Yu. L. Raikher

Motion of ferroparticles inside the polymeric matrix in magnetoactive elastomers

Ferroelastic composites are smart materials with unique properties including large magnetodeformational effects, strong field enhancement of the elastic modulus and magnetic shape memory. On the basis of mechanical tests, direct microscopy observations and magnetic measurements we conclude that all these effects are caused by reversible motion of the magnetic particles inside the polymeric matrix in response to an applied field. The basic points of a model accounting for particle structuring in a magnetoactive elastomer under an external field are presented.

Magnetic and optical properties of ionic ferrofluids based on nickel ferrite nanoparticles

New ionic ferrofluids containing NiFe2O4 nanoparticles of size ⩽10 nm are investigated. The crystalline structure of the particles is probed by transmission electron microscopy and x-ray scattering. Static magnetization and field-induced birefringence measurements are performed on three samples differing by particle volume fraction. Cross analyzing of the results of those two types of macroscopic tests completely rejects a simple single-domain particle model but readily supports the two-component scheme of a particle as consisting of a core with a uniform magnetization and a surface layer of comparable thickness stowed with a spin-glass-like arrangement.

Ferromagnetic resonance in ferrite nanoparticles with uniaxial surface anisotropy

Magnetization oscillations in a single-domain spherical ferromagnetic particle with uniaxial surface and bulk anisotropies are studied. In a linear approximation we consider a weakly nonuniform precession mode and derive the dispersion relationships for the cases of the applied field either parallel or normal to the particle easy axis. It is shown that in both situations the surface can produce a considerable shift of the precession frequency as observed with the conventional ferromagnetic resonance (FMR) technique. The data obtained for fine particle assemblies (frozen magnetic fluids based on γ-Fe2O3 nanosize grains) support the conclusion. Moreover, in the systems under study the surface anisotropy contribution to the FMR resonance field appears to entirely dominate the bulk one.

NiFe2O4 nanoparticles in ferrofluids: evidence of spin disorder in the surface layer

We show that surface magnetic properties of NiFe2O4 nanoparticles constituting ionic ferrofluids can be investigated in macroscopic experiments. Cross-analysis of static magnetization and field-induced birefringence prove that the particles consist of a uniformly magnetized core and a spin-disordered surface layer of comparable thickness. r 2002 Elsevier Science B.V. All rights reserved.

Magnetodeformational effect in ferrogel samples

Abstract Elongation of a magnetoelastic material under the action of a uniform external field is discussed. For an isotropic sphere this problem is the most simple and for it—assuming linear magnetization and Hookean elasticity of the magnetoelast—there exists a classical estimation of the initial magnetodeformational susceptibility. This estimation essentially implies that the shape of a deformed sphere is an ellipsoid of revolution. Changing this implication by an assumption that the shape must be just axisymmetrical, makes to solve the problem anew. The result shows that the true shape of the sample is not an ellipsoidal one and due to that the initial magnetodeformational susceptibility of a sphere is about 30% higher than the conventional prediction.

Magnetic properties of ferrocolloids: The effect of interparticle interactions

Abstract Several new experimental results on the initial susceptability of magnetic fluids are presented and discussed from the theoretical point of view.

Numerical modeling of large field-induced strains in ferroelastic bodies : a continuum approach

A consistent continuum model of a soft magnetic elastomer (SME) is presented and developed for the case of finite strain. The numeric algorithm enabling one to find the field-induced shape changes of an SME body is described. The reliability of the method is illustrated by several examples revealing specifics of the magnetostriction effect in SME samples of various geometries.

Modeling of particle interactions in magnetorheological elastomers

The interaction between two particles made of an isotropic linearly polarizable magnetic material and embedded in an elastomer matrix is studied. In this case, when an external field is imposed, the magnetic attraction of the particles, contrary to point dipoles, is almost wraparound. The exact solution of the magnetic problem in the linear polarization case, although existing, is not practical; to circumvent its use, an interpolation formula is proposed. One more interpolation expression is developed for the resistance of the elastic matrix to the field-induced particle displacements. Minimization of the total energy of the pair reveals its configurational bistability in a certain field range. One of the possible equilibrium states corresponds to the particles dwelling at a distance, the other—to their collapse in a tight dimer. This mesoscopic bistability causes magnetomechanical hysteresis which has important implications for the macroscopic behavior of magnetorheological elastomers.

Effect of a dc bias field on the dynamic hysteresis of single-domain ferromagnetic particles

Dynamic magnetic hysteresis in uniaxial superparamagnetic nanoparticles in superimposed ac and dc magnetic fields of arbitrary amplitude is considered using Brown’s model of coherent rotation of the magnetization. The dependence of the area of the dynamic hysteresis loop on the temperature, frequency, and ac and dc bias fields is analyzed. In particular, the dynamic hysteresis loop of a single-domain ferromagnetic particle is substantially altered by applying a relatively weak dc field. Furthermore, it is shown that at intermediate to low ac field amplitudes, the dc bias field permits tuning of the magnetic power absorption of the particles, while for strong ac field amplitudes the effect becomes entirely analogous to that produced by the exchange biased anisotropy. Simple analytical formulas are provided in the linear response regime for the steady-state magnetization and loop area, exhibiting perfect agreement with the numerical solution of Brown’s Fokker–Planck equation. Comparison with previous result...

Ferronematics: On the development of the continuum theory approach

Abstract Some basic features of the previously existing macroscopic theory of ferronematics (i.e. magnetic suspensions with a liquid-crystalline carrier) are analyzed and the limitations of its applicability to real thermotropic systems is demonstrated. The modified free-energy density expression is proposed and discussed.