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