arXiv subjects

J. -L. Déjardin

Publications and source records attributed to J. -L. Déjardin.

5 recordsLinked to original sources

arXiv2022

Time profile of temperature rise in assemblies of nanomagnets

We compute the heat generated by (non-interacting) nanomagnets subjected to an alternating magnetic field (AMF) and study its transfer to the hosting medium and environment. For the first task, we compute the heat generated by the nanomagnets (or the specific absorption rate) using the ac susceptibility in the linear regime. For the second task, the loss of heat to the environment is modeled with the help of a balance (macroscopic) equation based on Newton's law of cooling. This equation is solved both numerically and analytically for a generic ferrofluid and the analytical solution renders a very good approximation to the general balance equation. Then, we investigate the effects of AMF frequency and amplitude on the temperature elevation during its temporal evolution. Finally, using the available experimental data for maghemite and magnetite ferrofluids, we discuss the behavior of Newton's heat transfer coefficient in terms of the AMF amplitude and frequency. These results could trigger experimental investigations of this coefficient which characterizes the rate of heating in a ferrofluid, with the aim to build more refined models for the mechanisms of heat generation and its diffusion in ferrofluids used in magnetic hyperthermia.

physics.app-ph↗

arXiv2020

Specific absorption rate of magnetic nanoparticles: nonlinear AC susceptibility

In the context of magnetic hyperthermia, several physical parameters are used to optimize the heat generation and these include the nanoparticles concentration and the magnitude and frequency of the external AC magnetic field. Here we extend our previous work by computing nonlinear contributions to the specific absorption rate, while taking into account (weak) inter-particle dipolar interactions and DC magnetic field. In the previous work, the latter were shown to enhance the SAR in some specific geometries and setup. We find that the cubic correction to the AC susceptibility does not modify the qualitative behavior observed earlier but does bring a non negligible quantitative change of specific absorption rate, especially at relatively high AC field intensities. Incidentally, within our approach based on the AC susceptibility, we revisit the physiological empirical criterion on the upper limit of the product of the AC magnetic field intensity $H_{0}$ and its frequency $f$, and provide a physicist's rationale for it.

cond-mat.mes-hall↗

arXiv2018

Ferromagnetic resonance of a two-dimensional array of nanomagnets: Effects of surface anisotropy and dipolar interactions

We develop an analytical approach for studying the FMR frequency shift due to dipolar interactions and surface effects in two-dimensional arrays of nanomagnets with (effective) uniaxial anisotropy along the magnetic field. For this we build a general formalism on the basis of perturbation theory that applies to dilute assemblies but which goes beyond the point-dipole approximation as it takes account of the size and shape of the nano-elements, in addition to their separation and spatial arrangement. The contribution to the frequency shift due to the shape and size of the nano-elements has been obtained in terms of their aspect ratio, their separation and the lattice geometry. We have also varied the size of the array itself and compared the results with a semi-analytical model and reached an agreement that improves as the size of the array increases. We find that the red-shift of the ferromagnetic resonance due to dipolar interactions decreases for smaller arrays. Surface effects may induce either a blue-shift or a red-shift of the FMR frequency, depending on the crystal and magnetic properties of the nano-elements themselves. In particular, some configurations of the nano-elements assemblies may lead to a full compensation between surface effects and dipole interactions.

cond-mat.mes-hall↗

arXiv2017

Effect of dipolar interactions and DC magnetic field on the specific absorption rate of an array of magnetic nanoparticles

We address the issue of inter-particle dipolar interactions in the context of magnetic hyperthermia. More precisely, the main question dealt with here is concerned with the conditions under which the specific absorption rate is enhanced or reduced by dipolar interactions. For this purpose, we propose a theory for the calculation of the AC susceptibility, and thereby the specific absorption rate, for a monodisperse two-dimensional assembly of nanoparticles with oriented anisotropy, in the presence of a DC magnetic field, in addition to the AC magnetic field. We also study the competition between the dipolar interactions and the DC field, both in the transverse and longitudinal configurations. In both cases, we find that the specific absorption rate has a maximum at some critical DC field that depends on the inter-particle separation. In the longitudinal setup, this critical field falls well within the range of experiments.

cond-mat.mes-hall↗

arXiv2012

Comment on "Thermal fluctuations of magnetic nanoparticles" [arXiv:1209.0298]

We comment on some misleading and biased statements appearing in the manuscript arXiv:1209.0298 ("Thermal fluctuations of magnetic nanoparticles") about the use of the damped Landau-Lifshitz equation and the kinetic Langer theory for the calculation of the relaxation rate of magnetic nanoclusters. We reiterate simple scientific arguments, part of which is well known to the whole community, demonstrating that the authors' criticisms are unfounded and that they overstate the issue of damping in the Landau-Lifshitz equation with no unanimous experimental evidence.

cond-mat.mes-hall↗