Magnetic Friction Research Articles

Canted exchange bias in antiparallel biased spin valves

Conventional spin valves consist of two magnetic layers, one of which is pinned by exchange bias to act as a reference magnetic layer in giant magnetoresistance measurements. Antiparallel biased spin valves feature a Co/Ru/Co trilayer in place of this pinned layer, where the Co layers are strongly antiferromagnetically coupled. This leads to hugely enhanced pinning since an applied field will exert opposite torques on the Co layer moments. Spin valves are typically deposited in a magnetic field to define magnetic anisotropies, and the development of magnetic structure in the pinned AF couple is not trivial under these conditions. The resultant moment of the AF couple is found to slowly rotate as the final Co layer is deposited. This results in canted pinning once the exchange bias layer is added. There appears to be some magnetic friction as the Co layers rotate during growth. The rotation means that the pinning direction can take any angle with respect to the growth field, leading to positive or negative (or even orthogonal) exchange bias.

A prototype of a levitation-type PM motor with a millimeter-sized rotor

This paper describes a prototype of a levitation type PM (permanent magnet) motor with a millimeter-sized cylindrical rotor. The PM motor consists of a rotor (/spl phi/ 2.0 mm/spl times/10 mm), a pair of electromagnets, a pair of photodiodes, and an analog PD controller. The rotor successfully rotates levitating in the center of electromagnets. In this paper, driving characteristics such as rotation speed, acceleration characteristics, and magnetic friction of the rotor are discussed. It is shown that the levitation-type PM motor is very promising.

ANALYSIS OF LOSSES DUE TO ROTOR VIBRATIONS IN A HIGH-TcSUPERCONDUCTING FLYWHEEL SYSTEM

The loss mechanisms associated with the combined effects of magnetic unbalance and hysteretic damping in a superconducting flywheel system have been modelled under the assumption that the dynamic characteristics of the bearing can be approximated by a linear, elastic isotropic spring with structural damping. The theoretical rundown equation of motion of such systems has been obtained by a Lagrangian approach, neglecting the effects of angular acceleration. The unknown parameters of the theoretical model have been determined by two different identification procedures starting from experimental time versus rotational speed curves obtained during rundown tests performed on a plastic flywheel with embedded permanent magnet, suspended on a highTcsuperconducting stator. In addition to magnetic friction and eddy current losses, the hysteretic nature of the superconducting magnetic bearing gives a significant contribution to the overall losses. Accordingly, the efficiency of the system can be increased by minimizing the unbalance of the rotor.

Experimental Study on Friction by Magnetic Attractive Force.

The more there are applications with magnetic frictional or attractive force, the more the relationship between these forces under the magnetic field applied will be important. Actually, magnetic friction force in the BTM (Belt type Transit system by Magnet) is double or triple to mechanical frictional force. But another report says that magnetic friction is equal to mechanical One. So experimental study based on real driving model becomes more necessary. In this paper, we acquire that the friction coefficient is increased in inverse proportion to air gap length in the magnetic circuit. Furthermore, by simulations of magnetic field in the model with finite element method, the correlation between attractive force and air gap length is observed.

Eddy currents and magnetic friction

Eddy currents and magnetic friction

A magnetic internal friction peak in iron—rare earth alloys

physica status solidi (a)Volume 148, Issue 2 p. K99-K102 Short Note A magnetic internal friction peak in iron—rare earth alloys Dian-Qi Geng, Dian-Qi Geng Department of Material Science and Engineering, Northeastern University, Shenyang Search for more papers by this authorDian-Yu Geng, Dian-Yu Geng Institute of Metal Research, Academia Sinica, Shenyang Search for more papers by this authorTao Zhao, Tao Zhao Department of Physics, Northeastern University, Shenyang Search for more papers by this authorZhi-Dong Zhang, Zhi-Dong Zhang Institute of Metal Research, Academia Sinica, Shenyang Search for more papers by this authorGong-Qing Teng, Gong-Qing Teng Department of Physics, Northeastern University, Shenyang Search for more papers by this author Dian-Qi Geng, Dian-Qi Geng Department of Material Science and Engineering, Northeastern University, Shenyang Search for more papers by this authorDian-Yu Geng, Dian-Yu Geng Institute of Metal Research, Academia Sinica, Shenyang Search for more papers by this authorTao Zhao, Tao Zhao Department of Physics, Northeastern University, Shenyang Search for more papers by this authorZhi-Dong Zhang, Zhi-Dong Zhang Institute of Metal Research, Academia Sinica, Shenyang Search for more papers by this authorGong-Qing Teng, Gong-Qing Teng Department of Physics, Northeastern University, Shenyang Search for more papers by this author First published: 16 April 1995 https://doi.org/10.1002/pssa.2211480243 Shenyang 110006, People's Republic of China. Shenyang 110005, People's Republic of China. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Volume148, Issue216 April 1995Pages K99-K102 RelatedInformation

Decade Variations of the Earth's Rotation and Geomagnetic Core-Mantle Coupling

The variations of the length of day and the angular velocity of the relative rotation of the Earth's core derived from geomagnetic secular variation were compared from 1650 to 1985. Periods comparable with the well-known 60 to 70-years periods of the fluctuations in the Earth's rotation could be found within the spectrum of the core rotation parameters by lengthening of the previously used time series to epochs before 1900. Additionally, the torques of the geomagnetic core-mantle coupling were computed for this time interval. The results agree with previously derived values for epochs after 1900. The coupling behaves like a homogeneous and isotropic magnetic friction, and the difference between axial and equatorial mechanical torques derived from Earth rotation parameters cannot be explained by magnetic core-mantle coupling.

Influence of dry magnetic friction on the ferromagnetic resonance

Equations of motion for the ferromagnetic resonance in the presence of a dry magnetic friction are solved analytically. The width of the resonance line is found to depend on the amplitude h of the driving field h exp( iω t) and it diverges when h decreases towards a critical value. Below this value dry friction stops the motion of the magnetization.

Spinning rate decay of levitated high- Tc superconductors in rotational magnetic field

The rotation damping of a levitated superconductor was examined in the static field of a ring-shaped magnet and in the rotating field of coils. It was demonstrated that the pinning force mainly contributed to magnetic friction while the influence of a viscous component was negligible. The rotating magnetic field created a torque, reducing the angular deceleration under relaxation. Dependence of the rotational field-induced torque on the field-intensity was step-like. A relationship between the step-like behaviour of rotational field-induced torque and pinning center distribution is discussed. The origins of friction torque and rotational field-produced torque are discussed.

Atomic and magnetic ordering in CoPt alloys studied by magnetic and internal friction measurements

The high-temperature magnetic and mechanical relaxation behaviour of disordered equi-atomic CoPt alloys has been investigated during annealing up to the Curie temperature. It is shown, using a sensitive ac technique, that the magnetic relaxation spectrum is a superposition of two effects: a rapid, thermally-activated, reorientational relaxation of CoPt atomic pairs leading to short-range order and a slower, migrational, structural relaxation leading to long-range order with a lowering of the Curie temperature.

Rotational and transverse dynamics of some random anisotropy magnets (invited)

After rapid survey of the behavior of random anisotropy magnets, the time and frequency evolution of the magnetization vector of a rotating disk of amorphous DyNi is focused on, as measured by a two-SQUID detectors setup. The results show that the dynamics of this system is completely determined by two macroscopic variables: the magnetizations M1 and M2, respectively associated with short and long relaxation times. The first one is fixed with respect to the applied field and therefore responsible for dissipation (magnetic friction) whereas the second one is fixed with respect to the sample (rotating anisotropy). Furthermore, transverse ac susceptibility experiments are used to confirm some of the results obtained. In particular low-temperature dynamical scaling (T→0) permits the existence of a new type of frequency crossover, when the volume samples by the ac field becomes of the order of Imry and Ma domains, to be shown.

Stability of magnets levitated above superconductors

The stability of a permanent magnet levitated above a slab of hard superconductor is considered. The force on a dipole magnet over a perfectly diamagnetic disk is calculated. It is found that the radial component of the force is directed outward and is 10%–20% of the image (vertical) force near the edge. Estimates of the magnetic friction force due to flux motion in a hard superconductor are made using Bean’s model. The magnitude of the magnetic friction is large enough to stabilize the magnet over most of the disk for typical values of the critical current in ceramic superconductors (∼103 A/cm2), but too small for the highest values reported (>106 A/cm2). It is conjectured that flux trapping due to inhomogeneities gives rise to transient restoring forces.

Magnetic energy density and friction factor in magnetic amorphous ribbons

The domain wall size (DWS) model can be extended to the case of magnetic amorphous ribbons by taking into account the geometry of magnetic domain walls and assuming a systematic pinning of domain walls to the external surface of the ribbon. We show that the friction factor of the DWS can be represented by ƒ = ƒ 0 + α∥E m∥ , where ƒ is the friction factor of the domain walls, ƒ 0 and α are constants, and E m is the magnetic energy density. This model can thus represent the initial magnetization curve, i.e. the existence of a critical magnetic field H cr which separates the reversible, low-field range from the hysteretic, high-field range, as well as the slope of the magnetization curve. The very high field range approaching saturation can also be represented by the model. Calculated values are compared with experimental results obtained on commercially available amorphous ribbons (Metglas type).

Gravitational stability of finitely conducting two-component plasma through porous medium

The gravitational stability of magnetized self-gravitating two-component plasma of finite conductivity flowing through porous medium is studied. Effect of magnetic field, porosity, viscosity, finite conductivity, and neutral gas friction is considered on the stability of the system. Dispersion relations are derived from linearized equations using normal mode analysis. Longitudinal and transverse wave propagations are discussed. On the basis of Hurwitz criterion, the stability of the system is discussed. It is found that Jeans's criterion determines the stability of the system. Jeans's expression depends on the sonic speeds in both the components. For transverse wave propagation in perfectly conducting plasma. Jeans's expression is modified due to magnetic field and porosity but in case of finitely conducting plasma the Jeans's expression remains unaltered. Collisional frequency, viscosity, permeability of the porous medium have damping effect.

The effects of cathodic charging on the magnetic internal friction in Armco iron

Samples of Armco iron were cathodically charged in a 1 N H 2SO 4 solution while the resonant frequency and internal friction (damping) were measured. The internal friction as a function of vibratory strain amplitude was measured before and after cathodic charging. Each of these measurements was made both in and out of a strong steady state longitudinal magnetic field. The magnetic damping was obtained by subtraction of the damping at saturation from the total damping. The change in magnetic internal friction was found to be entirely in the vibratory strain-amplitude-dependent region and is attributed to plastic deformation resulting from hydrogen-induced cracking. The mechanical hysteresis coefficient varies with the amount of time that the sample was charged, the current used in the charging and the vibratory strain amplitude used during charging.

Measurement of surface topography of magnetic tapes by Mirau interferometry

Stylus-profiling techniques cannot be used for surface characterization of polymeric surfaces, such as magnetic tapes, because of their relatively low hardness. An interferometric-optical-profiling microscope system was used to obtain high-accuracy surface profiles of magnetic media, rapidly and without physical contact with the sample. The profilometer consists of a conventional, reflection-type optical microscope with a Mirau two-beam interferometer attachment. The interference patterns of the surface can be observed through the eyepieces and can be detected with a solid-state linear array of 1024 detector elements. By translating the reference surface of the interferometer with a piezoelectric transducer while taking consecutive measurements, accurate surface-height measurements can be obtained from each detector element. The microscope system is controlled by a microcomputer, which communicates with a desk-top computer for further analysis of the surface-profile data. A computer-controlled specimen stage is added to increase the sample size. The reasons for selecting the Mirau two-beam interferometry are also discussed. Sample data of magnetic tapes are presented. Experimental data presented in the paper show that optimization of surface roughness is necessary to obtain optimum magnetic amplitude, friction, and wear properties.

Magnetic Pressure Drop and Heat Transfer of Sodium-Argon Two-Phase Flow in Annular Channel under Transverse Magnetic Field

Based on simple models, magnetic pressure drop and heat transfer coefficient correlations were derived for two-component two-phase liquid metal flow in an annular channel under magnetic field. Magnetic pressure drop and heat transfer experiments of Na-Ar two-phase flow in an annular channel under magnetic field were conducted. It became clear from the experimental results that the magnetic pressure drop of two-component two-phase liquid metal flow was equal to or smaller than that of single-phase liquid metal flow under conditions of the same liquid flow rate and magnetic field strength. The measured magnetic friction factor multiplier was smaller than predictions by the correlations derived in this study. It was also revealed from the experimental results that the heat transfer coefficient of two-component two-phase liquid metal flow was equal to or slightly smaller than that of single-phase liquid metal flow under conditions of the same liquid flow rate and magnetic field strength. The tendency that the...

Metal-metal friction phenomena between ferromagnetic materials under magnetic attraction

The authors investigate metal-metal friction phenomena between ferromagnetic materials under magnetic attraction. In this paper, experimental results of magnetic friction phenomena are compared with Coulomb's friction. It was found out that the frictional coefficient increases remarkably under magnetic attraction and it is possible to control its value by means of an electrical signal. Such results suggest the need for a new theory describing frictional phenomena under magnetic attraction.

Interaction of fluid dynamics phenomena and generator efficiency in two-phase liquid-metal gas magnetohydrodynamic power generators

The two-phase-flow (liquid-metal and gas) magnetohydrodynamic (LMMHD) generator and its associated thermodynamic cycle offer a promising alternative to conventional means of electrical power generation. Several advantages can be gained from its application. The LMMHD cycle can be applied to a very wide range of operating temperatures (heat sources) either alone or as a topping or bottoming part of a binary cycle. The potential is high for extreme design simplicity and reliability, because the need for high-speed rotating machinery, as used in turbine power plants, is eliminated. The electrical conductivities, proportional to the power densities, are higher by three or four orders of magnitude than in plasma MHD generators; thus, generator efficiency is potentially higher. Also, because of the intimate contact between liquid-metal and gas in the generator, the expansion process is nearly isothermal (an ‘infinite reheat’ turbine). Therefore, with proper heat regeneration, the cycle is a quasi-Ericsson cycle. The choice of working fluids may be either an ideal gas (e.g. helium, argon) or a condensible fluid (e.g. steam, CO 2). This paper relates the potential loss mechanisms in the LMMHD generator to various fluid dynamic phenomena that are discussed. Among these are effects resulting from velocity slip, vorticity generation and suppression by the magnetic field, wall friction, and shunt currents in liquid boundary layers. Experimental data relating to the internal flow phenomena and to overall generator performance are shown. The relative importance of these effects is discussed in relation to experimental evidence and known theory. It is concluded that the slip loss, resulting from a churn-turbulent flow pattern, is the major source of loss. A proposed means of creating a stable, homogeneous foam flow with virtually no slip, using the surface active properties of liquid metals, is discussed. Means are also suggested for reduction of wall shunt and frictional losses. The effects of end losses in two-phase flows are also described.

Plastic deformation of FeNi invar alloys

We report on the deformation behaviour of FeNi single crystals oriented for single slip in the temperature region between 77 and 610 K. Below the Curie temperature the critical shear stress of the Invar alloys increases much more rapidly than that of any f.c.c. substitutional alloy investigated till now. Simultaneously the activation volume decreases down to 10–20 b 3. Below the Curie temperature the stress-strain curves of the Invar crystals differ from those of other f.c.c. alloys. All results taken together indicate a dependence of the dislocation mobility on the magnetic properties of FeNi Invar. It is shown that the experiments cannot be understood with a simple magnetic friction stress arising from the interaction between the hydrostatic stress field of the edge dislocations and the stress dependence of the magnetization [3]. However, they can be described with an empirical friction law which had been introduced originally [18,19] for the case of strongly localized thermal activation, e.g. kink motion.