Stability Of Permanent Magnet Research Articles

Effect of the passive damping plate on the vertical stability of permanent magnet electrodynamic suspension system

AbstractThe vertical stability of permanent magnet (PM) electrodynamic suspension (EDS) system with a passive damping plate is studied because of the well‐known under‐damped nature of the EDS suspension system. The passive damping plate is installed under the Halbach permanent magnet array. When the vehicle‐mounted Halbach array and passive damping plate move to cut the track conductive plate, the 2D eddy current force is derived by establishing equations of magnetic vector potentials and using the Maxwell stress tensor method. And a 2D finite‐element model (FEM) is built to validate the accuracy of the derived levitation force equation. Based on the derived levitation force equation, the vertical stability of the PM EDS system with a passive damping plate and the effects of the main parameters on the damping ratio are analysed.

Effect of intergrain exchange interaction on magnetic viscosity of nanocrystalline isotropic NdFeB magnets

The coefficient of magnetic viscosity, S, of NdFeB alloys with varied intergrain exchange interaction constant, KIEI, was measured. The observed dependences of fluctuation field, Hf, on the applied field, H, are strongly correlated with KIEI value. Cooperative magnetization reversal of single domain interacting particles leads to improve the temperature stability of permanent magnets and magnetic systems based on them.

Magnetically Levitated Linear Drive with Repulsive Magnetic Guidance

Linear direct drives are often used when high performance is required, because of their high dynamic and their good position accuracy. Usually direct drives are used with linear rolling guidance. In clean room and vacuum applications linear rolling guidance cannot be used as particles can be cause problems. In this paper a magnetically levitated linear direct drive with a combination of repulsive permanent magnet stabilization and Lorentz force based stabilization is presented. With the joint use of magnet fields high dynamic can be achieved in combination with a cost efficient hardware. Due to the use of repulsive permanent magnet forces, it is possible to levitate an armature with nearly no power dissipation. An ad-hoc control reduces the power dissipation to a value less than 10 mW.

Measurement method of magnetic field for the wire suspended micro-pendulum accelerometer.

Force producer is one of the core components of a Wire Suspended Micro-Pendulum Accelerometer; and the stability of permanent magnet in the force producer determines the consistency of the acceleration sensor’s scale factor. For an assembled accelerometer; direct measurement of magnetic field strength is not a feasible option; as the magnetometer probe cannot be laid inside the micro-space of the sensor. This paper proposed an indirect measurement method of the remnant magnetization of Micro-Pendulum Accelerometer. The measurement is based on the working principle of the accelerometer; using the current output at several different scenarios to resolve the remnant magnetization of the permanent magnet. Iterative Least Squares algorithm was used for the adjustment of the data due to nonlinearity of this problem. The calculated remnant magnetization was 1.035 T. Compared to the true value; the error was less than 0.001 T. The proposed method provides an effective theoretical guidance for measuring the magnetic field of the Wire Suspended Micro-Pendulum Accelerometer; correcting the scale factor and temperature influence coefficients; etc.

Stabilization of magnet assemblies of permanent magnet sodium flowmeters used in fast breeder reactors

Permanent magnet flow meters (PMFMs) are used to measure the sodium flow in sodium cooled Fast Breeder Reactor Circuits. Prototype fast breeder reactor (PFBR) which is under construction at Kalpakkam is a 500MWe, sodium cooled, pool type reactor. Sodium flow measurement in various loops of the reactor is of prime importance from operational and safety point of view. To measure the flow of electrically conducting liquid sodium, in primary and secondary circuit pipe lines of PFBR, permanent magnet flow meters are used. PMFM is a non-invasive device, which works on the principle of generation of motional EMF by magnetic forces exerted on the charges in a moving conductor. Flowmeters of different pipe sizes ranging from 10mm to 200mm pipe diameter are required for PFBR. Long term performance of the flowmeters mainly depends on stability of permanent magnets used in flowmeters to generate constant magnetic field in stainless steel (SS) pipes. This paper describes the effects of time and temperature on permanent magnet assemblies made of ALNICO-V used in PFBR flowmeters. The stabilization methodology for ALNICO-V permanent magnet assemblies is evolved and established. Loss of magnetic field strength with respect to time and temperatures is determined by experiments and found negligible.

Pinholes and spin transfer effect in magnetic tunnel junction heads

Spin-transfer-induced microwave excitations in magnetic tunnel junction (MTJ) read heads have been investigated via spin-torque-modified micromagnetic modeling. This study focuses on the effect of possible pinholes in the barrier layer for a MTJ read head with permanent magnet stabilization. It is found that the critical current threshold for the spin-transfer-excited microwaves is significantly reduced if small pinholes exist in the tunnel barrier layer. The smaller effective conductive area, the smaller the current threshold. For a single small pinhole in the center of the barrier surface, the resulted spin-transfer-excited microwaves are chaotic in nature, yielding a significant 1∕f noise below 1GHz. The microwave excitation also strongly depends on the exact location of the pinhole(s). The study suggests that by studying the spin-transfer-excited microwaves, one can obtain a good knowledge about the possible existence of pinholes as well as their spatial distributions for the magnetic tunnel junction read heads.

Optimization of PM stabilization for balanced sensitivities and variations of spin valve heads

The control of variations of amplitude and symmetry of synthetic antiferromagnetic spin valve heads becomes increasingly difficult with the shrinking of head sizes. Micromagnetic Monte Carlo simulations were carried out to investigate the correlations between different head parameters and head performance variations. It was found that the head performance variation is strongly correlated with the permanent magnet stabilization strength. In addition, the magnetization orientation of the reference layer in the biasing state varies more from head to head than that of the free layer, when the sizes of the spin valve heads are in the deep-submicron range. The conflict between the head sensitivity and the head performance variation is clearly shown by the modeling results.

Effect of seedlayer and junction geometry on permanent magnet stabilization of magnetoresistive heads

The dependence of CoPt stabilization properties on surface topography and seedlayer thickness has been investigated. The coercivity degrades drastically without a Cr seed, showing a dependence upon surface angle. Using the resulting data wafer level sensor structures are micromagnetic simulated. Poor seedlayer coverage on the junction edge yields open transfer curve loops and degraded sensor response.

Microtrack base line instability of spin valve heads

The baseline shift in a micro-track profile of spin valve heads was investigated. Experimental and micro-magnetic modeling results were compared and found in close agreement. It was confirmed that the nonuniformity of pinning field and variation of anisotropy were among the potential sources of the formation of multi-states and the baseline instability. It was shown that the resulting instability could be suppressed by applying a strong permanent magnet stabilization field.

Novel approach to collimated physical vapor deposition

In some applications of physical vapor deposition (PVD), it is desirable to have the atoms arriving at the substrate at angles close to the normal (collimation). Applications of collimated PVD include continuous coverage of high-aspect ratio vias, as used in multilevel integrated circuit metallization, or high resolution shadow masking as used in lift-off technology. Several approaches to collimation have been proposed in recent years: natural (or long throw) collimation, physical collimation, ion beam deposition, ionized PVD cathodic arc deposition and hollow cathode collimation. In this article, we introduce a novel approach to collimation. According to simulations of target-to-substrate atom transport using the SIMBAD program this approach provides narrow angular distribution functions of the sputtered flux at the substrate surface. Furthermore, it is shown experimentally that this approach leads to a strongly improved degree of collimation when compared to the conventional methods based on either physical or natural collimation, through formation of abutted junction permanent magnet stabilization and metallization layers as in applied magnetoresistive thin film recording heads, using a lift-off process.

Performance of NiO spin-valve tape heads for high recording densities

Shielded NiO biased spin-valve tape heads with permanent magnet stabilization are described. Heads with 5 and 9 μm trackwidths, shield-to-shield spacing of 0.43 and 0.53 μm, and height of 1 μm are characterized on DLT–IV tape (0.2 μm magnetic thickness, Hc=1850 Oe, Mr.δ=6.5 memu/cm2) up to 150 kfci. Maximum head output is ∼1 mVpp/μm (3–5 mA sense current), decaying to a maximum of 23 μVpp/μm at 150 kfci (the 40–150 kfci resolution is 5%), with signal loss with linear density of 0.34 dB/kfci. Distortion level is ∼−33 dB for second and third harmonics at high linear density. Harmonic distortion above −20 dB is observed for linear density under 60 kfci due to partial head saturation. Head output is compared with a linear model. This model is used to predict head performance with smaller gaps (calculated resolution increases to 35% with 500 Å half-gap, but head efficiency decreases) and with thinner, smoother media (calculated output level at 150 kfci increases by 13 dB, and resolution increases to 14%).

Spin-valve read heads for tape applications

Shielded spin-valve tape heads with permanent magnet stabilization are described. Final MR of 3.5% is shown on heads with trackwidths of 5 and 9 /spl mu/m, and sensor heights of 0.5, 1.0 and 1.5 /spl mu/m. Heads are characterized on 1/2 DLT-IV tape up to 180 kfci. For isolated transitions the heads show a PW/sub 50/ of 0.8 /spl mu/m (pulse broadening due to partial head saturation) and a baseline shift /spl sim/10% that can be positive or negative, and is related with shield remanence and transfer curve hysteresis. At 120 kfci, head output for a sense current of 3 to 4 mA is /spl sim/1 mV/sub pp/ (up to 30% of the output at 30 kfci). At high linear densities, signal loss is 0.24 dB/kfci for a 0.35 /spl mu/m shield-to-shield spacing (D/sub 50/ of 80/spl sim/100 kfci). At low linear densities the head is partially saturated, and above 60 kfci harmonic distortion is lower than -20 dB.

Impact of microstructure on stability of permanent magnet biased magnetoresistive heads

This work pertains to output voltage stability of soft adjacent layer biased magnetoresistive heads with abutted permanent magnetic films for domain stabilization. A micromagnetic model has been developed to study the effect of the permanent magnet film microstructure and the micromagnetic properties of the abutted junction interface on head stability. The impact on head stability is analyzed with respect to interfacial ferromagnetic coupling across the junction interface and intergranular exchange coupling in the permanent magnet films among other film properties.

Permanent magnet stabilization of an overlaid MR head

Higher area density in the rigid disk magnetic recording industry is being achieved with magneto-resistive (MR) heads biased with a soft adjacent layer (SAL). The abutted and the overlaid designs are the two approaches most commonly used. Reader stability is a result of a single domain structure due to boundary control stabilization (BCS) from exchange or magnetostatic coupling. The details of a particular design determine on track and off track performance. Recent reviews and products suggest that the magnetostatically coupled abutted head design is superior to published overlaid designs. This paper describes the electrical performance and physical characteristics of another overlaid design that performs on track and off track at the recording density of 650 Mb/in/sup 2/.

Discussion on “The stability of permanent magnets”

Discussion on “The stability of permanent magnets”

The stability of permanent magnets

The paper describes tests made to compare the magnetic stability of representative martensitic and precipitation-hardening (isotropic and anisotropic) permanent-magnet materials. The weakening of magnets with time after magnetization, when left as free from disturbance as possible, was measured over a period of about three years, both on unstabilized magnets and on magnets artificially stabilized by weakening them 1% or 5% by applying demagnetizing fields. Anisotropic alloys were found to be much more stable than isotropic alloys, the stability of Columax being outstanding. Alnico, in spite of its much higher coercive force, was not more stable than the martensitic steels.The effect on stability of the working-point of the permanent-magnet material was also examined and found to be small above the (BH)max point on the demagnetization curve. Below the (BH)max point, however, the stability of both isotropic and anisotropic materials was considerably reduced.Further tests were made on the effects of heating at various temperatures up to 220° C and of mechanical stress and impact. Stability in high-temperature treatment is only roughly related to stability in time tests. The results of the mechanical tests were inconclusive as the magnet assemblies available, designed primarily for long-term tests, were not suitable for subjection to severe mechanical treatment.

Stability of Magnets Composed of Elongated Single-Domain Iron Particles

The magnetic and physical stability of permanent magnets compacted from elongated single domain iron particles in metallic and organic matrices under various environmental conditions are reported. Changes resulting from two years exposure to 95% relative cycling humidity and 100°C were measured. The influence of powder compacting temperatures, pressures, powder size, and additives are reported. Compacts with lead matrices exposed to 100°C and 95% relative humidity showed gradual magnetic and dimensional changes from insignificant variations after one month to maximums of 0.5% over a two-year period. These results compared favorably with results obtained for Alnico V magnets subjected to identical conditions. Temperatures of 200° and 250°C produced magnetic changes in the order of 1% after one month with physical degradation occurring after two months at 250°C. Magnetic changes of 1% and weight and volume changes in the order of 2% occurred in organic matrix magnets after one month at 100°C. The magnetic changes resulted from a slow oxidation of the iron particles which caused a small decrease of intrinsic saturation induction and an increase in coercive force.

The high temperature stability of permanent magnets of the iron-nickel-aluminium system

The open circuit magnetization of rectangular bars of various modern permanent magnet alloys has been measured at temperatures up to 550° C. A differential ballistic method has been used and, after magnetization at room temperature, both reversible and irreversible losses occur on heating. These losses vary with material and dimension ratio, and in an attempt to explain these variations demagnetization curves at 500° C have been measured. To avoid the irreversible losses, stabilization at a higher temperature is better than ageing in an alternating field.

Effect of low temperature on the stability of permanent magnets

Magnetometer measurements have been made of the change in magnetization with temperature between +60 and -60° C for magnets with a range of dimension ratios. The materials tested were Alcomax III, Columax, Alnico, 35 and 15% cobalt steels. With decreasing temperatures the magnetization increases reversibly except for the shorter dimension ratio magnets in Alcomax III and Columax, for which losses occur. The influence of tempering on these losses is discussed. Losses occur in the lower coercivity materials only with increasing temperature. The losses, whether due to increase or decrease of temperature, can be minimized by a partial demagnetization by an alternating field. The results are correlated with the changes in the demagnetization curves of the materials with temperature.

The performance and stability of permanent magnets

Very little data is available on the stability of permanent magnets. This paper collates information of practical or empirical nature useful to the user of permanent magnets. The desirable characteristics which materially contribute to the long-term magnetic stability of a permanent magnet alloy are given.The latest developments in permanent magnet alloys are discussed and an indication is given of present trends in production.