Dynamics of a nonmagnetic drop suspended in a magnetic fluid in a rotating magnetic field

The behavior of a nonmagnetic drop suspended in a magnetic fluid and subjected to the action of a rotating magnetic field is studied experimentally. The configurations of the drop in the form of prolate and oblate ellipsoids of revolution in the rotating field are investigated, and disintegration of the drop in the rotating field and the development of comb instability of its surface under certain conditions are observed.     

Magnetization of nanoparticle systems in a rotating magnetic field

The investigation of a sizable thermal enhancement of magnetization is put forward for uniaxial ferromagnetic nanoparticles that are placed in a rotating magnetic field. We elucidate the nature of this phenomenon and evaluate the resonant frequency dependence of the induced magnetization. Moreover, we reveal the role of magnetic dipolar interactions, point out potential applications, and reason the feasibility of an experimental observation of this effect.     

Elasticity of a magnetic fluid in a strong magnetic field

Complex measurements of the following elastic-magnetic parameters of a magnetic fluid suspended by magnetic levitation within a horizontal tube in a strong magnetic field were performed: the oscillation frequency and decay coefficient; the static, ponderomotive, and dynamic elasticity coefficients; the fluid displacement under hydrostatic pressure; magnetization curve; and the magnetic field strength and gradient. Calculations based on a model of ponderomotive elasticity with correction for the resistance of a viscous fluid in motion and on the fluid column displacement for two magnetic fluid samples agree well with the experimental magnetization curve. The discussed technique holds promise for research into magnetophoresis and nanoparticle aggregation in magnetic colloids.     

Dynamics of self-organization in a thin layer of a magnetic fluid placed in a constant electric field

Structuring in the form of rotating rings, which has not been observed previously in magnetic fluids, is revealed, and the parameters of these structures are determined. Also, the evolution of vortices in a magnetic fluid layer is traced. The electrical properties of the layer exposed to a constant electric field are studied as functions of the field magnitude and exposure time.     

NMRON spin echo in a slowly varying magnetic field

Nuclear spin-spin relaxation of⁶⁰Co and⁵⁶Co in iron single crystals has been studied, using the three-pulse NMRON spin echo. A previously reported rapidT ₂ in⁶⁰CoFe is shown to have arisen from a modulation of the echo amplitude, caused by variations in the phase of the Larmor precession relative to the applied rf field. A lower limit ofT ₂∼0.2s is found in⁵⁶Co⁵⁶ Fe. Extension of this result to other CoFe samples is discussed.     

Conducting axisymmetric body in a coaxial varying magnetic field

We calculate the vector potential and magnetic field strength in an axisymmetric conductor introduced into a preset coaxial external magnetic field varying harmonically with time. The proposed method involves the representation of the vector potential as the sum of a converging series each term of which is a solution to the Helmholtz equation with constant coefficient at infinity. The next terms of each series are determined from the preceding terms using the known Green function. The adequacy of the numerical results is confirmed by their comparison with the exact values for a sphere in a uniform magnetic field and by comparison of the numerical results obtained using two different series for an ellipsoid in a nonuniform magnetic field.     

Energy absorption by a magnetic nanoparticle suspension in a rotating field

Heat generation by viscous dissipation in a dilute suspension of single-domain ferromagnetic particles in a rotating magnetic field is analyzed by assuming that the suspended particles have a high magnetic rigidity. The problem is solved by using a kinetic approach based on a rotational diffusion equation. Behavior of specific loss power (SLP) as a function of field strength H and frequency ω is examined at constant temperature. SLP increases as either of these parameters squared when the other is constant, eventually approaching a saturation value. The function SLP(H, ω) can be used to determine optimal and admissible ranges of magnetically induced heating.     

Interaction of a droplet magnetic fluid with a variable magnetic field

The subject of investigation is the effective magnetic susceptibility of a droplet magnetic fluid. It is found that the rotation of such a medium causes the intriguing behavior of its magnetization due to the probing-field-induced deformation of droplet aggregates followed by their reorientation under rotation. To confirm this, the deformation of droplet aggregates subjected to a variable magnetic field is studied. In the case of the variable magnetic field, the threshold strength causing deformation is much lower than in the case of the constant field and depends on the field frequency.     

Dynamics of a nanoparticle rotating in the near field of a heated solid surface

General expressions for the force of attraction to the surface and the projections of torque exerted on a particle rotating near a solid surface at an arbitrary orientation of the angular velocity vector have been found. It has been shown that the particle decelerates over time and the angular velocity vector tends to be oriented perpendicularly to the surface at any initial conditions.

     

Response of a magnetic nanoparticle lattice to a magnetic field pulse near the stability boundary

The dynamic response of a system being near the stable equilibrium boundary to an external magnetic field pulse is studied for 2D lattices of magnetic nanoparticles with cubic crystallographic anisotropy. The conditions under which magnetic moment oscillations from individual dipoles propagate to the entire system are revealed. This effect results in the lattice response are significantly larger in the external pulse duration and with an amplitude rather weakly depending on initial conditions and external field parameters, the processes during which the pulse results in reorientation of only individual lattice dipoles.     

Dynamics of a dielectric droplet suspended in a magnetic fluid in electric and magnetic fields

The behavior of a microdrop of dielectric liquid suspended in a magnetic fluid and exposed to the action of electric and magnetic fields is studied experimentally. With increasing electric field, the deformation of droplets into oblate ellipsoid, toroid and curved toroid was observed. At the further increase in the electric field, the bursting of droplets was also revealed. The electrorotation of deformed droplets was observed and investigated. The influence of an additional magnetic field on the droplet dynamics was studied. The main features of the droplet dynamics were interpreted and theoretically examined.     

Instability of a thin layer of a magnetic fluid in a perpendicular magnetic field

The instability and disintegration of a thin layer of a magnetic fluid in a perpendicular magnetic field are considered. New experimental findings for the dependence of the resulting surface structure of the layer on the external magnetic field and thickness of the layer are reported. Light diffraction by such structures is studied. Experimental data are compared with today’s theoretical concepts.     

Dynamics of the generalized Glauber-Ising chain in a magnetic field

A one-dimensional kinetic Ising model with nearest neighbor interactionJ and magnetic fieldH 0 is treated in both linear and nonlinear response, using the most general single spin-flip transition probabilities that depend on nearest neighbor states only. The dynamics is reformulated in terms of kinetic equations for the concentration n_l ⁺(t) [@#@ n_l(t) of clusters containingl up- [or down-] spins, which is exact in the homogeneous case. The initial relaxation time ^* of the magnetization is obtained rigorously for arbitraryJ, H, and temperatureT. The relaxation function is found by numerical integration forJ/T < 2. It is shown that coagulation of minus-clusters becomes negligible for bothJ/T andH/T large, and the resulting set of equations is solved exactly in terms of an eigenvalue problem. A perturbation theory is developed to take into account the neglected coagulation terms. The relaxation function is found to be non-Lorentzian in general, in contrast to the Glauber results atH = 0, which are recovered as a special case. In addition, nonlinear and linear relaxation functions differ forH 0. Consequences for the application to biopolymers are briefly mentioned.Supported in part by the Deutsche Forschungsgemeinschaft (SFB 130).     

Bifurcations of the shape of a magnetic fluid droplet in a rotating magnetic field

An analysis is made of the behavior of a magnetic droplet suspended in a liquid in a high-frequency uniform, rotating magnetic field. In weak fields the droplet is spheroidal while in strong fields it is disk-shaped. The observed change in the shape of the droplet as the amplitude of the field increases depends on the magnetic permeability μ of the liquid and takes place according to three scenarios: (a) for small μ the spheroidal droplet is continuously converted into a disk; (b) for intermediate μ there is a range of fields in which the droplet becomes a triaxial ellipsoid with its major axis lying in the plane of the field, and spheroid-triaxial ellipsoid-disk transitions take place as a result of a soft bifurcation; (c) at high μ both transitions are hard. Theoretical calculations are made of the stability curve for the various droplet shapes. It is predicted that a change in the types of droplet shape bifurcations will occur in strong fields. A comparison is made with the experimental data.