150 research outputs found
Dynamics of a magnetic dimer with exchange, dipolar and Dzyalozhinski-Moriya interaction
We investigate the dynamics of a magnetic system consisting of two magnetic
moments coupled by either exchange, dipole-dipole, or Dzyalozhinski-Moriya
interaction. We compare the switching mechanisms and switching rates as induced
by the three couplings. For each coupling and each configuration of the two
anisotropy axes, we describe the switching modes and, using the kinetic theory
of Langer, we provide (semi-)analytical expressions for the switching rate. We
then compare the three interactions with regard to their efficiency in the
reversal of the net magnetic moment of the dimer. We also investigate how the
energy barriers vary with the coupling. For the dipole-dipole interaction we
find that the energy barrier may either increase or decrease with the coupling
depending on whether the latter is weak or strong. Finally, upon comparing the
various switching rates, we find that the dipole-dipole coupling leads to the
slowest magnetic dimer, as far as the switching of its net magnetic moment is
concerned.Comment: 20 pages, 18 Figures, 2 table
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.Comment: 14 pages, 5 figure
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.Comment: 7 pages, 5 figure
Magnetic relaxation of a system of superparamagnetic particles weakly coupled by dipole-dipole interactions
The effect of long range dipole-dipole interactions on the thermal
fluctuations of the magnetization of an assembly of single-domain ferromagnetic
particles is considered. If orientational correlations between the particles
are neglected, the evolution of the magnetization orientations may be described
by a nonlinear Fokker-Planck equation (FPE) reducing to the usual linear one in
the limit of infinite dilution [W.F. Brown Jr, Phys. Rev. 130, 1677 (1963)].
The thermally activated relaxation time scale of the assembly is estimated,
leading to a simple modification of the axially symmetric asymptotes for the
superparamagnetic relaxation time.Comment: 31 pages, 3 figures, regular articl
Biométrie de la glossine : étude statistique des mensurations de l'aile dans diverses communautés
Des mensurations ont été faites dans quatre lots de tsé-tsé de la même sous-espèce: Glossina fuscipes quanzensis Pires; chacun de ces lots provenait d'une localité distincte. L'on constate qu'il existe des différences significatives de la taille moyenne entre certains de ces lots. Cette différence de taille est vraisemblablement attribuable principalement à la différence des biotopes selon les lieux de captur
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