79 research outputs found
Nonmonotonic Evolution of the Blocking Temperature in Dispersions of Superparamagnetic Nanoparticles
We use a Monte Carlo approach to simulate the influence of the dipolar
interaction on assemblies of monodisperse superparamagnetic
nanoparticles. We have identified a critical
concentration c*, that marks the transition between two different regimes in
the evolution of the blocking temperature () with interparticle
interactions. At low concentrations (c < c*) magnetic particles behave as an
ideal non-interacting system with a constant . At concentrations c > c*
the dipolar energy enhances the anisotropic energy barrier and
increases with increasing c, so that a larger temperature is required to reach
the superparamagnetic state. The fitting of our results with classical particle
models and experiments supports the existence of two differentiated regimes.
Our data could help to understand apparently contradictory results from the
literature.Comment: 13 pages, 7 figure
Self-consistent description of spin-phonon dynamics in ferromagnets
Several recently reported exciting phenomena such as spin caloritronics or ultrafast laser-induced spin dynamics
involve the action of temperature on spin dynamics. However, the inverse effect of magnetization dynamics on
temperature change is very frequently ignored. Based on the density matrix approach, in this work we derive
a self-consistent model for describing the magnetization and phonon temperature dynamics in ferromagnets
in the framework of the quantum Landau-Lifshitz-Bloch equation. We explore potential applicability of our
approach for two cases, inspired by magnetocaloric effect and magnetic fluid hyperthermia. In the first case, the
spin-phonon dynamics is governed by the longitudinal relaxation in bulk systems close to the Curie temperature;
while in the second case it is described by the transverse relaxation during the hysteresis cycle of individual
nanoparticles well below the Curie temperature
Chemical abundances and ionizing mechanisms in the star-forming double-ring of AM 0644-741 using MUSE data
We present the analysis of archival Very Large Telescope (VLT) Multi-Unit
Spectroscopic Explorer (MUSE) observations of 179 HII regions in the
star-forming double-ring collisional galaxy AM 0644-741 at 98.6 Mpc. We
determined ionic abundances of He, N, O and Fe using the direct method for the
brightest H II region (ID 39); we report
and . We also find the so-called
`blue-bump', broad He II , in the spectrum of this knot of massive
star-formation; its luminosity being consistent with the presence of
Wolf-Rayet (WR) stars of the Nitrogen late-type. We determined the O abundances
for 137 HII regions using the strong-line method; we report a median value of
. The location of three objects, including
the WR complex, coincide with that of an Ultra Luminous X-ray source. Nebular
He II is not detected in any H II region. We investigate the physical
mechanisms responsible for the observed spectral lines using appropriate
diagnostic diagrams and ionization models. We find that the H II regions are
being photoionized by star clusters with ages Myr and ionization
potential . In these diagrams, a binary
population is needed to reproduce the observables considered in this work.Comment: 20 pages. Accepted in MNRA
Controlling Magnetization Reversal and Hyperthermia Efficiency in Core-Shell Iron-Iron Oxide Magnetic Nanoparticles by Tuning the Interphase Coupling
Magnetic particle hyperthermia, in which colloidal nanostructures are exposed to an alternating magnetic field, is a promising approach to cancer therapy. Unfortunately, the clinical efficacy of hyperthermia has not yet been optimized. Consequently, routes to improve magnetic particle hyperthermia, such as designing hybrid structures comprised of different phase materials, are actively pursued. Here, we demonstrate enhanced hyperthermia efficiency in relatively large spherical Fe/Fe-oxide core-shell nanoparticles through the manipulation of interactions between the core and shell phases. Experimental results on representative samples with diameters in the range 30-80 nm indicate a direct correlation of hysteresis losses to the observed heating with a maximum efficiency of around 0.9 kW/g. The absolute particle size, the core-shell ratio, and the interposition of a thin wüstite interlayer are shown to have powerful effects on the specific absorption rate. By comparing our measurements to micromagnetic calculations, we have unveiled the occurrence of topologically nontrivial magnetization reversal modes under which interparticle interactions become negligible, aggregates formation is minimized and the energy that is converted into heat is increased. This information has been overlooked until date and is in stark contrast to the existing knowledge on homogeneous particles
Beyond the blocking model to fit nanoparticle ZFC/FC magnetisation curves
We consider the probability of a magnetic nanoparticle to flip its magnetisation near the blocking temperature, and use this to develop quasi-analytic expressions for the zero-field-cooled and field-cooled magnetisation, which go beyond the usual critical energy barrier approach to the superparamagnetic transition. The particles in the assembly are assumed to have random alignment of easy axes, and to not interact. We consider all particles to be of the same size and then extend the theory to treat polydisperse systems of particles. In particular, we find that the mode blocking temperature is at a lower temperature than the peak in the zero-field-cooled magnetisation versus temperature curve, in agreement with experiment and previous rate-equation simulations, but in contrast to the assumption many researchers use to analyse experimental data. We show that the quasi-analytic expressions agree with Monte Carlo simulation results but have the advantage of being very quick to use to fit data. We also give an example of fitting experimental data and extracting the anisotropy energy density K
Thermodynamics of interacting magnetic nanoparticles
We apply the concepts of stochastic thermodynamics combined with transition-state theory to develop a framework for evaluating local heat distributions across the assemblies of interacting magnetic nanoparticles (MPs) subject to time-varying external magnetic fields. We show that additivity of entropy production in the particle state-space allows separating the entropy contributions and evaluating the heat produced by the individual MPs despite interactions. Using MP chains as a model system for convenience, without losing generality, we show that the presence of dipolar interactions leads to significant heat distributions across the chains. Our study also suggests that the typically used hysteresis loops cannot be used as a measure of energy dissipation at the local particle level within MP clusters, aggregates, or assemblies, and explicit evaluation of entropy production based on appropriate theory, such as developed here, becomes necessary
Thermal, dielectrical and mechanical response of α and β-poly(vinilydene fluoride)/Co-MgO nanocomposites
Nanocomposites of the self-forming core-shell Co-MgO nanoparticles, which were of approximately 100 nm in diameter, and poly(vinylidene fluoride) (PVDF) polymer have been prepared. When the polymer is crystallized in the α-phase, the introduction of the nanoparticles leads to nucleation of the γ-phase of PVDF, increasing also the melting temperature of the polymer. With the introduction of the Co-MgO particles, the dielectric constant of the material slightly increases and the storage modulus decreases with respect to the values obtained for the pure polymer
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