Magnetic Damping Effect Research Articles

Three dimensional simulation of melt flow in Czochralski crystal growth with steady magnetic fields

Three-dimensional transient numerical simulations were carried out to investigate the melt convection and temperature fluctuations within an industrial Czochralski crucible. To study the magnetic damping effects on the growth process, a vertical magnetic field and a cusp magnetic field were considered. Due to our special interest in the melt convection, only local simulation was conducted. The melt flow was calculated by large-eddy simulation (LES) and the magnetic forces were implemented in the CFD code by solving a set of user-defined scalar (UDS) functions. In the absence of magnetic fields, the numerical results show that the buoyant plumes rise from the crucible to the free surface and the crystal–melt interface, which indicates that the heat and mass transfer phenomena in Si melt can be characterized by the turbulent flow patterns. In the presence of a vertical magnetic field, the temperature fluctuations in the melt are significantly damped, with the buoyant plumes forming regular cylindrical geometries. The cusp magnetic field could also markedly reduce the temperature fluctuations, but the buoyant plumes would break into smaller vortical structures, which gather around the crystal as well as in the center of the crucible bottom. With the present crucible configurations, it is found that the vertical magnetic field with an intensity of 128 mT can damp the temperature fluctuations more effectively than the 40 mT cusp magnetic field, especially in the region near the growing crystal.

Spatial manipulation of magnetic damping in ferromagnetic-antiferromagnetic films by ion irradiation

The spatial manipulation of the effective magnetic damping parameter in ferromagnetic-antiferromagnetic-ferromagnetic film systems is shown. By applying ultrathin antiferromagnetic layers in Ni${}_{81}$Fe${}_{19}$/IrMn/Ni${}_{81}$Fe${}_{19}$ sandwich structures in combination with low fluence Ni-ion irradiation, a lateral control of the effective magnetic damping parameter is achieved. With irradiation, an interfacial intermixing and roughening is introduced, by which the interfacial coupling mechanisms and the magnetic state of the interlayer are altered. We find an exponential decay of all relevant magnetic property parameters with irradiation. Local irradiation is then applied to generate a magnetic layer with spatially distributed regions of different values of damping. The resulting overall relaxation time of the mixed property film is a direct superposition of the individual relaxation contributions. Thereby, the ratio of the phases with individual damping parameter determines the resulting overall damping.

Magnetic damping and stiffness effects on rod translation by active magnetic bearing

The purpose of this article is to apply the wavelet transform algorithm to identify the magnetic damping and magnetic stiffness coefficients of the drive rod with which a set of 4-pole active magnetic bearing (AMB) is equipped. By taking advantage of time–frequency analysis feature, the ridge curve of rod free response after wavelet transformation can be extracted to find the natural frequency of the rod/AMB system. In other words, due to the influence of magnetized field by AMB, the stiffness of the rod dynamics is not linear any more and can be estimated from the curve of the amplitude versus frequency by wavelet transformation. On the other hand, the non-linear damping coefficients can be estimated from the derivative of amplitude versus amplitude by wavelet transformation of rod free vibration. It is found that the non-linear magnetic damping coefficients are up to second order in polynomial and the stiffness coefficient is mainly of third order, respectively. In addition, the identified second-order damping coefficient is negative and hence implies that under specific rod displacement and speed, the dynamic of rod/AMB system in axial direction is unstable.

Phenomenology of Current-Induced Dynamics in Antiferromagnets

We derive a phenomenological theory of current-induced staggered magnetization dynamics in antiferromagnets. The theory captures the reactive and dissipative current-induced torques and the conventional effects of magnetic fields and damping. A Walker ansatz describes the dc current-induced domain-wall motion when there is no dissipation. If magnetic damping and dissipative torques are included, the Walker ansatz remains robust when the domain wall moves slowly. As in ferromagnets, the domain-wall velocity is proportional to the ratio between the dissipative torque and the magnetization damping. In addition, a current-driven antiferromagnetic domain wall acquires a net magnetic moment.

Non-contact control of elastic vibration with magnetic damping for non-ferromagnetic plate

This paper describes a method for non-contact vibration control of non-ferromagnetic conductive structures using electromagnetic force. This method has potential applications in conventional engineering and medical engineering. Elastic vibration is induced by an electromagnetic force, which is produced by the interaction between a magnetic field and the ed dy currents induced by a transient magnetic field. Since the int ensity of the electromagnetic force changes with the intensity or time variation of the transient magnetic field, the amplitud e or time variation of the elastic vibration can be controlle d by the transient magnetic field. On the other hand, the electromoti ve force induced by the deformation velocity and magnetic fie ld reduces the vibration, which is referred to as the magnetic damping effect. Since the magnetic damping effect reduces vibration and induces a time delay in the vibration response, consideration of the magnetic damping effect is required for non-contact vibration control of non-ferromagnetic structures using e lectromagnetic force. In this study, a method for controlling the vibration of non- ferromagnetic conductive structures using electromagnetic force is proposed, which is based on theoretical and one-degree-of-freedom models of vibration with magnetic damping. The parameters of the model are determined on the basis of the coupled eigenvalues obtained from the coupled finite element equation. Fee dback control is also used to realize vibration control. In order t o examine the validity and performance of the proposed vibration control method, vibration control experiments are carried out for various desired amplitudes and frequencies of vibration.

The Magnetic Viscous Damping Effect on the Natural Frequency of a Beam Plate Subject to an In-Plane Magnetic Field

Several magnetic force models were developed to interpret various phenomena of a soft ferromagnetic beam plate subjected to a uniform external magnetic field with different incident angles. In this paper, a new transverse magnetic force model for the interface between a ferromagnetic material and the air is derived with the continuation of magnetoelastic stress across the material boundary. It is noted that both the normal and the tangential components of magnetic field on the material boundary are considered in this model. By applying such a transverse magnetic force and the effect of magnetic viscous damping, a new theoretical model is constructed in this study to predict the natural frequency of a soft ferromagnetic beam plate placed in an in-plane magnetic field. The numerical results of the present study are displayed graphically and compared with the experimental data, which appeared in literature to assure the exactness of the present work.

Multiaxis Maglev Positioner With Nanometer Resolution Over Extended Travel Range

This paper presents a novel multiaxis positioner that operates on the magnetic-levitation (maglev) principle. This maglev stage is capable of positioning at the resolution of a few nanometers over a planar travel range of several millimeters. A novel actuation scheme was developed for the compact design of this stage that enables six-axis force generation with just three permanent magnets. We calculated the forces with electromagnetic analysis over the whole travel range and experimentally verified them with a unit actuator. The single-moving part, namely, the platen, is modeled as a pure mass due to the negligible effect of magnetic spring and damping. There are three laser interferometers and three capacitance sensors to sense the six-axis position/rotation of the platen. A lead-lag compensator was designed and implemented to control each axis. A nonlinear model of the force was developed by electromagnetic analysis, and input current linearization was applied to cancel the nonlinearity of the actuators over the extended travel range. Various experiments were conducted to test positioning and loading capabilities. The 0.267kg single-moving platen can carry and precisely position an additional payload of 2kg. Its potential applications include semiconductor manufacturing, microfabrication and assembly, nanoscale profiling, and nanoindentation.

Investigation of magnetic damping on an air track

A more effective experimental method is used to analyze the effect of magnetic damping on a nonferromagnetic air track. Due to the continuous interaction between the horizontal air track and the moving magnets fixed on the air-track glider, the experiment can be easily done with low-cost photoelectric detecting techniques. A simple model is proposed based on Faraday’s law, the Lorentz force law, and Ampere’s law. Analytic expressions for the position, the velocity, and the magnetic damping force of the moving magnets as a function of time are obtained. Systematic measurements were performed and the results are in good agreement with the model. The method provides a simple teaching platform for introductory physics demonstrations and undergraduate courses in experimental physics.

Dynamic stability of ferromagnetic plate under transverse magnetic field and in-plane periodic compression

The dynamic stability of a soft ferromagnetic rectangular and simply supported plate immersed in an applied transverse magnetic field, as well as subjected to an in-plane periodic compression is presented in this paper. The fundamental equations involving magnetoelastic interaction and magnetic damping effect for the ferromagnetic plate are developed. In the theoretical model, the expression of induced magnetic force is based on a generalized magnetoelastic variational model, and the magnetic damping is due to the Lorentz body force arising from eddy current in the ferromagnetic material. By means of a linearized magnetoelastic theory and perturbation technique, the motion equation of the ferromagnetic plate is reduced to a damped Mathieu's equation and solved. The dynamic stability of the magnetoelastic system without in-plane compression is theoretically analyzed first, to show that there exist two stable states: magnetic damped stable oscillation, and over-damped asymptotically stable motion before static divergence instability of the ferromagnetic plate occurs. The dynamic instability and stability regions for the parametric excitation of the ferromagnetic plate due to the harmonically excited in-plane compression are obtained next. The effects of magnetic damping and excitation frequency of the in-plane periodic compression on the stability regions are discussed in detail.

An experimental study of thermally induced convection of molten gallium in magnetic fields

This paper presents an experimental investigation on natural convection in a molten metal subject to a uniform magnetic field. The working fluid is molten gallium, which is contained in a rectangular box with the two opposite vertical walls held at different temperatures. The imposed magnetic fields are parallel to the temperature gradient. Both the temperature and velocity fields are measured with and without an imposed magnetic field. Numerical simulations of convective flows in the system are also performed using the numerical model developed in a previous study. Good agreement exists between the measured and computed results for the conditions studied. Results show that natural convection is suppressed with an imposed magnetic field and the magnetic damping effect increases with an increase in the applied field strength.

Magnetic force-induced damping effect for magnetic bearing motor

An innovative damping induced by magnetic force was designed successfully for a totally passive magnetic bearing motor. A magnetic ring of high permeability and an annular-shaped rubber pad were mounted on the stator 0.55 mm below the permanent magnet of the rotor. Computer simulations were compared with experimental measurements to decide on the material and configuration of the critical components. The natural frequencies for lateral and rotational modes of the rotor are around 22 Hz measured by impulse method. Both the magnetic bearing motor with and without magnetic damping are rotated at a rated speed of 3840 rpm, which is far above the first critical speed of 1305 rpm. Without magnetic damping, the natural damping ratio in the radial direction of the rotor is 0.0655. After damping, it increases to 0.1401. We have demonstrated by both experimental measurement and theoretical calculation that the antishock performance is significantly improved by the innovative damping technology in a passive magnetic bearing motor.

Influence of gravitational sedimentation of magnetic particles on ferrofluid convection in experiments and numerical simulations

In this study the effect of gravity sedimentation on ferrofluid convection has been studied both by experiments and numerical simulations. To take into account the sedimentation density gradient a two-phase mixture model was used in the simulations. Temperature difference across the layer and concentration of magnetic phase were varied in order to control the driving magnetic forces and sedimentation damping effects. Both the experiments and the simulations showed that sedimentation gradient within the fluid layer may lead to oscillatory convection.

Dynamic magnetic anisotropy at the onset of exchange bias: TheNiFe∕IrMnferromagnet/antiferromagnet system

The strength of exchange bias and rotatable anisotropy in polycrystalline $\mathrm{Ni}\mathrm{Fe}\ensuremath{-}\mathrm{Ir}\mathrm{Mn}$ ferromagnet/antiferromagnet systems is quantified from dc down to the picosecond time scale by regular quasistatic and microwave magnetometry, as well as magnetic domain observation. A transition from superparamagnetic to antiferromagnetic behavior with increasing IrMn thickness is derived from the magnetic resonance frequency and the effective magnetic damping parameter. A discrepancy between magnetic loop shift and dynamically obtained exchange bias strength is explained by asymmetric rotatable anisotropy contributions with different relaxation times in the antiferromagnetic layer. The time-dependent relaxation is directly confirmed by magnetic domain observations. Partially switching in the IrMn layer even with strong exchange bias is concluded. The increase of coercivity rises solely from the rotatable anisotropy contribution.

NUMERICAL SOLUTION OF MELTING PROCESSES FOR UNFIXED PHASE-CHANGE MATERIAL IN THE PRESENCE OF ELECTROMAGNETICALLY SIMULATED LOW GRAVITY

Electromagnetic simulation of low-gravity environment has been numerically investigated to study the transport phenomena associated with melting of an unfixed and electrically conducting phase-change material (PCM) inside a rectangular enclosure. Electromagnetic fields are configured such that the resulting Lorentz force can be used to damp and/or counteract the natural convection as well as the flow induced by sedimentation and/or floatation, thereby simulating the low-gravity environment of outer space. The governing equations are discretized using a control-volume-based finite-difference scheme. Numerical solutions are obtained for true low-gravity environment as well as for the simulated low-gravity conditions due to electromagnetic forces. The results show that when the Lorentz force is caused by the presence of magnetic field alone, the low-gravity condition is simulated by the magnetic damping effect, which is shown to have a profound effect on the flow field. On the other hand, it is shown that under electromagnetic field simulation where the Lorentz force is caused by the transverse electric and magnetic fields, it is possible to minimize the flow field distortion caused by the high magnetic field and thereby achieve a much better simulation of low gravity.

Influence of magnetic properties on magnetization dynamics of high-p films

The influence of magnetic properties on the magnetization dynamics of high-resistivity amorphous CoZrTa thin films was investigated. A strong correlation with magnetic coercivity was found. Even small values of coercivity have an effect on the observed FMR frequency and the effective magnetic damping parameter /spl alpha/. The increased coercivity is due to a locally changing magnetic anisotropy distribution acting as a trap for the domain walls. The inhomogeneous anisotropy distribution in the films leads to additional frequency components observed during the dynamic remagnetization processes. The anisotropy field is measured directly from the dynamically obtained data. The observed dynamic response of the films makes them suitable for applications in the gigahertz regime.

A study on the vibration analysis of a maglev vehicle -- A theoretical investigation of the effect of magnetic damping on a vibration control system

A study on the vibration analysis of a maglev vehicle -- A theoretical investigation of the effect of magnetic damping on a vibration control system

Evaluation on fracture toughness of low activation ferritic steel (JLF-1)

Low activation ferritic steels have been developed for a candidate of structural materials in nuclear fusion reactor applications. This paper dealt with the effects of rolling direction and side groove on the fracture toughness of low activation ferritic steel, JLF-1 Steel (9Cr-2W-V-Ta). The fracture toughness of JLF-1 steel showed the anisotropy in the rolling direction. In the case of J-R curve, JQ and slope of the longitudinal JLF-1 steel (BML) for the rolling direction were a slight larger than those of transverse JLF-1 Steel (BMT) at room temperature. In order to obtain the valid fracture toughness of JLF-1 Steel through the UC method, the CT specimen with a side groove of 40% was recommended. As one of low activation ferritic steels for the IEA test program, the JLF-1 (9Cr-2W-V-Ta) steel has been developed and investigated by the active cooperation of Japanese Universities (1,3,4). The low activation ferritic steels mounted in a high magnetic field such as fusion reactor components may be affected by the operating cycle and the mechanical vibration that is associated with the magnetic damping. For the sound design of fusion reactor, it is necessary to investigate the effect of magnetic damping on the mechanical properties of JLF-1 steel. Prior to its widespread, practical use, however, several fracture properties of the JLF-1 steel including tensile property, fatigue property and fracture toughness must be evaluated, preferentially. Especially, the investigation of fracture toughness and J-R curve is essential for practical applications of the JLF-1 steel into fusion areas, since it suffered from the elastic-plastic fracture behavior caused by the history of operation temperature. When the JLF-1 steel is used as the blanket structural material of fusion reactors, the experimental data for tensile properties and fracture toughness are very important to estimate safety life span of the blanket by critical crack length with the irradiation. The tensile properties and fracture toughness characteristics of unirradiated materials are required before the estimation of irradiated materials (2,5). Finally, the correlativity between magnetic field and fracture properties of JLF-1 steel should be established for its active application. Due to the limited amount of materials, the unloading compliance method with single-specimen method (ASTM E813-89), which was the alternative of multi-specimen method, was recommended for the examination of fracture toughness (6,9).

MAGNETIC DAMPING OF g-JITTER INDUCED DOUBLE-DIFFUSIVE CONVECTION

This article describes a numerical study of g-jitter driven double-diffusive convective flows and thermal and concentration distributions in binary alloy melt systems subject to an external magnetic field. The study is based on the finite element solution of transient magnetohydrodynamic equations governing the momentum, thermal, and solutal transport in the melt pool. Numerical simulations are conducted using synthesized single- and multi-frequency g-jitter as well as real g-jitter data taken during space flights with or without an applied magnetic field. It is found that for the conditions studied, the main melt flow follows approximately a linear superposition of velocity components induced by individual g-jitter components, regardless of whether a magnetic field exists or not. The flow field is characterized by a recirculating double-diffusive convection loop oscillating in time with a defined frequency equal to that of the driving g-jitter force. An applied magnetic field has little effect on the oscillating recirculating pattern, except around the moment when the flow reverses its direction. The field has no effect on the oscillation period, but it changes the phase angle. It is very effective in suppressing the flow intensity and produces a notable reduction of solute striations and time fluctuations in the melt. For a given magnetic field strength, the magnetic damping effect is more pronounced on the velocity associated with the largest g-jitter component present and/or the g-jitter spiking peaks. A stronger magnetic field is more effective in suppressing the melt convection and also is more helpful in bringing the convection in phase with the g-jitter driving force. The applied field is particularly useful in suppressing the effect of real g-jitter spikes on both flow and solutal distributions. With appropriately selected magnetic fields, the convective flows caused by g-jitter can be reduced sufficiently, and it is possible that diffusion dominates the solutal transport in the melt.

An analysis of variable magnetic damping of a cantilever beam-plate with end coils in transverse magnetic fields

This paper presents a non-linear effect of magnetic damping of conducting structures moving in magnetic fields. For this purpose, a simply typical model, i.e. a non-conducting and non-magnetizable cantilevered beam-plate with end coils in a transverse magnetic field, is employed in the discussion of this paper. After the second power of the rotation at the free end is taken into account, a nonlinear relation between the magnetic couple exerted on the coils and the rotation is gained. A formula of the effective damping factor for the nonlinear dynamic system with electromagneto–mechanical interaction is obtained by applying the method of energy balance. The numerical results show that the effective damping factor is changeable with the amplitude of the beam-plate or time. The larger the amplitude, the bigger the effective magnetic damping.

Effect of non-uniform magnetic field on crystal growth by floating-zone method in microgravity

The magnetic damping effect of the non-uniform magnetic field on the floating-zone crystal growth process in microgravity is studied by numerical simulation. The results show that the non-uniform magnetic field with designed configuration can effectively reduce the flow near the free surface and then in the melt zone. At the same time, the designed magnetic field can improve the impurity concentration non-uniformity along the solidification interface. The primary principles of the magnetic field configuration design are also discussed.