78 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
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
Spectroscopic and evolutionary analyses of the binary system AzV 14 outline paths toward the WR stage at low metallicity
The origin of the observed population of Wolf-Rayet (WR) stars in
low-metallicity (low-Z) galaxies, such as the Small Magellanic Cloud (SMC), is
not yet understood. Standard, single-star evolutionary models predict that WR
stars should stem from very massive O-type star progenitors, but these are very
rare. On the other hand, binary evolutionary models predict that WR stars could
originate from primary stars in close binaries. We conduct an analysis of the
massive O star, AzV 14, to spectroscopically determine its fundamental and
stellar wind parameters, which are then used to investigate evolutionary paths
from the O-type to the WR stage with stellar evolutionary models. Multi-epoch
UV and optical spectra of AzV 14 are analyzed using the non-LTE stellar
atmosphere code PoWR. An optical TESS light curve was extracted and analyzed
using the PHOEBE code. The obtained parameters are put into an evolutionary
context, using the MESA code. AzV 14 is a close binary system consisting of two
similar main sequence stars with masses of 32 Msol. Both stars have weak
stellar winds with mass-loss rates of log = -7.7. Binary evolutionary
models can explain the empirically derived stellar and orbital parameters. The
model predicts that the primary will evolve into a WR star with T = 100 kK,
while the secondary, which will accrete significant amounts of mass during the
first mass transfer phase, will become a cooler WR star with T = 50 kK and are
predicted to have compared to other WR stars increased oxygen abundances. This
model prediction is supported by a spectroscopic analysis of a WR star in the
SMC. We hypothesize that the populations of WR stars in low-Z galaxies may have
bimodal temperature distributions. Hotter WR stars might originate from primary
stars, while cooler WR stars are the evolutionary descendants of the secondary
stars if they accreted a significant amount of mass.Comment: 21 pages (13 main body + 8 appendix), 16 figures, 9 table
- …