V. I. Stepanov

Frequency dependence and long time relaxation of the susceptibility of the magnetic fluids

Abstract A simple dynamic theory of nonequilibrium phenomena in magnetic fluids is suggested. It is shown that temperature maximum of the magnetic susceptibility and special character of its dispersion (1/ω−noise) are explained by the one-particle effects. A conception which links these phenomena with transition of the magnetic fluid into the spin-glass state is criticized.

Magnetic properties of biomineral particles produced by bacteria Klebsiella oxytoca

Ferrihydrite nanoparticles (2–5 nm in size) produced by bacteria Klebsiella oxytoca in the course of biomineralization of iron salt solutions from a natural medium exhibit unique magnetic properties: they are characterized by both the antiferromagnetic order inherent in a bulk ferrihydrite and the spontaneous magnetic moment due to the decompensation of spins in sublattices of a nanoparticle. The magnetic susceptibility enhanced by the superantiferromagnetism effect and the magnetic moment independent of the magnetic field provide the possibility of magnetically controlling these natural objects. This has opened up the possibilities for their use in nanomedicine and bioengineering. The results obtained from measurements of the magnetic properties of the ferrihydrite produced by Klebsiella oxytoca in its two main crystalline modifications are reported, and the data obtained are analyzed theoretically. This has made it possible to determine numerical values of the magnetic parameters of real biomineral nanoparticles.

Rotational viscosity of magnetic fluids: contribution of the Brownian and Néel relaxational processes

Abstract The general Fokker-Planck equation is derived, which takes into account two simultaneous diffusion processes: rotation of the magnetic moment of the particle around its crystal axes and rotation of the particle together with its magnetic moment with respect to the liquid. The influence of the joint diffusion on additional (rotational) viscosity arising in the flow of magnetic fluid in the presence of magnetic field is studied.

Magnetic Relaxation in a Suspension of Antiferromagnetic Nanoparticles

A kinetic model is proposed to describe the low-frequency magnetodynamics of antiferromagnetic nanoparticles suspended in a fluid. Because of their small size, apart from an anisotropic magnetic susceptibility typical of antiferromagnets, these particles also have a constant magnetic moment caused by sublattice decompensation. An orientational crossover takes place in such a nanosuspension (colloid) when magnetized by a constant field: the axes of easy particle magnetization that were initially aligned along the field become oriented perpendicularly. This effect changes significantly the characteristics of the system’s magnetic response: the dynamic susceptibility spectrum and the relaxation time in a pulsed field.

Magnetic resonances in ferrofluids: Temperature effects

The joint influence of superparamagnetic and powder-pattern effects on the formation of FMR absorption lines in ferrofluids is discussed. Consistent consideration of these mechanisms yields specific contours of the absorption lines and affects substantially the temperature dependence of the linewidth.

Transient field-induced birefringence in a ferronematic

Time-dependent orientational response of a nematic liquid crystal doped with single-domain ferromagnetic particles is studied. A set of coupled macroscopic equations describing evolution of the director texture and the magnetization distribution under the change of the external field is derived. For the case of a plane layer, the relaxation rates are obtained and analyzed.

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

Dynamic optical probing of the magnetic anisotropy of nickel-ferrite nanoparticles

Field dependence of dynamic magneto-orientational birefringence in a ferrocolloid based on the nickel-ferrite nanoparticles is examined. The nanoparticles are electrostatically stabilized and suspended in glycerin at low-volume fractions Φ⩽0.75%. The colloids are tested under crossed magnetic fields: an alternating weak (probing) and a constant strong (bias) one. By comparison to a theoretical model of the birefringence relaxation, an evaluation of the mean nanoparticle magnetic anisotropy energy, Ea, is done. We get σ=Ea∕kT∼10, given that the particles are rather polydisperse: a log-normal distribution with exp[⟨lnd⟩]=6.5nm and the width s=0.55. With the parameter σ in such a range, the NiFe2O4 grains make a first example of a nanodisperse material (ferrofluid) whose anisotropy properties differ substantially from both magnetically soft (e.g., maghemite with σ 50) nanosubstances of the same grain size.

Low-frequency dynamics of the orientational birefringence in a suspension of dipolar particles

Abstract On the basis of the rotation-diffusion model the orientational dynamics of a dipolar particle suspension (ferrocolloid, stiff-chain polymer solution, etc.) in an alternating field is studied. The frequency spectrum of the field-induced oscillations of the suspension optical anisotropy is obtained. It is demonstrated that, providing the field strength is sufficiently large (the nonlinear regime), the characteristic extrema appear on the field dependences of the optical signal harmonics. This peculiar amplitude-frequency behavior may be employed for estimation of the dispersed particle parameters.

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.