Showing papers in "IEEE Transactions on Magnetics in 1999"

Thermal effect limits in ultrahigh-density magnetic recording

Abstract: In current longitudinal magnetic recording media, high areal density and low noise are achieved by statistical averaging over several hundred weakly coupled ferromagnetic grains per bit cell. Continued scaling to smaller bit and grain sizes, however, may prompt spontaneous magnetization reversal processes when the stored energy per particle starts competing with thermal energy, thereby limiting the achievable areal density. Charap et al. have predicted this to occur at about 40 Gbits/in/sup 2/. This paper discusses thermal effects in the framework of basic Arrhenius-Neel statistical switching models. It is emphasized that magnetization decay is intimately related to high-speed-switching phenomena. Thickness-, temperature- and bit-density dependent recording experiments reveal the onset of thermal decay at "stability ratios" (K/sub u/V/K/sub B/T)/sub 0//spl sime/35 /spl plusmn/ 2. The stability requirement is grain size dispersion dependent and shifts to about 60 for projected 40 Gbits/in/sup 2/ conditions and ten-year storage times. Higher anisotropy and coercivity media with reduced grain sizes are logical extensions of the current technology until write field limitations are reached. Future advancements will rely on deviations from traditional scaling. Squarer bits may reduce destabilizing stray fields inside the bit transitions. Perpendicular recording may shift the onset of thermal effects to higher bit densities. Enhanced signal processing may allow signal retrieval with fewer grains per bit. Finally, single grain per bit recording may be envisioned in patterned media, with lithographically defined bits.

A general framework for the analysis and design of tubular linear permanent magnet machines

Abstract: A general framework for the analysis and design of a class of tubular linear permanent magnet machines is described. The open-circuit and armature reaction magnetic field distributions are established analytically in terms of a magnetic vector potential and cylindrical coordinate formulation, and the results are validated extensively by comparison with finite element analyses. The analytical field solutions allow the prediction of the thrust force, the winding emf, and the self- and mutual-winding inductances in closed forms. These facilitate the characterization of tubular machine topologies and provide a basis for comparative studies, design optimization, and machine dynamic modeling. Some practical issues, such as the effects of slotting and fringing, have also been accounted for and validated by measurements.

Ferromagnetic shape memory in the NiMnGa system

Abstract: Strain versus field measurements for a ferromagnetic shape memory alloy in the NiMnGa system demonstrate the largest magnetostrictive strains to date of nearly 1.3%. These strains are achieved in the martensitic state through field-induced variant rearrangement. An experimental apparatus is described that provides biaxial magnetic fields and uniaxial compressive prestress with temperature control while recording microstructural changes with optical microscopy. The magnetostrictive response is found to be sensitive to the initial state induced by stress-biasing the martensitic variant structure, and exhibits rate effects related to twin boundary mobility. Experiments performed with constant stress demonstrate work output capacity. Experimental results are interpreted by using a theory based on minimization of a micromagnetic energy functional that includes applied field, stress, and demagnetization energies. It is found that the theory provides a good qualitative description of material behavior, but significantly overpredicts the amount of strain produced. Issues concerning the martensitic magnetic anisotropy and variant nucleation are discussed with regard to this discrepancy.

Piezoelectric microactuator for dual stage control

Abstract: This paper presents a piezoelectric-based microactuator concept that could provide an economical and easily integrated option for dual stage actuation used in rigid disk drives (RDDs). The design and operation of a prototype version is described. Manufacturability of the actuator and its integration in the RDD assembly is assessed. Approaches to providing the voltages required to drive the microactuator to achieve displacement specifications are shown. A model for the dynamic performance is shown, and experimental results are presented. Issues involving dual-input single-output control loop integration are discussed.

Dual-stage actuator system for magnetic disk drives using a shear mode piezoelectric microactuator

Abstract: We developed a novel piezoelectric microactuator for dual-stage actuator systems in magnetic disk drives. This microactuator is based on the shear deformation of piezoelectric elements, and drives the head suspension assembly. The actuator is suitable for thin devices, and is easily manufactured because of its simple stack configuration. We installed the microactuator in a 2.5" prototype drive, and evaluated the servo system of the dual-stage actuator. This paper describes the structure of our piezoelectric actuator, its mechanical characteristics, and the evaluation of the dual-stage actuator servo system.

Analytical calculation of the switched reluctance motor's unaligned inductance

Abstract: This paper develops an analytical model for the unaligned inductance of the switched reluctance motor. The analytical model is useful as long as the machine's stator and rotor poles do not overlap and its iron does not saturate. Iron saturation is almost never experienced when the rotor is in the unaligned position for practical winding currents. This paper expands on results presented previously to include the contribution to the unaligned inductance of the flux in both the stator slot and the rotor slot. The approximations that are inherent to the analytical model are stated, and a detailed development of the model is provided. Finally, the paper compares predictions of the unaligned inductance computed by the analytical model to those obtained by finite-element analysis for three different machines with different pole counts and rotor diameters.

Large air-gap coupler for inductive charger [for electric vehicles]

Abstract: A novel magnetic coupler with large air gap is presented. It is developed for the electric vehicle's automatic inductive charger. The new inductive coupler proposed has sufficient exciting inductance even if it has a large air gap. The calculated exciting inductance is 40 /spl mu/H at 2 turns of winding and 5 mm air gap, which agrees well with the measured value. In order to assess the generation of heat caused by the eddy current, the magnetic flux densities in the inductive charger and also a flat iron plate, to which the inductive charger is attached, are calculated. The conversion efficiency with the coupler and a MOSFET full-bridge inverter of 100 kHz is 97% at 8.3 kW output.

Ion beam deposition of Mn-Ir spin valves

Abstract: Half-biased spin valves have been prepared by ion beam deposition. The magnetoresistance (MR) signal reaches 7.7% and the exchange field is 350 Oe with a coupling field of 15 Oe and a coercivity of the free layer equal to 4 Oe. The [111] texture induced by a very thin Ta buffer layer (thickness <10 /spl Aring/) has a strong effect in increasing the MR signal and coupling field, while decreasing the exchange field and coercivity. The blocking temperature of the MnIr-biased spin valves is 250/spl deg/C and a thermal stability study shows that the exchange field is constant up to 300/spl deg/C, under consecutive 5-h anneals at each temperature. After these anneals, the MR signal is still equal to 5%. These films show better thermal stability than equivalent samples prepared by sputtering.

High-frequency small-signal model of ferrite core inductors

Abstract: A circuit model of ferrite core inductors is presented. The behavior of the model parameters versus frequency is considered. The total power loss in inductors consisting of the winding resistance loss and the core loss, is modeled by a frequency-dependent equivalent series resistance. The total inductance given by the sum of the main inductance and the leakage inductance is obtained as a function of frequency. In order to study the core equivalent resistance and main inductance versus frequency, the magnetic field distribution in the core is derived from Maxwell's equations for a long solenoid. The complex permeability and permittivity of the ferrite core are introduced in the electromagnetic field equations. Experimental results are also given.

Piezoelectric piggy-back microactuator for hard disk drive

Abstract: A new piezoelectric piggy-back microactuator for high density hard disk drives was designed and fabricated to achieve more accurate positioning of the magnetic head and increasing servo bandwidth. The microactuator is made of stiff piezoelectric ceramic material to actuate the slider on which the head-element is formed. It is designed to amplify the actuating displacement. The basic mechanical performance of the microactuator was evaluated. The stroke of displacement was /spl plusmn/0.5 /spl mu/m with a drive voltage of /spl plusmn/lO V and a dc bias of +10 V. The resonant frequency of the microactuator was high at 34 kHz. The fundamental resonant frequency of an assembled head with microactuator was high at around 20 kHz. The head variation was suppressed to O.1 /spl mu/m by the microactuator with feedback control. These results suggest the feasibility of increasing servo bandwidth up to several kHz and positioning the head more precisely.

Precision positioning using a microfabricated electrostatic actuator

Abstract: This paper presents a microfabricated actuator designed for high precision servo-positioning in a magnetic hard disk drive. The device is actuated using electrostatic force generated with parallel-plate capacitive electrodes. The displacement of these electrodes is measured using a dedicated capacitive sensing interface, allowing closed-loop control to be used to extend the servo bandwidth. Using the sensing electronics and a simple phase-lead compensator, a prototype device was used to actuate a 1.6 mg ceramic slider over a 1.2 kHz bandwidth. Using optical position measurements, the same actuator was used to achieve a 2.5 kHz bandwidth.

Spin valve and dual spin valve heads with synthetic antiferromagnets

Abstract: Analytical and micromagnetic analysis have been performed for synthetic antiferromagnet (SAF) biased single and dual spin valve heads. It is found that for a un-pinned SAF under an external field, the antiparallel axis will flop to the direction orthogonal to the field direction. Introducing a thickness differential in the SAF can help to prevent the magnetization flop in addition to the exchange pinning. Design principles for SAF biased single and dual spin valve heads are presented. An SAF biased dual spin valve head with appropriate design can have significant advantages over the single spin valve besides higher GMR values.

Micromachined magnetic actuators using electroplated Permalloy

Abstract: Results on design, fabrication, and testing of silicon micromachined magnetic actuators are presented. Electroplated, low-stress Permalloy (Ni/sub 80/Fe/sub 20/) material is the medium for magnetic interaction and force generation. The Permalloy piece is supported by a structural plate, which consists of polycrystalline silicon thin film prepared by low-pressure chemical vapor deposition (LPCVD). Magnetic actuators supported both by cantilever beams and by torsional beams can provide large force (on the order of 100 /spl mu/N) and large displacement (on the order of 100 /spl mu/m). The vertical loading force of magnetic actuators under external bias has been experimentally determined. Applications of such actuators in magnetically assisted levitation and parallel assembly of three-dimensional structures are demonstrated.

Measurement and interpretation of magnetic time effects in recording media

Abstract: Magnetic time effects are highly relevant to magnetic information storage because of the large difference (up to 16 or 17 orders of magnitude) between the time scales of the recording process and the required storage stability. Magnetic time effects become more pronounced as the volume of the switching unit becomes smaller, and thus become of more practical importance as the microstructure of recording media is made finer in the pursuit of greater information storage density. Magnetic time effects taking place on time scales longer than about 10/sup -9/ s can be explained by a model of thermally assisted crossing of an energy barrier (Arrhenius-Neel formalism). Many recording phenomena can be explained within this regime. Observation of magnetization changes during exposure to a constant field ("magnetic viscosity") can be interpreted to yield an estimate of the switching volume; in most cases, this volume is larger than the frequently cited "activation volume". The dependence of coercivity on the time scale of the magnetic reversal precess (e.g., on the field sweep rate of a hysteresis loop) can also be used to deduce the volume of the switching unit and to estimate time effects relevant to information storage. A model and procedure for analysis of time-scale dependence of coercivity are described here and applied to typical advanced tape media, of both metal particulate (MP) and metal-evaporated (ME) composition. The resulting switching volume for the MP tape (where the microstructure is well defined) is in approximate agreement with the particle size seen by electron microscopy. The magnetic time effects are significantly stronger in the ME tape than in the MP tape. Measurement of coercivity ion at least two very different time scales (e.g., by vibrating-sample magnetometer and 60-Hz hysteresis loops) provides a convenient means of estimating the effective switching-unit volume, and hence the magnitude of the time effects to be expected in a recording application. The model also allows the estimation of the minimum particle or grain sizes that could be used with adequate stability of magnetic transitions in high-density information storage. The practical lower limit for metal-particle volume is found to be about one-fifth to one-fourth of the volume of current advanced particles.

Modeling of electromagnetic phenomena in soft magnetic materials under unidirectional time periodic flux excitations

Abstract: We report on recent advances in the modeling of magnetic losses in steel laminations used in electromagnetic devices. The integrated-lamination moving dynamic Preisach model, used to evaluate the dynamic magnetization loops under distorted unidirectional flux patterns, is described. The main goal is the comparison of two numerical procedures, using the finite element-finite difference technique and the finite element-fixed point technique, respectively, each properly taking into account the hysteresis characteristics by the Preisach theory. Moreover, attention is paid to the identification of the material parameters entering the moving dynamic Preisach model. Finally, the two techniques are validated by the comparison of numerical experiments and measurements on two different materials. Here, global as well as local quantities in the lamination structure are evaluated.

Batch-fabricated microinductors with electroplated magnetically anisotropic and laminated alloy cores

Abstract: Although many integrated inductors have been made by integrated circuit and electronic packaging batch-fabrication techniques, their magnetic characteristics are inferior to their discrete counterparts, in part because of the relatively poor magnetic properties of integrated magnetic cores. If the permeability of an integrated magnetic core can be increased, the magnetic characteristics of microinductors based on these cores will improve. To address this issue, batch-fabricated, integrated magnetic devices incorporating electroplated magnetically anisotropic cores and electroplated copper coils are investigated. These devices are made by micromachining and electroplating techniques at low temperature. Three different geometries of inductors, each possessing two different core materials [permalloy (NiFe) and supermalloy (NiFeMo)] are presented. The cores have been rendered magnetically anisotropic by application of a magnetic field during electrodeposition, resulting in easy and hard axis orientations. In addition, some cores consist of a two-layer electrodeposit separated by a polyimide thin-film lamination. At low frequencies (less than several hundred kHz), the easy-axis devices have higher inductance than the hard-axis devices. However, the hard-axis devices have better performance at higher frequencies because of a far less steep falloff of material permeability as a function of frequency in the hard axis direction.

Major HDD TMR sources and projected scaling with TPI

Abstract: This paper identifies the major sources of ontrack, self-induced, non-synchronous TMR (track mis-registration) in a HDD (hard-disk drive) with a track-density of 4400 TPI (tracks per inch) and a rotational rate of 90 Hz Experimental measurements of the drive's PES (position-error signal) in ontrack mode are used to determine the spectrum of the perceived TMR, which is then broken down into component parts The total RMS nonsynchronous perceived TMR of 056% of a track is found to be composed of 017%, due to torque disturbances, 033%, due to disk motions, 042% due to PES demodulation noise, with 004% unaccounted for (the total being the root-sum-of-squares of the components) With the assumption of a doubling of the PES noise, a factor of two reduction in the disk vibrations, a quadrupling in the servo sample-rate, and a doubling of the loop bandwidth, the authors show that an RMS ontrack, nonsynchronous, self-induced TMR of 167% of a track is possible with a track-density of roughly 72000 TPI

Technology assessment for the implementation of magnetoresistive elements with semiconductor components in magnetic random access memory (MRAM) architectures

Abstract: We describe the DRAM-like approach towards a non-volatile magnetoresistive memory integrating magnetic and semiconductor devices into one cell. The speed at which the magnetic memory signal can be read depends on many factors. An important factor is the magnetic element itself, the size, magnetic characteristics and absolute resistance. Secondly, the design of the read-out electronics is a key issue. A third determining factor is the technology in which the electronics are fabricated. Some features are indicated that are essential in optimizing MRAM in future.

AC impedance and circular permeability of slab and cylinder

Abstract: The spectra of internal ac impedance and fluxmetric and energetically effective circular permeabilities are calculated for infinite slab and cylinder in classical and domain models. The domain models assume transverse bar domains for a slab and transverse circular disk domains for a cylinder, with technical magnetization being carried out by domain wall displacements. For each case, a general formula is given together with the low and high-frequency limits. Numerical results are computed accurately from analytical formulas. Quantitative comparisons among different cases are made. Physical concepts and problems of practical significance are discussed on the basis of the results.

Electrostatic microactuator and design considerations for HDD applications

Abstract: A flexure-based rotary electrostatic microactuator is described. This microactuator is for application in a high track-density hard disk drive (HDD). It is fabricated using a high aspect-ratio stencil electroplating process. This paper emphasizes the practical system-level issues that affect the device design. The micro-actuator is demonstrated in a 3.5" form-factor HDD. With a servo bandwidth of 5 kHz, a one-track seek can be completed in under 0.2 ms. This batch-fabricated electro-static microactuator provides a low-cost, high-performance solution for achieving very high track-densities.

Nonlinear ferromagnetic resonance and foldover in yttrium iron garnet thin films-inadequacy of the classical model

Abstract: A thin-film resonator structure has been used for quantitative measurements of ferromagnetic resonance foldover and the associated bistable power response for yttrium iron garnet (YIG) thin films. The resonator consisted of a 1-mm by 1-mm-square, 4.9 /spl mu/m-thick epitaxial YIG film on top of a 50 /spl mu/m-wide, 3-mm-long microstrip transducer. A static magnetic field of 3200 Oe was applied perpendicular to the film. Low- order magnetostatic forward volume wave standing modes were excited at low power levels in the -20-dBm range and detected as resonance dips in reflected power versus frequency spectra over the range 4-5 GHz. At powers in the 0- to +15-dBm range, these dips showed foldover and bistable response characteristics for increasing and decreasing frequency or power sweeps. The use of 1-10-/spl mu/s-wide pulses instead of continuous-wave (CW) excitation resulted in the consistent disappearance of the foldover and bistability characteristics. The frequency sweep pulse data at fixed power reproduced the down-sweep CW results, and the pulse data for both increasing and decreasing power at fixed frequency reproduced the increasing-power CW results. A quantitative theoretical analysis demonstrates that observed foldover and bistable response characteristics are much weaker than predicted from the classical precession foldover mechanism proposed by Anderson and Suhl, in which the decrease in the static component of the magnetization drives the response. The up-sweep and down-sweep foldover frequency jumps both occur sooner than predicted by this classical mechanism and the calculated foldover profiles are much more severe than the data show.

Experiments to measure wear in aluminum armatures [in railguns]

Abstract: Although wear in sliding electric contacts has been studied extensively at low velocities (100 m/s or less), the phenomenon constitutes uncharted territory at conditions of interest in railguns. A fundamental understanding of wear in railguns is important because loss of material from the armature is one cause of transition from solid to arcing contact, and the resulting deleterious effects on performance. Sixteen experiments have been conducted at the IAT this year to address two fundamental questions related to wear: (1) what is the relative importance of electrical vs. mechanical heat generation at the rail/armature interface?; and (2) what are the governing parameters that affect material loss from the armature? This paper describes the experimental techniques used to make wear measurements and discusses results from the experiments.

Performance of metallic antiferromagnets for use in spin-valve read sensors

Abstract: A summary of the electrical, magnetic and corrosion properties of Mn-based metallic antiferromagnets for use in spin-valve devices is presented. Two families are considered. One is the family of FeMn, IrMn and others having a fcc structure. They are in the antiferromagnetic phase in the as-deposited state. The other is the family of NiMn, PtMn and others which do require a high temperature anneal and large thickness in order to undergo a phase transformation into an antiferromagnetic fct lattice. Alloying Mn with precious metals such as Pt or Pd leads to the best corrosion properties, superior even to permalloy.

Effect of rail/armature geometry on current density distribution and inductance gradient

Abstract: The distribution of current in the conductors which is affected by the geometry of the armature and the velocity of the armature plays an important role in the performance of an electromagnetic launcher. In the early launching stage the current tends to flow on the outer surfaces of the conductors, resulting in high local current densities. Later in the launch, the tendency for current to concentrate on the surface is driven by the velocity skin effect. High current densities produce high local heating and, consequently, increased armature wear. This paper investigates the effects of rail/armature geometry on current density distribution and launcher inductance gradient (L'). Three geometrical parameters are used to characterize the railgun systems. These are the ratio of contact length to root length, relative position of contact leading edge to root trailing edge, and the ratio of rail overhang to the rail height. The distribution of current density and L' for various configurations are compared.

Optimal design of cylindrical air-gap synchronous permanent magnet couplings

Abstract: Different types of structures for ironless synchronous cylindrical air-gap permanent magnet couplings are studied. In these structures, radially and tangentially oriented magnets are placed in ironless cores. The structures present a very small inertia and offer important benefits in many cases. An efficient method to calculate the torque of these types of couplings is presented. It is based on analytical formulas for the forces between parallelepipedical magnets. Optimal designs for each type of coupling and each number of pole pairs are determined by using a nonlinear optimization method. The design results are analyzed. The different types of structure are compared. The influence of the number of pairs of poles and of the main characteristic dimensions of each coupling is discussed. The results show that an ironless coupling structure where both radially and tangentially oriented magnets are used is a very advantageous solution.

Improvements of magnetic properties of Sr ferrite magnets by substitutions of La and Co

Abstract: The effect of the simultaneous partial substitution of Co/sup 2+/ for Fe/sup 3+/ and of La/sup 3+/ for Sr/sup 2+/ ion in Sr ferrite on the magnetic properties of anisotropic Sr ferrite magnets was investigated. It was found that the coercive force of Sr ferrite magnets is increased without significant decrease in residual flux density by La-Co substitution. Temperature coefficients of coercive force were found to be also improved by La-Co substitution.

Analysis of the combustion driven linear generator for electric gun applications

Abstract: Based on the studies of the combustion driven linear generator for the electric gun application, this paper presents a model that can describe the interdependence of the pressure in the combustion chamber and the output current. A computer code is finished and then the results of calculations and analysis are given, which provide guidelines to the design of generators.

Performance enhancement of multi-rate controller for hard disk drives

Abstract: This paper describes a new multi-rate controller for hard disk drives, which has a modified state estimator to correct the estimated states not only at measurement instances of the position error signal but also at every control instance when updating the control input. This modification makes the control input smoother compared to the conventional multi-rate scheme. This estimator is obtained as a stochastic optimal estimator for a linear dynamic system with a 'random walk' input noise process. The effectiveness of the modified scheme is demonstrated by experiments.

Magnetic recording on FePt and FePtB intermetallic compound media

Abstract: Near-contact recording on high-coercivity FePt intermetallic compound media using a high B/sub sat/ write element was investigated. Untextured FePt media were prepared by magnetron sputtering on ZrO/sub 2/ disks at a substrate temperature of 450/spl deg/C, with postannealing at 450/spl deg/C for 8 h. Both multilayer and cosputtered precursors produced the ordered tetragonal L1/sub 0/ phase with high coercivity between 5 and 12 kOe. To improve readback noise and decrease magnetic domain size, FePtB media were subsequently prepared by cosputtering. Overwrite, roll-off, signal-to-noise ratio (SNR), and nonlinear transition shift (NLTS) were measured by both metal-in-gap and merged magneto-resistive heads. FePtB media showed NLTS similar to that of commercial CoCrPtTa longitudinal media, but 5 dB lower SNR. By operating recording transducers in near contact, reasonable values of overwrite (>30 dB) could be obtained.

Analysis of flux leakage in a brushless permanent-magnet motor with embedded magnets

Abstract: This paper analyzes the magnetic characteristics of brushless permanent-magnet motors with embedded magnets. It shows that the flux leakage has a substantial effect on the air gap flux density interacting directly with the armature current to produce torque. The flux leakage parameters /spl eta/ and /spl lambda/ are expressed analytically in terms of the magnetic material properties and the motor dimensions. They are essential quantities for the accurate prediction of the average flux densities within the air gap and the magnet. The finite element numerical method is adopted to verify the analytical model. For the numerical example in this paper, the average deviation between the finite element result and the analytical one is about 1-2%. This finding confirms the suitability of the analytical model for the design purpose. Although design is not particularly emphasized in this paper, one section gives a simple explanation of flux leakage considerations in magnet design.