reversal in bulk BiFe0.5Mn0.5O3
We report on the synthesis and characterization of BiFe0.5Mn0.5O3, a
potential type-I multiferroic compound displaying temperature induced
magnetization reversal. Bulk samples were obtained by means of solid state
reaction carried out under the application of hydrostatic pressure at 6 GPa and
1100{\deg}C. The crystal structure is an highly distorted perovskite with no
cation order on the B site, where, besides a complex scheme of tilt and
rotations of the TM-O6 octahedra, large off-centering of the bismuth ions is
detected. Below T1 = 420 K the compound undergoes a first weak ferromagnetic
transition related to the ordering of iron rich clusters. At lower temperatures
(just below RT) two distinct thermally activated mechanisms are superimposed,
inducing at first an enhancement of the magnetization at T2 = 288 K, then a
spontaneous reversal process centered at T3 = 250 K, finally giving rise to a
negative response. The application of fields higher than 1500 Oe suppresses the
process, yielding a ferromagnetic like behaviour. The complementary use of
SQuID magnetometry and M\"ossbauer spectroscopy allowed the interpretation of
the overall magnetic behaviour in terms of an uncompensated weak competitive
coupling between non-equivalent clusters of interactions characterized by
different critical temperatures and resultant magnetizations. PACS numbers:
75.85.+t, 75.60.Jk, 76.80.+y, 75.30.Et, 75.30.KzComment: 30 pages, 13 figure
Comparison of tensile strength and fracture toughness under mode I and II loading of co-cured and co-bonded CFRP joints
Adhesive bonding is the elective joining system between Carbon-Fiber Reinforced Polymer (CFRP) parts because, with respect to fastening, it allows a large connection area, no additional parts (hence weight saving) and no need to drill holes into the composite, that is always detrimental for the strength due to the possibility of developing damage. However, the choice of bonding CFRP should be evaluated as alternative to direct curing in terms of strength and durability, compared to cost and manufacturing time and complexity. In this work, a comparison between co-cured and co-bonded CFRP is done with respect to tensile strength and mode I and mode II fracture toughness, in order to understand whether co-bonding guarantees the same performance of a co-cured composite part.Adhesive bonding is the elective joining system between Carbon-Fiber Reinforced Polymer (CFRP) parts because, with respect to fastening, it allows a large connection area, no additional parts (hence weight saving) and no need to drill holes into the composite, that is always detrimental for the strength due to the possibility of developing damage. However, the choice of bonding CFRP should be evaluated as alternative to direct curing in terms of strength and durability, compared to cost and manufacturing time and complexity. In this work, a comparison between co-cured and co-bonded CFRP is done with respect to tensile strength and mode I and mode II fracture toughness, in order to understand whether co-bonding guarantees the same performance of a co-cured composite part
Conditions for the growth of smooth La0.7Sr0.3MnO3 thin films by pulsed electron ablation
We report on the optimisation of the growth conditions of manganite
La0.7Sr0.3MnO3 (LSMO) thin films prepared by Channel Spark Ablation (CSA). CSA
belongs to pulsed electron deposition methods and its energetic and deposition
parameters are quite similar to those of pulsed laser deposition. The method
has been already proven to provide manganite films with good magnetic
properties, but the films were generally relatively rough (a few nm
coarseness). Here we show that increasing the oxygen deposition pressure with
respect to previously used regimes, reduces the surface roughness down to unit
cell size while maintaining a robust magnetism. We analyse in detail the effect
of other deposition parameters, like accelerating voltage, discharging energy,
and temperature and provide on this basis a set of optimal conditions for the
growth of atomically flat films. The thicknesses for which atomically flat
surface was achieved is as high as about 10-20 nm, corresponding to films with
room temperature magnetism. We believe such magnetic layers represent appealing
and suitable electrodes for various spintronic devices.Comment: original paper, thin film optimization, 25 pages, 9 figure
From direct to inverse Giant Magnetocaloric Effect in Co-doped NiMnGa multifunctional alloys
We report the magnetic and magnetocaloric properties in Co-doped Ni–Mn–Ga Heusler alloys around the Mn-rich composition
Ni50Mn30Ga20. The presence of Co affects profoundly the critical temperatures and alters the exchange interactions of martensite and
austenite to different extents; by varying the composition it is possible to tune the critical temperatures and to induce a paramagnetic
gap between the magnetically ordered martensite and magnetic austenite, thus giving rise to a reverse magnetostructural transformation.
Contrary to the Co-free alloys, the saturation magnetization moment of austenite is strongly enhanced by Co with respect to the martensitic one: thus the magnetocaloric effect turns from direct into inverse. Remarkable values of the magnetic properties related to the magnetocaloric effect, e.g. the saturation magnetization jump at the transformation (DM) and the field dependence of the transformation temperature (dT/dH) are reported, together with high positive values of the isothermal magnetic entropy change (DS). The conditions for
enhancing the magnetocaloric properties and triggering the sign reversal of the magnetocaloric effect are discussed
Design of the EnVisS instrument optical head
The EnVisS (Entire Visible Sky) instrument is one of the payloads of the European Space Agency Comet Interceptor
mission. The aim of the mission is the study of a dynamically new comet, i.e. a comet that never travelled through the
solar system, or an interstellar object, entering the inner solar system.
As the mission three-spacecraft system passes through the comet coma, the EnVisS instrument maps the sky, as viewed
from the interior of the comet tail, providing information on the dust properties and distribution. EnVisS is mounted on a
spinning spacecraft and the full sky (i.e. 360°x180°) is entirely mapped thanks to a very wide field of view (180°x45°)
optical design selected for the EnVisS camera.
The paper presents the design of the EnVisS optical head. A fisheye optical layout has been selected because of the
required wide field of view (180°x45°). This kind of layout has recently found several applications in Earth remote
sensing (3MI instrument on MetOp SG) and in space exploration (SMEI instrument on Coriolis, MARCI on Mars
reconnaissance orbiter). The EnVisS optical head provides a high resolved image to be coupled with a COTS detector
featuring 2kx2k pixels with pitch 5.5µm. Chromatic aberration is corrected in the waveband 550-800nm, while the
distortion has been controlled over the whole field of view to remain below 8% with respect to an Fθ mapping law. Since
the camera will be switched on 24 hours before the comet closest encounter, the operative temperature will change
during the approaching phase and crossing of the comet’s coma.
In the paper, we discuss the solution adopted for reaching these challenging performances for a space-grade design,
while at the same time respecting the demanding small allocated volume and mass for the optical and mechanical design.
The view expressed herein can in no way be taken to reflect the official opinion of the European Space Agency
Comparison of Tensile Strength and Fracture Toughness of Co-Bonded and Cold-Bonded Carbon Fiber Laminate-Aluminum Adhesive Joints
The purpose of this work is to compare the co-bonding vs. cold-bonding route on the adhesive joint performance of a CFRP (Carbon Fiber Reinforced Polymer) laminate–aluminum connection. In particular, the overlap shear, tensile strength and Mode I and Mode II fracture toughness will be evaluated. The adhesives for co-bonding and cold-bonding are, respectively, a thermosetting modified epoxy, unsupported structural film and a two-component epoxy adhesive, chosen as representative of applications in the high-performance/race car field. The emerging trend is that, in tensile e Mode I fracture tests, the failure path is predominantly in the composite. Mode II fracture tests instead resulted in a cohesive fracture, meaning that, under pure shear loading, the weakest link may not be the composite. The lap-shear tests are placed midway (cohesive failure for co-bonding and composite delamination for cold-bonding, respectively), probably due to the different peel stress values related to the different adhesive Young’s modulus. The exploitation of the full capacity of the adhesive joint, hence the possibility of highlighting better, different performances of co-bonding vs. cold-bonding, would require consistent improvement of the out-of-plane strength of the CFRP laminate and/or to someway redistribute the peel stress on the bondline
Polymorphism and multiferroicity in Bi1-x/3(Mn3III)(Mn4-xIIIMnxIV)O12
We report on structural and magnetic characterizations performed on the
quadruple perovskite Bi1x/3(MnIII3)(MnIII4xMnIVx)O12, showing dramatic changes on
the magnetic properties of different polymorphs produced by small compositional
variations (0 < x < 0.3). Besides the stoichiometric monoclinic phase, rhombohedral
(0.10 < x < 0.27) and cubic (x > 0.27) polymorphs are detected, being related to the loss of
orbital ordering originated by the introduction of Mn4+ ions within the structure. For the
monoclinic and rhombohedral phases, ferroelectric properties are suggested by singlecrystal
X-ray diffraction and TEM studies, while their magnetic behavior reveals weak
ferromagnetism, induced by DzyaloshinskiiMoriya (DM) interaction, characterized by
high coercive fields, not easily explained by the soleDMeffect. The pinning of the magnetic
domains to the electric ones, mediated by strong magnetoelectric coupling, could be the
mechanism at the origin of such coercive fields. This hypothesis finds further support in
the low switching fields detected for the cubic phase, where just local polar character
is observed
Convergence of direct and indirect methods in the magnetocaloric study of first order transformations: the case of Ni-Co-Mn-Ga Heusler alloys
The study of two aspects of the magnetocaloric effect (MCE), namely, the matching between isothermal
entropy change and direct adiabatic temperature change, is not straightforward since huge differences between
these two quantities have often been reported. Here we put in relation the direct and indirect measurements on the
first order magnetostructural martensitic transformation occurring in Ni-Co-Mn-Ga alloys. In order to complete
the characterization of the MCE and to find an explanation of these mismatches, differential scanning calorimeter
measurements have been performed at different applied magnetic fields