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.     

Current-induced domain formation in magnetic junctions

A junction between two ferromagnetic metal layers with fixed spins in one of them and free spins in the other (spin valve) is considered. The junction is placed in an external magnetic field that orients the free layer oppositely to the fixed layer. It is shown that the spin-polarized electron flow from the fixed layer to the free layer gives rise to stable motionless magnetic domains in the free layer, provided that the magnetic field and the thickness of the free layer are large enough.     

Effect of magnetic field with helical lines of force on the capillary instability of magnetic fluids with the interface surrounding a current-carrying conductor

The subject of investigation is a configuration made up of immiscible magnetic fluids surrounding a long current-carrying conductor in the presence of an external longitudinal magnetic field. The axisymmetric problem of capillary instability of this configuration is considered in a linear statement. A dispersion relation is derived and analyzed, and neutral stability curves are constructed. The effect of the magnetic field and a step change in the permeabilities of the bordering fluids on the characteristic size of drops arising as a result of capillary disintegration of a continuous cylindrical layer of the magnetic fluid is discussed.     

On the influence of a horizontal magnetic field on the Rosensweig instability of a nonlinear magnetizable ferrofluid

The Rosensweig instability induced by magnetic forces of the flat free surface of the layer of a stationary nonlinearly magnetizable ferrofluid is considered. The fluid covers a horizontal plate of a nonmagnetic material, located in a tilted magnetic field. The critical value of the vertical component of the magnetization vector is calculated in the linear formulation for a deep magnetic fluid for any physically admissible magnetization law. The influence of the horizontal component of the applied magnetic field on the onset of instability upon the modified Langevin magnetization is analyzed.     

Optical diode based on a highly anisotropic layer of a helical periodic medium subjected to a magnetic field

The effect of an applied magnetic field on the optical properties of a layer of a helical periodic medium is studied in view of magnetooptic activity. The case when the radiation is normally incident on the layer and the magnetic field is aligned with the axis of the medium is considered. Irreversibility (nonreciprocity) effects in such a system are discussed. The situations with weak and high anisotropy are investigated. It is shown that the system can function as an optical shutter, optical diode, or one-side reflector. Reasons for the high irreversibility of transmission (reflection) are found.     

Optics of magnetic multilayers

Optical response of layered lossy anisotropic media displaying magnetic order is considered. The analysis is based on classical optical terms. Planar magnetic multilayers (ML) are represented by Yeh's 4×4 matrices which relate the field amplitudes in the isotropic half spaces on both sides of a ML structure. Each layer is characterized by a complex permittivity tensor of general form while magnetic permeability assumes its vacuum value. The basic assumption is that the electromagnetic field in a layer can be represented by a linear combination of four eigenmodes following from the wave equation of the layer. Then the boundary conditions at interfaces along with a set of four field amplitudes completely determine ML electric and magnetic field distributions. Alternatively, two orthogonally polarized incident wave amplitudes in each of the isotropic half spaces may be specified. The approach is more general than those published so far. Assuming minimum previous experience in the ML optics the case of polar magnetization is treated in detail to provide a simple way to the solution of the problems most often encountered in practice.     

Switching of a spin valve with three magnetic layers

A spin valve with two pinned ferromagnetic layers sandwiching a free ferromagnetic layer with a thickness smaller than the spin diffusion length in the same layer and than the domain wall thickness is considered. The instability conditions are determined for various mutual orientations of the magnetization of the layers. The possibility of a considerable decrease in the instability threshold due to joint action of spin-polarized current and an external magnetic field is indicated. It is shown that in addition to collinear states, a nonequilibrium noncollinear state can exist, into which the system is switched after exceeding the instability threshold.     

The equatorial asymmetry of a magnetic field

Solution of the inverse problem for Parker’s one-dimensional mean-field dynamo model in a thin spherical layer is considered. The method allows the spatial distribution of energy sources, the α- and Ω-effects, to be found provided specified constraints occur on the solution. The highest ratio of the magnetic energies for the Northern and Southern hemispheres is discussed as such a constraint. The method is a modification of the Monte-Carlo technique; it is convenient for parallel computations and based on minimization of the cost function that characterizes the deviation of the model solution properties from the desired ones. The calculations show that the ratio of the energies in the hemispheres may exceed an order of magnitude for both poloidal and toroidal components of the magnetic energy. The ratio depends on the distance of the effective zone of the generation of the magnetic field from the equator and the number of harmonics in the spectrum. The greater this distance is and the higher the number of harmonics is, the stronger the magnetic field asymmetry can be.     

Tunnel magnetoresistance of magnetic junctions with cubic symmetry of the layers

A tunnel magnetic junction is considered with magnetic hard and magnetic soft layers of cubic symmetry. The magnetic switching of the layers is analyzed for a magnetic field perpendicular to the initial magnetizations. In such a situation, an additional peak in the tunnel magnetoresistance ratio appears at the magnetic field value that is substantially lower than the anisotropy field of the soft layer.     

Rossby waves in the magnetic fluid dynamics of a rotating plasma in the shallow-water approximation

We have studied rotating magnetohydrodynamic flows of a thin layer of astrophysical plasma with a free boundary in the β-plane. Nonlinear interactions of the Rossby waves have been analyzed in the shallow-water approximation based on the averaging of the initial equations of the magnetic fluid dynamics of the plasma over the depth. The shallow-water magnetohydrodynamic equations have been generalized to the case of a plasma layer in an external vertical magnetic field. We have considered two types of the flow, viz., the flow in an external vertical magnetic field and the flow in the presence of a horizontal magnetic field. Qualitative analysis of the dispersion curves shows the presence of three-wave nonlinear interactions of the magnetic Rossby waves in both cases. In the particular case of zero external magnetic field, the wave dynamics in the layer of a plasma is analogous to the wave dynamics in a neutral fluid. The asymptotic method of multiscale expansions has been used for deriving the nonlinear equations of interaction in an external vertical magnetic field for slowly varying amplitudes, which describe three-wave interactions in a vertical external magnetic field as well as three-wave interactions of waves in a horizontal magnetic field. It is shown that decay instabilities and parametric wave amplification mechanisms exist in each case under investigation. The instability increments and the parametric gain coefficients have been determined for the relevant processes.     

Instability of the flat surface of a magnetic fluid in a cylindrical cavity in the presence of a vertical magnetic field

The problem of a magnetic liquid which completely fills a vertical cylindrical cavity in an undeformable horizontal layer of a magnet having the same magnetic properties as the liquid is considered. The entire system is immersed in a uniform vertical magnetic field. in a linear formulation of the problem an approximate solution in the form of series is obtained for the evolution of an initial small deviation of the free surface of the liquid from its flat equilibrium shape. An experiment is performed which shows that the initially flat free surface takes on a stable domed shape as the field strength is increased (from zero) and that a further increase in the field in a certain restricted range leads to the formation of an annular corrugation. The structures observed, which are the result of the nonlinear stage in the development of the initial perturbation, are qualitatively similar to the first two modes of the solution obtained. Zh. Tekh. Fiz. 68, 23–30 (January 1998)     

Periodic structure of acicular magnetic clusters in a magnetic liquid

The behavior of magnetic clusters in a magnetic liquid placed in a circular capillary is considered. When a uniform magnetic field is applied to the system, acicular clusters grow from the sediment, being aligned with the field. The interaction of the clusters as magnetic dipoles with one another and with an external gradient magnetic field is considered theoretically. In a nonuniform symmetric magnetic field with a peak, the cluster distribution is uniform near the peak. Such a distribution is fairly stable when the magnetic field gradient is varied over certain limits. The ordered (periodic) cluster configuration is realized experimentally, and it is shown that its period can be controlled. As the magnetic field gradient exceeds a certain threshold, the clusters are redistributed, forming a multirow hexagonal array.     

On possibility of the spontaneous magnetic flux in a Josephson junction containing magnetic impurities

The Josephson junction containing localized magnetic moments in a dielectric layer between two superconductors is considered. Conditions are studied under which the phase difference between superconductors in the state with energy minimum is equal to π (such a junction we call π-junction). In addition we consider “one-dimensional” Josephson junction one part (2) of which is π-junction, the other (1) being the usual Josephson junction ( 0 - junction). Conditions are found under which in such a system there is a spontaneous vortex with the centre at the boundary between the parts 1 and 2 and magnetic flux associated with this vortex. The vortex appears by second order phase transition as temperature decreases for T_c.     

Continuous spin-reorientation phase transition in the surface region of an inhomogeneous magnetic film

A continuous spin-reorientation transition from a uniform magnetic state with the in-plane orientation of the moments of all atomic layers to a nonuniform canted state in the surface region is considered. This transition was discovered in experiments on the divergence of magnetic susceptibility in a perpendicular magnetic field at a temperature of about 240 K, which is lower than the Curie point of gadolinium, equal to 292.5 K. These experiments were carried out on an ultrathin iron magnetic film deposited on the (0001) surface of a thin gadolinium film. It is shown that, in the vicinity of the spin-reorientation transition, the thermodynamic potential has a form characteristic of the Landau theory of second-order phase transitions. The orientation angle of the moment of the surface atomic layer with respect to the plane of the film, which is chosen as an order parameter, exhibits anomalous behavior and increases with temperature. Expressions are derived for the magnetic susceptibility of each atomic layer. It is shown that, in the vicinity of the transition, the irregular part of the magnetic susceptibility of each atomic layer exhibits behavior characteristic of the susceptibility in the Landau theory: it is less by a factor of two in the low-symmetry phase and diverges at the transition point. The regular part of the magnetic susceptibility of each atomic layer makes an additional contribution to the asymmetry of the total susceptibility in the vicinity of the transition point; this result follows from the fact that the inhomogeneous magnetic system considered is semi-infinite.     

磁场对二元合金凝固过程中糊状层稳定性的影响

利用线性稳定性方法研究了外加磁场对二元合金凝固过程中糊状层稳定性的影响,且模型同时考虑了温度场、浓度场和流动的耦合作用.利用计算得出的色散关系式分析了磁场对糊状层稳定性的影响,其中包括直接模式和振荡模式.给出了不同情况下外加磁场对糊状层稳定性的影响,发现磁洛伦兹力可以减小由浮力引起的失稳效应.振荡模式下外加磁场对糊状层产生稳定作用,但直接模式下外加磁场对糊状层的稳定作用具有不确定性.本文所给出结果为工业中利用外加磁场改善产品的质量提供了重要的理论参考.     

Current induced exchange switching of magnetic junctions with cubic anisotropy of a free layer

The stability of equilibrium configurations of magnetic junctions with a free layer of cubic anisotropy with two axes in the layer plane has been analyzed. Variants of switching between different configurations have been considered. It has been demonstrated that the possibility exists of considerably decreasing the threshold current density necessary for switching. Numerical simulation of the dynamics of switching between different configurations has been performed.     

Ground magnetic state of an assembly of single-domain particles confined in a spherical layer

The ground magnetic state of systems of finite number N of single-domain particles confined in a spherical monolayer is investigated by numeric simulations. Two model situations are considered. In the first, the particle positions are imposed and fixed, in the second, the particles are able to move freely within the layer; in the latter case the excluded volume effect is taken into account. It is found that in all the range studied (N?200) the ground state of the system retains a considerable extent of magnetic vorticity (toroid moment). Moreover, the magnetic and toroid moments are correlated: they are approximately perpendicular to one another.     

Reflection and transmission of light through a layer possessing dielectric and magnetic helicities: II. Principal axes of dielectric and magnetic helicities are rotated relative to each other

The transmission and reflection of light are considered in the case of its normal incidence on a layer of a periodically nonuniform medium possessing dielectric and magnetic helicities, with the axes of electrical and magnetic permittivities being rotated relative to each other. A simple algorithm for constructing the solution of the boundary problem for a layer of a helical periodic medium (HPM) of finite thickness is proposed. Jones matrices are constructed and natural polarizations and eigenvalues of the transfer function for the complex amplitude corresponding to natural polarizations are calculated. Numerical analysis is carried out for the case of large (and very large) local anisotropy of the medium. It is shown in particular that a domain of transmission appears between domains of diffraction and specular reflection under certain conditions.     

Instability and breakup of a thick layer of a viscous magnetic fluid in a tilted magnetic field

A linear partial differential equation describing the evolution of an initial disturbance of a flat free surface of a thin layer of a viscous magnetic fluid covering a horizontal plate in the presence of a uniform magnetic field, is derived within a system of ferrohydrodynamic and magnetostatic equations. The effect of magnetizing the plate on the stability of the flat free surface is investigated. An estimate is obtained for the minimum value of the tangential component of the magnetization vector of the fluid sufficient to radically alter the pattern of the final breakup of the continuous layer. Zh. Tekh. Fiz. 69, 14–22 (October 1999)     

Influence of the segregation effect on the magnetic and optical properties of a compensated ferronematic liquid crystal

The Freedericksz transition in splay and bend geometry in a ferronematic (a suspension of single-domain magnetic particles in a nematic liquid crystal) is considered in the framework of the continuum theory. In zero magnetic field, the ferronematic was assumed to be compensated (i.e., having equal fractions of an impurity with magnetic moments directed parallel and antiparallel to the local director). Spatial distortions of the director and the concentration redistribution of the magnetic impurity in the ferronematic layer are studied as functions of the applied magnetic field and the segregation parameter. It is shown that the magnetic-field induced Freedericksz transition from the homogeneous to the inhomogeneous state has a threshold nature. The transition field as a function of the material parameters of the ferronematic is determined analytically. Magnetization of the ferronematic and its optical properties for a strong and weak segregation as functions of the magnetic field are studied.