Intrinsic magnetic resonance in nanoparticles: Landau damping in the collisionless regime

Abstract In a uniaxially anisotropic particle the frequency of intrinsic ferromagnetic resonance depends on the angle deviation of the magnetic moment from its equilibrium position. At low spin-lattice relaxation and in the presence of superparamagnetism, the regime of relaxation in an assembly of such particles closely resembles the Landau damping regime, which is well known in the kinetic theory of plasma. In the high-temperature limit in the case of polydisperse systems, particle volume and surface anisotropy affect the absorption lines differently. r 2002 Published by Elsevier Science B.V

Basic magnetic properties of magnetoactive elastomers of mixed content

The results of theoretical and experimental investigations of the polymer composites that belong to a class of magnetoactive elastomers with mixed magnetic content (MAEs-MC) are presented. The fundamental distinction of such composites from ordinary magnetoactive elastomers is that the magnetic filler of MAEs-MC comprises both magnetically soft (MS) particles of size 3–5 µm and magnetically hard (MH) particles whose size is an order of magnitude greater. Since MH particles of the magnetic filler are mixed into a composition in a non-magnetised state, this can ensure preparation of samples with fairly homogeneous distribution of the filler. The 'initiation' process of a synthesised MAE-MC is done by its magnetisation in a strong magnetic field that imparts to the sample unique magnetic and mechanical properties. In this work, it is shown that the presence of MS particles around larger MH particles, firstly, causes an augmentation of magnetic moments, which the MH particles acquire during initiation, and secondly, enhances the magnetic susceptibility and remanent magnetisation of MAEs-MC. These magnetic parameters are evaluated on the basis of the macroscopic magnetostatics from the experimental data of spatial scanning of the field over the space around MAEs-MC made in the shape of a spheroid. A set of samples with a fixed MH and varying MS volume contents that are initiated in two different fields, is used. The developed mesoscopic model of magnetic interactions between the MH and MS phases is able to explain the experimentally observed dependencies of the magnetic parameters on the concentration of the MS phase. The problem is solved numerically under the assumption that the elastic matrix of MAEs-MC is rigid, i.e. the mutual displacements of the particles are negligible. The model helps to elucidate the interaction of the magnetic phases and to establish that the MS phase plays thereby a dual role. On the one hand, the MS phase screens out the field acting inside MH particles, and on the other hand, it forms mesoscopic magnetic bridges between adjoining MH particles, which in turn enhance their field. The combined interplay of these contributions defines the resulting material properties of MAEs-MC on the macroscopic scale

Strong spin-orbit induced Gilbert damping and g-shift in iron-platinum nanoparticles

The shape of ferromagnetic resonance spectra of highly dispersed, chemically disordered Fe_{0.2}Pt_{0.8} nanospheres is perfectly described by the solution of the Landau-Lifshitz-Gilbert (LLG) equation excluding effects by crystalline anisotropy and superparamagnetic fluctuations. Upon decreasing temperature, the LLG damping $\alpha(T)$ and a negative g-shift, g(T)-g_0, increase proportional to the particle magnetic moments determined from the Langevin analysis of the magnetization isotherms. These novel features are explained by the scattering of the $q \to 0$ magnon from an electron-hole (e/h) pair mediated by the spin-orbit coupling, while the sd-exchange can be ruled out. The large saturation values, $\alpha(0)=0.76$ and $g(0)/g_0-1=-0.37$, indicate the dominance of an overdamped 1 meV e/h-pair which seems to originate from the discrete levels of the itinerant electrons in the d_p=3 nm nanoparticles.Comment: 8 pages, 4 figures, accepted for publication in Phys. Rev. B (http://prb.aps.org/

Stochastic resonance in a superparamagnetic particle

Abstract The stochastic resonance (SR) effect in a single-domain particle is investigated for the case where the exciting field is imposed not parallel to the anisotropy axis but at an arbitrary angle. We show that despite the fact that for the transverse case there is no SR at all, the intermediate cases yield signal-to-noise ratios much higher than the wellinvestigated longitudinal case. The frequency range over which the effect is observable is estimated. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Superparamagnetic dynamics; Stochastic resonance; Signal-to-noise ratio The phenomenon of stochastic resonance (SR) is inherent to noise-driven multistable systems. As it always happens with the effects related to Brownian motion, it has a very wide range of applicability In magnetism, the SR effect turns up in several situations. In particular, in a single-domain ferromagnetic particle with uniaxial anisotropy. In the absence of interaction with the neighbours the particle orientationdependent energy is where e; n and h are the unit vectors of the particle magnetic moment, anisotropy axis and the external field, respectively; K is the effective anisotropy constant (for uniaxial anisotropy it is essentially positive), m ¼ I s V is the magnetic moment of a single-domain particle, I s its magnetization and V its volume. As Eq. Reducing the problem to a one-dimensional equation (as is typical for the basic SR theory), the authors find that in the limit o-0 and in the linear response theory approximation, magnetic stochastic resonance is described by some universal curve, SNR 0 ð1=sÞ: This curve corresponds to the cross-section b ¼ 0 of the surface i

Ferrohydrodynamics: testing a new magnetization equation

A new magnetization equation recently derived from irreversible thermodynamics is employed to the calculation of an increase of ferrofluid viscosity in a magnetic field. Results of the calculations are compared with those obtained on the basis of two well-known magnetization equations. One of the two was obtained phenomenologically, another one was derived microscopically from the Fokker-Planck equation. It is shown that the new magnetization equation yields a quite satisfactory description of magnetiviscosity in the entire region of magnetic field strength and the flow vorticity. This equation turns out to be valid -- like the microscopically derived equation but unlike the former phenomenological equation -- even far from equilibrium, and so it should be recommended for further applications.Comment: 4 pages, 3 figures, Submitted to Phys. Rev.