Yuriy L. Raikher

Modelling of magnetodipolar striction in soft magnetic elastomers

The origin of the so called magnetostriction effect, i.e., the deformation of a soft magnetic elastomer (SME) in response to a uniform magnetic field is discussed. We note that up to now there exists a number of cases, where the theory is unable to explain the facts well established for real SMEs. In our view, the essential issue, formerly not adequately accounted for, is the type of structural short-range order in the magnetic microparticle assembly. Two-dimensional model problems are formulated and solved, which demonstrate the validity of the hypothesis.

Linear and nonlinear superparamagnetic relaxation at high anisotropy barriers

Institute of Continuous Media Mechanics, Ural Division of RAS, 614013, Perm, Russia~Received 20 May 2002; revised manuscript received 17 July 2002; published 11 December 2002!The micromagnetic Fokker-Planck equation is solved for a uniaxial particle in the low-temperature limit.Asymptotic series in the parameter that is the inverse barrier height-to-temperature ratio are derived. With theaid of these series, the expressions for the superparamagnetic relaxation time and the odd-order dynamicsusceptibilities are presented. The obtained formulas are both quite compact and practically exact in the low~with respect to FMR! frequency range that is proved by comparison with the numerically exact solution of themicromagnetic equation. The susceptibility formulas contain angular dependencies that allow to considertextured as well as randomly oriented particle assemblies. Our results advance the previous two-level model fornonlinear superparamagnetic relaxation.DOI: 10.1103/PhysRevB.66.214406 PACS number~s!: 75.30.Cr, 75.30.Gw, 75.50.TtI. INTRODUCTION

Modeling of the Field-Induced Plasticity of Soft Magnetic Elastomers

A phenomenological model is proposed to describe the plasticity of soft magnetic elastomer composites under an external field. Along with high elasticity, internal dry friction is introduced whose origin is attributed to the dipole-dipole interaction of the embedded particles. Numerical estimates of the model parameters are obtained from comparison with the measurements performed on iron carbonyl dispersions in soft silicon-rubber matrices.

Mean-field description of the order–disorder phase transition in ferronematics

A mean-field theory is developed describing in what way a small amount of ferromagnetic nanoparticles embedded in a nematic liquid-crystalline matrix affects the order parameters and the temperature of the nematic–isotropic phase transition. In such a suspension, often termed ferronematic, the ability of the particles to be aligned by the magnetic field results in occurrence of a partially oriented (paranematic) state above the transition temperature. This fact entails the possibility of field-tuning the transition temperature in both ways: up and down. The pertinent field-dependencies are obtained and analyzed. The same description applies to liquid-crystalline suspensions of ferroelectric nanoparticles tuned by the electric field.

Structure organization and magnetic properties of microscale ferrogels: The effect of particle magnetic anisotropy

The equilibrium structure and magnetic properties of a ferrogel object of small size (microferrogel(MFG)) are investigated by coarse-grained molecular dynamics. As a generic model of a microferrogel (MFG), a sample with a lattice-like mesh is taken. The solid phase of the MFG consists of magnetic (e.g., ferrite) nanoparticles which are mechanically linked to the mesh making some part of its nodes. Unlike previous models, the finite uniaxial magnetic anisotropy of the particles, as it is the case for real ferrogels, is taken into account. For comparison, two types of MFGs are considered: MFG-1, which dwells in virtually non-aggregated state independently of the presence of an external magnetic field, and MFG-2, which displays aggregation yet under zero field. The structure states of the samples are analyzed with the aid of angle-resolved radial distribution functions and cluster counts. The results reveal the crucial role of the matrix elasticity on the structure organization as well as on magnetization of both MFGs. The particle anisotropy, which plays insignificant role in MFG-1 (moderate interparticle magnetodipole interaction), becomes an important factor in MFG-2 (strong interaction). There, the restrictions imposed on the particle angular freedom by the elastic matrix result in notable diminution of the particle chain lengths as well as the magnetization of the sample. The approach proposed enables one to investigate a large variety of MFGs, including those of capsule type and to purposefully choose the combination of their magnetoelastic parameters.

Effect of Surface Anisotropy on the Magnetic Resonance Properties of Nanosize Ferroparticles

Fine particles are unique physical objects with remarkable magnetic properties which differ greatly from their parent massive ferromagnets or ferrites. The source of this difference, which has numerous and diverse manifestations, is the fact that in fine particles surface effects are not at all negligible in comparison with the bulk ones. Notably, just this condition suffices to outline the range of spatial scales where the notion of a fine magnetic particle turns up. All throughout this paper we discuss, estimate and analyze the equilibrium and dynamic properties of low-dimensional objects, thin films and fine particles made of a ferromagnet or ferrite. That is why from the very beginning we introduce the most important notations for “global” usage. In particular, we adopt the macroscopic (continuum) approach to magnetodynamics assuming simultaneously that the temperature is well below the Curie point. This framework had been proposed by Brown [1] and since then is known as micromagnetism. In this framework one may treat the magnetization of the ferromagnetic material as a vector of a constant length and denote it as

Magnetorelaxometry in the Presence of a DC Bias Field of Ferromagnetic Nanoparticles Bearing a Viscoelastic Corona

With allowance for orientational Brownian motion, the magnetorelaxometry (MRX) signal, i.e., the decay of magnetization generated by an ensemble of ferromagnet nanoparticles, each of which bears a macromolecular corona (a loose layer of polymer gel) is studied. The rheology of corona is modelled by the Jeffreys scheme. The latter, although comprising only three phenomenological parameters, enables one to describe a wide spectrum of viscoelastic media: from linearly viscous liquids to weakly-fluent gels. The “transverse” configuration of MRX is considered where the system is subjected to a DC (constant bias) field, whereas the probing field is applied perpendicularly to the bias one. The analysis shows that the rate of magnetization decay strongly depends on the state of corona and slows down with enhancement of the corona elasticity. In addition, for the case of “transverse” MRX, we consider the integral time, i.e., the characteristic that is applicable to relaxation processes with an arbitrary number of decay modes. Expressions for the dependence of the integral time on the corona elasticity parameter and temperature are derived.

SANS investigation of a ferrofluid based silicone elastomer microstructure

Structural peculiarities of ferroelastomers composed of polydimetylsiloxane with magnetite particles embedded during polymerization are studied by small-angle neutron scattering. The effects of ferroparticle concentration in the range up to 6 mass.%. and external magnetic fields of induction up to 1 kG, applied while curing, on the structure of the polymer and the particle spatial distribution in matrix are analyzed.

Collective Magnetic Moment Relaxation in an Assembly of Ferromagnetic Nanoparticles

The idea of establishing coherent relaxation in an assembly of magnetic moments by placing it in a passive resonator [1, 2] is applied to the case of single-domain ferromagnetic nanoparticles. The magnetodynamics of a particle is governed by the Landau-Lifshitz equation modified by adding the resonator feedback field. The particle uniaxial magnetic anisotropy and the interparticle dipole-dipole interactions are taken into account.

Theory of Magneto‐Inductive Hyperthermia Under a Rotating Field

Generation of heat due to viscous friction in a dilute suspension of single‐domain ferromagnetic particles subjected to a rotating field is considered; the particles are assumed to be magnetically rigid. The problem is solved in the framework of a kinetic approach. The behavior of specific loss power (SLP) as a function of field amplitude and frequency is studied. We find that for either of these parameters (while the other is kept constant) SLP first grows quadratically and then saturates. A simple analysis of SLP behavior is performed that enables one to determine the optimal and allowable parameter ranges for a given magnetic suspension intended for heat generation.