Showing papers in "IEEE Transactions on Magnetics in 2004"

A phenomenological theory of damping in ferromagnetic materials

Abstract: In 1955, a phenomenological theory of ferromagnetism was well established and had been corroborated by a considerable amount of experimental data. However, there were problems in the phenomenological theory of the dynamics of the magnetization field. The Landau-Lifshitz equation for damping of the motion of the magnetization field could not account for the large noneddy-current damping in thin Permalloy sheets. The problem undertaken herein is a reformulation of the theory in a way that is more consistent with the theory of damping in other physical systems in order to be able to take large damping into account.

A dynamic core loss model for soft ferromagnetic and power ferrite materials in transient finite element analysis

Abstract: A dynamic core loss model is proposed to estimate core loss in both soft ferromagnetic and power ferrite materials with arbitrary flux waveforms. The required parameters are the standard core loss coefficients that are either directly provided by manufacturers or extracted from the loss curve associated with sinusoidal excitation. The model is applied to calculating core loss in both two-dimensional and three-dimensional transient finite element analysis, and the results are compared with measured data.

70% TMR at room temperature for SDT sandwich junctions with CoFeB as free and reference Layers

Abstract: Spin dependent tunneling (SDT) wafers were deposited using dc magnetron sputtering. SDT junctions were patterned and connected with one layer of metal lines using photolithography techniques. These junctions have a typical stack structure of Si(100)-Si/sub 3/N/sub 4/-Ru-CoFeB-Al/sub 2/O/sub 3/-CoFeB-Ru-FeCo-CrMnPt with the antiferromagnet CrMnPt layers for pinning at the top. High-resolution transmission electron microscopy (HRTEM) reveals that the CoFeB has an amorphous structure and a smooth interface with the Al/sub 2/O/sub 3/ tunnel barrier. Although it is difficult to pin the amorphous CoFeB directly from the top, the use of a synthetic antiferromagnet (SAF) pinned layer structure allows sufficient rigidity of the reference CoFeB layer. The tunnel junctions were annealed at 250/spl deg/C for 1 h and tested for magneto-transport properties with tunnel magnetoresistive (TMR) values as high as 70.4% at room temperature, which is the highest value ever reported for such a sandwich structure. This TMR value translates to a spin polarization of 51% for CoFeB, which is likely to be higher at lower temperatures. These junctions also have a low coercivity (Hc) and a low parallel coupling field (Hcoupl). The combination of a high TMR, a low Hc, and a low Hcoupl is ideal for magnetic field sensor applications.

Predicting loss in magnetic steels under arbitrary induction waveform and with minor hysteresis loops

Abstract: We have studied ways of predicting power losses in soft magnetic laminations for generic time dependence of the periodic magnetic polarization J(t). We found that, whatever the frequency and the induction waveform, the loss behavior can be quantitatively assessed within the theoretical framework of the statistical loss model. The prediction requires a limited set of preemptive experimental data, depending on whether or not the arbitrary J(t) waveform is endowed with local slope inversions (i.e., minor hysteresis loops) in its periodic time behavior. In the absence of minor loops, such data reduce, for any peak polarization value J/sub p/, to the loss figures obtained under sinusoidal J(t) at two different frequency values. In the presence of minor loops of semiamplitude J/sub m/, the two-frequency loss experiment should be carried out for both peak polarization values J/sub p/ and J/sub m/. Additional knowledge of the quasi-static major loop, to be used for modeling hysteresis loss, does improve the accuracy of the prediction method. A more general approach to loss in soft magnetic laminations is obtained in this way, the only limitation apparently being the onset of skin effect at high frequencies.

Rotor Eddy-Current Loss in Permanent-Magnet Brushless AC Machines

Abstract: This paper analyzes rotor eddy-current loss in permanent-magnet brushless ac machines. It is shown that analytical or finite-element techniques published in literature for predicting rotor eddy-current loss using space harmonic based approaches may not yield correct results in each magnet segment when one magnet-pole is circumferentially segmented into more than two pieces. It is also shown that the eddy-current loss in each equally segmented piece may differ by a large margin, which implies that the temperature distribution in the magnets will be uneven and the risk of demagnetization has to be carefully assessed. The theoretical derivation is validated by time-stepped transient finite-element analysis.

Advantages of ferromagnetic nanoparticle composites in microwave absorbers

Abstract: Composites with nanosize ferromagnetic particles can be useful in microwave absorbers, since nanoparticles exhibit distinct magnetic properties compared to bulk materials. This paper examines theoretically the properties of composites with nanoparticles, the key material parameters, and the characteristics of a single-layer absorber made from a nanoparticle composite. In such an absorber, high magnetic losses over a wide frequency range induce a series of strong and wide absorption peaks at increasing frequencies. By using metallic (iron and cobalt) nanoparticles, absorbers with relatively low volume fraction of metallic inclusions can be made. The paper compares the characteristics of nanocomposite absorbers to those of common dielectric or ferromagnetic absorbers and identifies the potential advantages of nanocomposite absorbers. Because nanocomposite absorbers combine advantages of both dielectric and ferromagnetic absorbers and have significantly better characteristics, they could become essential for production of microwave and millimeter-wave absorbers.

Kriging: a useful tool for electromagnetic device optimization

Abstract: This paper deals with the use of the Kriging models for objective function approximations and their use on electromagnetic device optimization. The models are compared with some radial basis function neural networks and with the diffuse element method. Comparative tests are done on an analytical function and on the TEAM workshop problem 25.

Analytical magnetic field analysis of Halbach magnetized permanent-magnet machines

Abstract: We develop analytical models for predicting the magnetic field distribution in Halbach magnetized machines. They are formulated in polar coordinates and account for the relative recoil permeability of the magnets. They are applicable to both internal and external rotor permanent-magnet machines with either an iron-cored or air-cored stator and/or rotor. We compare predicted results with those obtained by finite-element analyses and measurements. We show that the air-gap flux density varies significantly with the pole number and that an optimal combination of the magnet thickness and the pole number exists for maximum air-gap flux density, while the back iron can enhance the air-gap field and electromagnetic torque when the radial thickness of the magnet is small.

Pareto optimality and particle swarm optimization

Abstract: Real-world optimization problems often require the minimization/maximization of more than one objective, which, in general, conflict with each other. These problems (multiobjective optimization problems, vector optimization problems) are usually treated by using weighted sums or other decision-making schemes. An alternative way is to look for the pareto-optimal front. In this paper, the particle swarm algorithm is modified to detect the pareto-optimal front.

Encapsulation of spherical iron-particle with PMMA and its magnetorheological particles

Abstract: Composite magnetic particles (CMP) with carbonyl iron (CI) core and poly(methyl methacrylate) shell were prepared by an in-situ dispersion polymerization method via surface treated CI with acrylic acid. These CMP were adopted as dispersed phase of magnetorheological (MR) fluids, and has better MR fluids characteristics than fluid with CI alone as they have severe sedimentation and poor dispersion quality. Flow properties of the MR fluids were analyzed via a rotational rheometer equipped with a magnetic field supplier in parallel plate geometry.

Classical model of extrinsic ferromagnetic resonance linewidth in ultrathin films

Abstract: This paper describes a classical version of the two-magnon model of ferromagnetic resonance linewidth in inhomogeneous magnetic thin films. The ferromagnetic resonance line broadening due to inhomogeneity is described in terms of film properties and the statistical properties of the inhomogeneity. Analytical results for the case of ultrathin films in the limit of zero damping are compared with numerical results computed with finite damping.

Design optimization of radially magnetized, iron-cored, tubular permanent-magnet machines and drive systems

Abstract: In this paper we deduce, from analytical field solutions, the influence of leading design parameters on the performance of a radially magnetized, iron-cored, tubular permanent-magnet machine and its drive system. We derive analytical formulas for predicting the open-circuit electromotive force, the thrust force, the iron loss, and the winding resistance and inductances, as well as the converter losses. The force density, the machine and drive system efficiencies, and the power factor and converter volt-ampere (VA) rating are established as functions of a set of machine dimensional ratios, with due account of magnetic saturation and subject to a specified thermal constraint. We validate the utility and accuracy of the analytically derived formulas by finite-element calculations. Finally, we show that the design optimization of such a linear drive system must account for the losses and VA rating of the converter as well as the design parameters of the tubular machine.

Optimization of permanent magnet shape for minimum cogging torque using a genetic algorithm

Abstract: The paper presents an approach to minimization of the cogging torque in permanent magnet (PM) machines using surface-mounted magnets with discrete skew angle. For the purpose of determining the proper arrangement of PM-pole slices, an optimization procedure based on a genetic algorithm is applied. The torque and objective function are determined from a simplified model for torque calculation only partially supported by three-dimensional (3-D) field solution. The results are validated against the 3-D finite-element model as well as experimental data obtained from a prototype machine. A new outer-rotor brushless dc motor motor for an electric fan is considered as a sample model.

Thermally assisted switching in exchange-biased storage layer magnetic tunnel junctions

Abstract: A thermally assisted writing procedure is proposed in a tunnel junction based magnetic random access memory cell. The magnetic layers of the tunnel junction are both exchange-biased with antiferromagnetic layers, the reference layer having a much higher blocking temperature than the storage layer. In the operating mode, a current pulse sent through the junction generates enough heat to raise the temperature of the storage layer above its blocking temperature, without affecting the pinning of the reference layer. The concept is demonstrated here for an isolated junction using an homogeneous external magnetic field.

Investigation of benchmark model for estimating iron loss in rotating machine

Abstract: In order to examine estimation methods of iron losses of rotating machines, a verification model of the surface permanent magnet (SPM) motor is manufactured and measured by the investigation committee of the IEE of Japan. The effect of stress due to the insertion to aluminum case on magnetic characteristics can also be investigated by using the model. In this paper, features of the benchmark model are discussed, and some results of calculation and measurement are shown. Various kinds of methods for estimating iron losses due to distorted elliptical rotating flux are discussed. It is illustrated that the difference between results estimated using those methods and measured iron loss is within 10%.

MR ratio enhancement by NOL current-confined-path structures in CPP spin valves

Abstract: We have compared the magnetoresistance (MR) performance of current-confined-path (CCP) current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) spin valve films with a nano-oxide-layer (NOL), made between natural oxidation (NO) and ion-assisted oxidation (IAO). For the NO, an MR ratio was only 1.5% at an RA of 370 m/spl Omega//spl mu/m/sup 2/, whereas for the IAO, an MR ratio was greatly increased to 5.4% at an RA of 500 m/spl Omega//spl mu/m/sup 2/. Fitted data by the Valet-Fert model showing larger MR enhancement effect by the IAO is explained by the improvement of the metal-purity of the Cu inside the CCP structure. By further improvement of metal-purity of the Cu, a large MR ratio of more than 30% can be expected at a small RA of 300 m/spl Omega//spl mu/m/sup 2/. The CCP-CPP spin valve film is a promising candidate for realizing high-density recording heads for 200 to 400-Gbpsi recording.

Off-diagonal impedance in amorphous wires and its application to linear magnetic sensors

Abstract: We investigated the magnetic-field behavior of the off-diagonal impedance in Co-based amorphous wires under sinusoidal (50 MHz) and pulsed (5 ns rise time) current excitations. For comparison, we measured the field characteristics of the diagonal impedance as well. In general, when an alternating current is applied to a magnetic wire, the voltage signal is generated not only across the wire but also in a pickup coil wound on it. These voltages are related to the diagonal and off-diagonal impedances, respectively. We demonstrate that these impedances have a different behavior as functions of axial magnetic field: the diagonal impedance is symmetrical, whereas the off-diagonal one is antisymmetrical with a near-linear portion within a certain field interval. For the off-diagonal response, the dc bias current is necessary to eliminate circular domains. In the case of the sinusoidal excitation without a dc bias current, the off-diagonal response is very small and irregular. In contrast, the pulsed excitation, combining both high- and low-frequency harmonics, produces the off-diagonal voltage response without additional biasing. This behavior is ideal for a practical sensor circuit design. We discuss the principles of operation of a linear magnetic sensor based on a complementary metal-oxide-semiconductor transistor circuit.

Modeling and Simulation of a salient-pole synchronous Generator with dynamic eccentricity using modified winding function theory

Abstract: This paper models and simulates a salient-pole synchronous generator using a modified winding function theory and more precise stator and rotor winding distribution with dynamic eccentricity between the stator and rotor. Air-gap permeance is also computed more accurately compared to currently available methods. Inductances with this method are compared to those obtained from other methods and it is shown that the results are closer to those obtained from finite element computations. Finally, the calculated inductances are used in a coupled electromagnetic model for simulation and studying the frequency spectrum of the stator line current in the presence of dynamic eccentricity.

Magnetic and structural properties of FePt-FeRh exchange spring films for thermally assisted magnetic recording media

Abstract: Recently a novel media structure for thermally assisted magnetic recording was proposed consisting of a layer of FePt exchange coupled to a FeRh layer. The FePt forms a high magnetocrystalline anisotropy, high coercivity ferromagnetic layer. The FeRh layer is antiferromagnetic at room temperature, but upon heating above a transition temperature becomes ferromagnetic with a large magnetic moment and low magnetocrystalline anisotropy. The coupled ferromagnetic FePt and FeRh layers form an exchange-spring system significantly lowering the coercive field of the composite system compared to a single layer of FePt. This feature opens intriguing possibilities for media applications for thermally assisted magnetic recording where the ferromagnetic phase of FeRh is exploited to help write the media while the low-temperature antiferromagnetic phase supports the long-term stability. Here temperature-dependent structural and magnetic measurements of undoped and doped FeRh single layer and FePt-FeRh bilayer films are presented and the promises and challenges of the exchange spring media structure are discussed.

Characterization of noise and vibration sources in interior permanent-magnet brushless DC motors

Abstract: This work characterizes electromagnetic excitation forces in interior permanent-magnet (IPM) brushless direct current (BLDC) motors and investigates their effects on noise and vibration. First, the electromagnetic excitations are classified into three sources: 1) so-called cogging torque, for which we propose an efficient technique of computation that takes into account saturation effects as a function of rotor position; 2) ripples of mutual and reluctance torque, for which we develop an equation to characterize the combination of space harmonics of inductances and flux linkages related to permanent magnets and time harmonics of current; and 3) fluctuation of attractive forces in the radial direction between the stator and rotor, for which we analyze contributions of electric currents as well as permanent magnets by the finite-element method. Then, the paper reports on an experimental investigation of influences of structural dynamic characteristics such as natural frequencies and mode shapes, as well as electromagnetic excitation forces, on noise and vibration in an IPM motor used in washing machines.

Inverse Jiles-Atherton vector hysteresis model

Abstract: In this paper, we present a new vector hysteresis model, which is derived from the original Jiles-Atherton scalar one. The model presents the magnetic vector induction as the independent variable. A modified Langevin equation is used to represent the anhysteretic vector magnetization, and some model aspects are discussed. A comparison between measured and calculated curves for an anisotropic soft magnetic material is performed to validate the model.

Modeling of solid conductors in two-dimensional transient finite-element analysis and its application to electric machines

Abstract: We present an approach for directly coupling transient magnetic fields and electric circuits. The circuit can contain arbitrarily connected solid conductors located in the magnetic field region. A systematic procedure suitable for both nodal method and loop method is used to couple fields and circuits. The structures of the system equations of the two methods are analogous. The formulations allow the equations in stranded windings and solid conductors to be unified and the coefficient matrix of the system equations to be symmetrical. In order to reduce the solution domain, the periodic boundary conditions are still applicable when the solid conductors are involved. Our approach has been applied to the simulation of electric machines. We give four examples: 1) calculation of the input phase current and output torque when a single-phase induction motor with shaded rings is in locked-rotor operation; 2) simulation of the sudden short circuit of a synchronous generator with starting cage; 3) study of the phase current waveform of an induction motor when the rotor bars are broken; and 4) investigation of the parasitic capacitive impact of the surge voltage on a winding due to drive switching and cable ring.

Model of laminated iron-core inductors for high frequencies

Abstract: We propose a model with frequency-dependent lumped parameters to represent the behavior of laminated iron-core inductors that are used in switching converters, i.e., for excitation frequencies above several kilohertz. The model applies to laminated iron cores with air gaps. We show that the core parameters can be calculated by using the same analytical expressions as those valid for gapless cores if a properly defined equivalent magnetic permeability is introduced instead of the iron sheet permeability. We compare the inductor model parameters calculated as functions of frequency with those obtained by measurements for two test inductors. The behavior of the inductors showed that the effects due to the eddy currents in the laminated iron core and the turn-to-turn and turn-to-core stray capacitances become significant in different frequency ranges.

New rotor shape design for minimum torque ripple of SRM using FEM

Abstract: A major problem of switched reluctance motors (SRMs) is torque ripple, which causes undesirable acoustic noise and vibration. It is caused by the saliency of the stator and rotor. In this paper, the geometry for low torque ripple is researched and a motor having notched teeth is proposed. Its characteristics are simulated by finite-element method (FEM) analysis and compared with SRMs having conventional shape.

Electromagnetic launch science and technology in the United States enters a new era

Abstract: Electromagnetic launch science and technology in the United States is on the threshold of a major new advance in its development. Significant developments in hypervelocity electromagnetic launch and in hypervelocity high-G guidance and control components have provided the impetus for exploring advanced electromagnetic weapon systems capable of providing revolutionary new capabilities. These systems include the electromagnetic launch of aircraft from navy aircraft carriers, hypervelocity electromagnetic guns on ships to provide protection and to launch guided projectiles to extraordinary ranges, and lightweight ground combat vehicles propelled by hybrid electric power protected by electromagnetic armor and capable of defeating advanced armor targets with lightweight hypervelocity penetrators from electromagnetic railguns. This presentation will summarize many of the technical advances leading to the decision to implement these new engineering efforts.

Real coded genetic algorithm for Jiles-Atherton model parameters identification

Abstract: The parameters set of the Jiles-Atherton hysteresis model is identified by using a real coded genetic algorithm. The parameters identification is performed by minimizing the mean squared error between experimental and simulated magnetic field curves. The procedure is validated by comparing experimental and simulated results.

Numerical modeling of magnetic devices

Abstract: We present a general approach to directly couple finite-element models with arbitrary electric circuits for application to electromagnetic devices. We describe both two-dimensional (2-D) and three-dimensional (3-D) transient finite-element models, with emphasis on 3-D using a T-/spl Omega/ formulation. For 3-D transient and circuit coupling, the derivation of the induced voltage is an integral part of the coupling approach, and the induced voltage links the magnetic field and the electrical circuit together. The system of electric circuits is created automatically. Then graph theory is used to deduce the circuit by tree/cotree and loop analysis. The resulting field equations and circuit equations are coupled together and solved simultaneously at each time step in the time domain. We give three examples of applications: a brushless dc motor drive, a permanent-magnet synchronous motor drive, and a three-phase power transformer with rectifier and load circuit.

Demonstration of thermomechanical recording at 641 Gbit/in/sup 2/

Abstract: Ultrahigh storage areal densities can be achieved by using thermomechanical local-probe techniques to write, read back, and erase data in the form of nanometer-scale indentations in thin polymer films. This paper presents single-probe experimental results in which large data sets were recorded at 641 Gbit/in/sup 2/ and read back with raw bit-error rates better than 10/sup -4/. (d,k) modulation coding is used to mitigate the effect of partial erasing, occurring when subsequent indentations are spaced too closely together, and to increase the effective areal density. The physical indentation profile, the sensitivity of the probe in readback mode, and noise sources that affect data detection are also discussed. Quantitative measurements of the partial erasing effect in both the on-track and cross-track directions are reported.

Design characteristics of the induction motor used for hybrid electric vehicle

Abstract: Design characteristics of the induction motor used for hybrid electric vehicle (HEV) are discussed. Equivalent circuits corresponding to the starting, operating performance, and harmonics are given. The selecting rules of the starting voltage and frequency are got. The relations between the iron core and copper loss versus frequency are analyzed and compared. The influence of harmonics on the induction motor is analyzed. The design principles of the induction motor used for HEV are given considering the impacts of the starting, operating performance, and harmonics.

Spin transfer induced noise in CPP read heads

Abstract: In this paper, spin transfer induced magnetic noise in current-perpendicular-to-plane (CPP) spin valve heads is investigated via micromagnetic modeling with spin transfer torque included. It is found that there exists of a critical sense current density beyond which the spin transfer effect yields a substantial noise with a pronounced (1/f/sup n/) spectral content at low frequencies. This magnetic noise arises from the essentially chaotic magnetization precessions excited by the spin transfer effect that can effectively acts as energy pumping to the spin system. The onset of the noise occurs when the pumping rate becomes equal to the rate of energy dissipation. The critical current density is found to be proportional to the damping constant /spl alpha/ as well as the longitudinal bias field.