Distribution of micrometer-size particles in magnetic fluids in the presence of steady uniform magnetic field

In this paper, distribution of suspended micrometer-size particles in magnetic fluids is investigated. Microstructure formation of particles in magnetic fluids is simulated by using the discrete particle method based on the simplified Stokes dynamics. Not only magnetic particles but also nonmagnetic particles are rearranged in the field direction and form chain-like clusters due to the apparent magnetization in magnetic fluids in the presence of magnetic field. When the diameter of nonmagnetic particles is smaller than that of magnetic particles, nonmagnetic particles move into the empty space of microstructure of magnetic particles, and they are rearranged in the field direction. Uniformity of distribution of particles on the plane perpendicular to the field direction is maintained even after microstructure formation.     

System of magnetic particles in an external magnetic field: Surface and bulk structure

A system of particles with spin interaction in an external magnetic field is studied using a general formalism applicable to the solid, liquid, or glassy state. Explicit results are given in the mean spherical, LOGA, and EXP approximations. The Laplace transform of the wall-particle correlation function near a surface is obtained. From it, analytical expressions for the orientational density profile are derived and used to calculate the magnetization and magnetic susceptibility in the bulk. Magnetostriction is also discussed.     

Self-diffusion in one-dimensional lattice gases in the presence of an external field

We study the motion of a tagged particle in a one-dimensional lattice gas with nearest-neighbor asymmetric jumps, withp (respectively,q),p > q, the probability to jump to the right (left). It was shown in Ref. 6 that the fluctuations in the position of the tagged particle behave normally; (X)²Dt. Here we compute explicitly the diffusion coefficient. We findD=(1-)(p-q). where is the gas density. The result confirms some recent conjectures based on theoretical arguments and computer experiments.Partially supported by NSF grant No. DMR81-14726.Partially supported by CNR.Partially supported by CNPq, grant No. 201682-83.     

Ground states of spin-orbit-coupled Bose Einstein condensates in the presence of external magnetic field

Two different kinds of spin-orbit (SO) coupling are often investigated theoretically and experimentally in atomic Bose-Einstein condensates (BECs), namely, Rashba and Dresselhaus SO couplings. We show that ground states for these two SO-coupled BECs share lots of similarities and it is impossible to distinguish them from the observation of ground states. We find that an Ioffe-Pritchard magnetic field can be utilized as a tool to distinguish them. In the presence of the Ioffe-Pritchard magnetic field, ground states manifest distinctively for the Rashba and Dresselhaus SO-coupled BECs.     

强磁场作用下Cu熔体中富Fe颗粒的迁移与排列

凝固界面前沿颗粒间的相互作用决定了颗粒的运动轨迹、分布和材料的性能,控制熔体中颗粒的迁移可用于材料的净化和提纯.在Cu-30%Fe合金液固两相区施加不同的强磁场条件,富Fe颗粒的分布和排列不尽相同.当无强磁场作用时,富Fe颗粒较均匀地分布在Cu熔体中;随着施加稳恒强磁场磁感应强度的增加,富Fe颗粒向远离重力方向的试样上端迁移,样品底部几乎无富Fe颗粒;而施加向下的梯度磁场作用后,富Fe颗粒沿重力方向向下迁移.结合强磁场作用下颗粒的受力情况,分析了Fe颗粒的迁移行为.不同磁场条件和不同区域的颗粒直径统计分析表明,随磁感应强度增加,Fe颗粒聚合增加,但施加梯度强磁场后颗粒的团聚又逐渐减弱,对此从影响颗粒运动的Stokes和Marangoni凝并速度进行了讨论.从能量最低的角度解释了富Fe相沿平行磁场方向的取向排列.     

Lattice Boltzmann simulation of the two-dimensional flow of a magnetic suspension between two parallel walls under a non-uniform magnetic field

We have developed a lattice Boltzmann method based on fluctuation hydrodynamics that is applicable to the flow problem of a particle suspension. In this method, we have introduced the viscosity-modifying method, rather than the velocity-scaling method, in which a modified viscosity is used for generating random forces in lattice Boltzmann simulations. The viscosity-modifying method is found to be applicable to the simulation of a magnetic particle suspension. We have applied this method to the two-dimensional Poiseuille flow of a magnetic suspension between two parallel walls in order to investigate the behavior of magnetic particles in a non-uniform applied magnetic field. From the results of the snapshots, the pair correlation function between the magnetic pole and the magnetic particles and the averaged local particle velocity and magnetization distributions, it was observed that the behavior of the magnetic particles changes significantly depending upon which factor dominates the phenomenon in the balance between the magnetic particle–particle interaction, the non-uniform applied magnetic field and the translational and rotational Brownian motion.     

Thermopower anisotropy in magnetic films of Co and Ni plated in presence of an external magnetic induction

Electroplated films of cobalt and nickel were grown on copper substrate both in absence and presence of external magnetic induction ranging from 25 to 72 gauss. The latter films showed a magnetic behaviour and an anisotropy in thermopower with respect to the direction of magnetisation.     

The effect of magnetic field on plasma particles in dusty plasma

In this work, we present the investigation of the formation features and internal structure of dust clouds created in plasma of glow discharge in the external magnetic field corresponding to a range of moderate and strong fields, at which the ion component is magnetized. The analysis of the plasma magnetization in the presence of dust components is carried out. We defined the values of magnetic induction at which the changes in dynamics of plasma particles in magnetic field in light inert gases are expected. The experimental setup was built in two versions. For the purpose of generating of magnetic field, the first setup was equipped with ordinary magnetic coils, and the second one included a superconducting solenoid. The discharge tubes, the main chambers where plasma was ignited and maintained in a glow discharge in lowered pressure, also have certain peculiarities, which we describe below. While using helium as a bulk gas, our study focused only on the dust trap in the region of narrowing discharge current. For neon, we used two traps: the striation trap and one just mentioned above placed in the narrowing of the discharge tube. As a result, the steady dust structures in a glow discharge under the magnetization of ions and electrons were obtained for the first time. Dust structures were rotated and tended to form a dust cluster and shell structure. A number of parameters of magnetization achieved in experiments were calculated.     

Oscillations of charged particles in an external magnetic field about steady motion

We develop a Hamiltonian formalism that can be used to study the particle dynamics near stable equilibria. The construction of an original canonical transformation allowed us to prove the conservation of the linear momentum P₃, which permitted the expansion of the Hamiltonian about a fixed point. The definition of the rotational variable h whose Poisson algebra properties played the essential role in the diagonalization of the quadratic Hamiltonian yielding two uncoupled oscillators with definite frequencies and amplitudes. It is through applying this variable near a fixed point that come to light Heisenberg's and Harmonic Oscillator equations of motion of the particles, leading thus the association of the fixed point trajectories with arbitrary trajectories in its immediate neighborhood. The present formalism succeeded to treat the problem of free-electron laser dynamics and may be applied to similar cases. Received 20 October 2001     

Ground state of finite arrays of magnetic dots in the presence of an external magnetic field

The ground state of an array of magnetic particles (magnetic dots), which are ordered in a square 2D lattice and whose magnetic moment is perpendicular to the lattice plane, in the presence of an external magnetic field has been analyzed. Such a model is applicable for sufficiently small dots with perpendicular anisotropy that are in a single-domain state and for dots in a strongly inhomogeneous vortex state whose magnetic moment is determined by the vortex core. For the magnetic field perpendicular to the system plane, the entire set of the states has been analyzed from the chessboard antiferromagnetic order of magnetic moments in low fields to the saturated state of the system with the parallel orientations of the magnetic moments of all dots in strong fields. In the presence of the border, the destruction of the chessboard order first occurs at the edges of the system, then near the extended sections of the surface, and finally expands over the entire interior of the array. The critical field at which this simplest state is destroyed is much more weakly than the value characteristic of the ideal infinite system. In contrast to this scenario, the destruction of the saturated state with decreasing field always begins far from the borders. Despite such different behaviors, the magnetic structure in the intermediate range of fields that is obtained with both increasing and decreasing field for finite arrays strongly differs from that characteristic of the ideal infinite system. The role of simple stacking faults of the magnetic dot lattice (such as single vacancies or their clusters) in the remagnetization of the system has been analyzed. The presence of such faults is shown to give rise to the appearance of local destructions of the chessboard antiferromagnetic order at fields that are much weaker than those for an ideal lattice.     

Stability of small planar dust cluster in the presence of an external magnetic field

Oscillations and stability of planar symmetric dust clusters shaped as a regular polygon and a regular polygon with a particle at the center in the presence of an external uniform magnetic field are analyzed. The structure stability against small perturbations is considered. To this end, the equation of motion was linearized with respect to small coordinate variations. As a result, a dispersion relation (quartic algebraic equation) was derived and numerically studied for particular types of interactions between particles. A comparison of stability regions for a quadrangle and a triangle with a particle at the center, as well as a pentagon and a quadrangle with a particle at the center shows that there are ranges of parameters, where configurations without particle at the center are stable and configurations with a particle at the center are unstable, and vice versa.     

The collision efficiency of spherical dioctyl phthalate aerosol particles in the Brownian coagulation

The collision efficiency in the Brownian coagulation is investigated. A new mechanical model of collision between two identical spherical particles is proposed, and a set of corresponding collision equations is established. The equations are solved numerically, thereby obtaining the collision efficiency for the monodisperse dioctyl phthalate spherical aerosols with diameters ranging from 100 to 760 nm in the presence of van der Waals force and the elastic deformation force. The calculated collision efficiency, in agreement with the experimental data qualitatively, decreases with the increase of particle diameter except a small peak appearing in the particles with a diameter of 510 nm. The results show that the interparticle elastic deformation force cannot be neglected in the computation of particle Brownian coagulation. Finally, a set of new expressions relating collision efficiency to particle diameter is established.     

Relativistic excitation of beat waves in a plasma in the presence of an external magnetic field

Summary A theoretical investigation of the nonlinear relativistic excitation of the longitudinal electron plasma wave at the beat frequency of two colinear laser beams has been made in a hot, collisionless and homogeneous plasma in the presence of an external magnetic field. The relativistic Vlasov equation has been solved to obtain the nonlinear response of magnetized electrons. It is observed that the power conversion efficiency of the excited-electron plasma wave at the difference frequency is much higher for the relativistic calculation than that for the nonrelativistic calculations. It is further noticed that the power conversion efficiency of the beat wave excitation increases slowly with the external magnetic field when the colinear laser beams propagate transverse to the direction of the external magnetic field, whereas it is almost unaffected when the incident waves propagate parallel to the direction of the magnetic field.

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Riassunto Si è effettuata una ricerca teorica dell'eccitazione relativistica nonlineare dell'onda longitudinale del plasma dell'elettrone ad una frequenza di battimento di due raggi laser colineari in un plasma omogeneo, caldo e senza collisioni, in presenza di un campo magnetico esterno. è stata risolta l'equazione di Vlasov relativistica per ottenere la risposta nonlineare di elettroni magnetizzati. Si osserva che l'efficacia di conversione di potenza dell'onda di plasma degli elettroni eccitati alla frequenza di differenza è molto superiore per il calcolo relativistico che quella per i calcoli non relativistici. Si nota inoltre che l'efficacia di conversione di potenza dell'eccitazione d'onda di battimento aumenta lentamente con il campo magnetico esterno quando i raggi laser colineari si propagano trasversalmente rispetto alla direzione del campo magnetico esterno, mentre rimane quasi immutata quando le onde incidenti si propagano parallelamente alla direzione del campo magnetico.

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Резюме Проводится теоретическое исследование нелинейного релятивистского возбуждения продольных электронных плазменных волн на частоте биений двух соосных лазерных пучков в торячей, бесстолкновительной и однородной плазме в присутствии внешнего магнитного поля. Чтобы получить нелинейный отклик электронов в магнитном поле, решается релятивистское уравнение Власова. Получается, что эффективность конверсии мощности для возбужденной электронной плазменной волны на разностной частоте оказывается много большей для релятивистских вычислений, чем для нерелятивистских вычислений. Затем отмечается, что эффективность возбуждения биений увеличивается медленно с ростом внешнего магнитного поля, когда соосные лазерные пучки распространяются поперек направления внешнего магнитного поля, тогда как эффективность возбуждения почти не изменяется, когда падающие волны распространяются параллельно направлению магнитного поля.

     

用外磁场控制电流感应磁化翻转效应中的临界电流方法

应用基于磁动力学方程的宏观唯象模型,研究了弱外磁场下纳米尺度赝自旋阀结构的电流感应磁化翻转效应.在统一考虑铁磁/非磁界面的自旋相关散射以及铁磁层中的自旋积累和弛豫过程后,给出了赝自旋阀结构在弱外磁场下的磁化翻转条件和临界电流.对该效应的数值计算解释了弱外磁场下赝自旋阀结构的电阻-电流回线的偏移,并给出了用外磁场控制电流感应磁化翻转效应中的临界电流方法. 关键词： 电流感应磁化翻转 外磁场 临界电流 赝自旋阀     

Switching behaviour of unijunction transistor in the presence of magnetic field

The influence of magnetic field on some switching parameters (turn-on time, turn-off time and amplitude of the current pulse appearing at base 1 terminal) of a unijunction transistor has been theoretically and experimentally investigated. The various switching parameters are shown to be governed by the magneto-concentration effect.     

Temperature dependence of the critical magnetic field of the structural transition in MBBA-based ferronematics

We studied the structural transitions in ferronematics based on the thermotropic nematic liquid crystal MBBA (4^′ -methoxybenzylidene-4-n-butylaniline) having a nematic-to-isotropic transition temperature T _N–I?=?48.0^○C and in MBBA-based ferronematics doped with a magnetic suspension consisting of Fe₃O₄ particles (10?nm in diameter) coated with oleic acid as a surfactant. The ferronematic samples were prepared with different volume concentrations of magnetic particles φ?=,1× 10^?4, 2× 10^?4 and 5×10^?4. The temperature dependences of the critical magnetic fields in a bias electric field under strong applied magnetic fields are presented. We calculated the surface density of anchoring energy W at the nematic–magnetic particle boundary. Scaling of the structural transition in the MBBA and MBBA-based ferronematics with the temperature of the nematic-to-isotropic transition was observed.     

Quasi-2D Monte Carlo simulations of a colloidal dispersion composed of magnetic plate-like particles with magnetic moment normal to the particle axis

We have investigated aggregation phenomena of a colloidal dispersion composed of magnetic plate-like particles by means of Monte Carlo simulations. Such plate-like particles have been modelled as disk-like particles with magnetic moment normal to the particle axis at the particle centre, with the section shape of a spherocylinder. The main objective of the present study is to clarify the influences of the magnetic field strength and magnetic interactions between particles on particle aggregation phenomena. We have concentrated our attention on a quasi-2D system from an application point of view such as the development of surface quality changing technology using such magnetic plate-like particles. A magnetic field is applied along the direction perpendicular to the plane of the monolayer. Internal structures of particle aggregates are discussed quantitatively in terms of radial distribution and orientational pair correlation functions. For the case of strong magnetic interactions between particles, particles form long column-like clusters with their magnetic moments alternating in direction between the neighbouring particles. These tendencies appear under circumstances of a weak applied magnetic field. However, as the magnetic field strength increases, particles incline towards the magnetic field direction, so that particles do not form such clusters.     

Generation of an external magnetic field with the spin orientation effect in a single layer Ising nanographene

In this work, the magnetic properties of the single layer Ising nanogaphene (SLING) are investigated by using Kaneyoshi approach (KA) within the effective field theory for different spin orientations of its magnetic atoms. We find that the magnetizations of the SLING has no phase transition, certain Curie temperature and distinct peak of susceptibility at T_c for the some spin orientations at the zero external magnetic field (H=0.0). Because these behaviors occur at H≠0.0, we suggest that the SLING generates an external magnetic field and behaves as an external magnetic field generator for these spin orientations. However, the SLING exhibits ferromagnetic behaviors for only one spin orientations. But, it exhibits antiferromagnetic behaviors for the others. For the AFM cases, diamagnetic susceptibility behaviors and type II superconductivity hysteresis behaviors are obtained. We hope that these results can open a door to obtain new class of single layer graphene and graphene-based magnetic field generator devices with the spin orientation effect.     

The magnetic hyperfine interaction of iron atoms isolated in a xenon matrix in the presence of an external magnetic field

The Mössbauer effect has been used to measure the magnetic hyperfine interaction of isolated ⁵⁷Fe atoms in solid xenon with an applied external magnetic field. A field dependent Mössbauer absorption spectrum is observed. The ground state of these iron atoms is a triplet, which is split in the external field. The Mössbauer spectrum was analyzed taking into consideration relaxation effects. For an applied external field of 28 kOe an internal magnetic field at the ⁵⁷Fe nucleus of 700± 15 kOe was observed (external field included).