IEEE Transactions on Magnetics (IEEE T MAGN)

Publications in this journal

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  Available from: nctu.edu.tw

  Article: H∞ repetitive control for pickup head flying height in near-field optical disk drives

  J W Chen, T S Liu
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  ABSTRACT: The slider of a flying pickup head in a near-field op-tical disk drive has to fly at a stable height above the disk surface. An air bearing force between the slider and optical disk is affected by disk rotation speed and deformation, which represent low-fre-quency periodic disturbance to the slider. The repetitive control is adopted in this study to eliminate the periodic disturbance. H control inherited with robust properties can deal with model un-certainty and disturbances. Therefore, to solve model uncertainty and disturbance, this study designs a repetitive controller cascaded by an H controller. The proposed controller is validated by ex-perimental results.
  IEEE Transactions on Magnetics ; 41.
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  Available from: nctu.edu.tw

  Article: Frequency-shaped sliding mode control for flying height of pickup head in near-field optical disk drives

  W C Wu, T S Liu
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  ABSTRACT: To achieve stable flying height of a pickup head in near-field optical disk drives, this study investigates a discrete sliding mode control with a frequency-shaped sliding surface. A compensator is introduced in a sliding mode using a fre-quency-shaped performance index with linear quadratic optimal and sliding mode control. As an actuator, a piezoelectric bender is used in this study to complement an air bearing at the head/disk interface. Experiments are carried out to demonstrate that the proposed controller performs better in flying height control than an optimal sliding mode controller. Accordingly, the controller can achieve stable flying height control in the presence of certain frequency vibration of optical disks.
  IEEE Transactions on Magnetics ; 41.
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  Available from: tohoku.ac.jp

  Article: Analysis of orthogonal-core type linear variable inductor and application to VAr compensator

  Kenji Nakamura, Osamu Ichinokura, Mitsuru Maeda, Shigeaki Akatsuka, Kazuo Takasugi, Hiromichi Sato
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  ABSTRACT: This paper presents a quantitative analysis of the orthogonal-core type linear variable inductor. The orthogonal-core has wedge gaps for reduction of harmonics of the output currents. The analysis is based on a 3-dimensional nonlinear magnetic circuit of the orthogonal-core. Using the magnetic circuit, we can calculate accurately the operating characteristics of the variable inductor. Furthermore, we develop a var compensator for 6.6 kV ac distribution system using the three-phase variable inductor. Index Terms—Linear variable inductor, orthogonal-core, reluc-tance network analysis, var compensator.
  IEEE Transactions on Magnetics ; 36.
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  Available from: psu.edu

  Article: Pulsed-current-induced domain wall propagation in permalloy patterns observed using magnetic force microscope

  L Gan, S H Chung, K H Aschenbach, M Dreyer, R D Gomez
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  ABSTRACT: Pulsed-current controlled wall motion in 20 m wide 200 m long 160 nm thick patterned Permalloy strips was studied using magnetic force microscopy. By sequential imaging, the displacement of Bloch walls as far as 200 m along the strip was observed. The direction of motion was in the same direction as the carrier velocity, which reversed with current polarity. The displacement per pulse was dependent upon the sample thickness and current density, which suggests that the mechanism is a combination of s-d exchange and hydromagnetic domain drag forces.
  IEEE Transactions on Magnetics ; 36.
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  Available from: uts.edu.au

  Article: Z-transform-based FDTD analysis of perfectly conducting cylinder covered with unmagnetized plasma

  Ming Yan, K R Shao, X W Hu, Youguang Guo, Jianguo Zhu, J D Lavers
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  ABSTRACT: In this paper, a novel and normalized Z-transform-based finite-difference time-domain (ZTFDTD) method is presented for simulating the interaction of the electromagnetic (EM) wave with unmagnetized plasma. The 2-D ZTFDTD formulations for unmagnetized plasma are derived. Using a simplified 2-D model for a perfectly conducting cylinder covered with unmagnetized plasma, the stealth effect of unmagnetized plasma is studied in different thicknesses of plasma, electron densities of plasma, EM wave frequencies, and plasma colli-sion frequencies. Numerical results indicate that plasma stealth is effective in theory and reasonable selection for the plasma parameters can greatly enhance its effectiveness. Index Terms—Finite-difference time-domain (FDTD), plasma stealth, radar cross section (RCS), Z-transform.
  IEEE Transactions on Magnetics 01/2968; 43.
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  Available from: unl.edu

  Article: Magnetization reversal and giant coercivity in Sm-Co/Cu-Ti particulate films

  Jian Zhou, Arti Kashyap, Yi Liu, Ralph Skomski, David J Sellmyer
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  ABSTRACT: Highly coercive granular SmCo/CuTi films were produced by thermal processing of sputtered SmCo 5 (CuTi) multilayers and investigated experimentally and theoretically. Electron microscopy shows that the processed films consist of spherical grains that have diameters of 5–10 nm and are embedded in a matrix. Both the grains and the matrix phase exhibit the CaCu 5 structure, but the matrix is probably Cu-rich. The films have high coercivities of up to 50.4 kOe at 300 K. Analytical calculations and micromagnetic simulations yield a transition from a nucleation-type weak-interaction regime to a discrete-pinning-type strong-interaction regime. The transition occurs at packing fractions similar to those encountered in the investigated films and is accompanied by a coercivity maximum. Index Terms—Coercivity, discrete domain-wall pinning, gran-ular films, micromagnetic simulations, samarium–cobalt films.
  IEEE Transactions on Magnetics 01/2940; 40.
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  Available from: mst.edu

  Article: Study of structural and magnetic properties of iron-rich mixed rare-Earth NdDyFe (17-yx) CoxSiy compounds

  K Kamaraju, J B Yang, W B Yelon, O A Pringle, M Kim, W J James, Q Cai
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  ABSTRACT: A series of NdDyFe 17 Co Si solid solutions with = 2 and 3 and = 0 5 1 0 and 1 5 were prepared by induction melting stoichiometric amounts of high-purity elements. The postannealed samples consist of two phases belonging to the space groups R 3 m and P6 3 mmc . The lattice parameters and the unit cell volumes were calculated from the refinements of the magnetic and structural unit cells using the FULLPROF version of the Rietveld program. For a fixed content of Co, the maximum Curie temperatures (305 C to 405 C) were observed in samples with = 1 and having two phases, a disordered rhombohedral (DR) structure and a disordered hexagonal (DH) structure. An increase in the Curie temperature of 70 C per atom of cobalt is observed in NdDyFe 17 Co Si with = 1 and 3, suggesting that with a suitable choice of rare earths this DR phase may be a promising candidate for high-energy product permanent magnets. The magnetization versus temperature (M versus T) plots of the solid solutions, which consist of two phases, exhibit only a single magnetic ordering transition temperature. Index Terms—Disordered hexagonal (DH), disordered rhombo-herdral (DR).
  IEEE Transactions on Magnetics 01/2910; 40.
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  Available from: usp.br

  Article: Effect of frequency on the iron losses of 0.5% and 1.5% Si nonoriented electrical steels

  Marcos F De Campos, Taeko Yonamine, Marcos Fukuhara, Fernando J G Landgraf, Carlos A Achete, Frank P Missell
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  ABSTRACT: The effect of grain size on iron losses were compared for two electrical steels with 0.5 and 1.5 wt% Si. The results confirm that the optimum grain size for minimizing the energy losses decreases when the electrical resistivity decreases or when the frequency increases. Experimental results are compared to a model which considers the influence of grain size. The recrystallization texture of the alloys varies little with grain size and consists mainly of the fibers 111 uvw and 110 RD.
  IEEE Transactions on Magnetics 01/2812; 42.
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  Available from: cmu.edu

  Article: Magnetic Properties of Co-Substituted Fe-Nb-Ta-Mo-B Amorphous Alloys

  C.-Y Um, M E Mchenry
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  ABSTRACT: Fe Co Nb Ta Mo B 0 41 amor-phous alloys were produced by rapid solidification. The Co substitutions for Fe on the magnetic and structural properties of as-cast and crystalline alloys were studied. Co substitutions were shown to enhance the saturation magnetization of as-quenched ribbon samples from 108 emu/g at 0 to a maximum of 171 emu/g at 18. Co substitutions resulted in only slight changes in the primary and secondary crystallization onset temperatures, and . The Curie temperature of the amorphous phase, increased with and exceeded at 18, where nanocrystalline –FeCo phase was observed. The depen-dence of the magnetization, with decreasing temperature (after secondary crystallization), was used to determine the demarcation between the Curie transition of the –FeCo phase and the loss of magnetization due to the phase transition at temperatures between 940 and 970 C. In the range 18 41, except for 20.5, an abrupt drop in magnetization was observed at corresponding to the secondary crystallization of a FeCo B phase. For the 20. 5 composition, the magnetization was observed to increase at , fall gradually, and then drop abruptly 140 C above . Therefore, a large magnetic moment was retained to about 790 C in this alloy. This composition was the best candidate for the soft magnetic application due to their high moments up to high temperature and low measured coercivity.
  IEEE Transactions on Magnetics 01/2724; 40.
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  Available from: tue.nl

  Article: Photoconductance in magnetic tunnel junctions

  P H P Koller, F W M Vanhelmont, R Coehoorn, W J M De Jonge
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  ABSTRACT: The photoconductance of magnetic tunnel junctions has been studied in order to obtain directly determined values of the tunnel barrier height. Experiments for different types of junc-tions show that the presence of an aluminum layer close to the oxide barrier is crucial for the observation of a large photocur-rent. For Ni 80 Fe 20 -based magnetic tunnel junctions, the effective barrier height increases with increasing magnetic layer thickness between the aluminum source layer and the aluminum oxide bar-rier. This trend is also visible in the barrier height determined from a Simmons fit to the current–voltage characteristics.
  IEEE Transactions on Magnetics 01/2712; 38.
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  Available from: hanyang.ac.kr

  Article: Development of an HDD spindle motor with increased stiffness and damping coefficients by utilizing a stationary permanent magnet

  G H Jang, C I Lee
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  ABSTRACT: This research proposes a novel design of a hard-disk-drive spindle motor with increased stiffness and damping coefficients. It has the permanent magnet in the sleeve, which generates the stationary attractive magnetic force. It results in the static eccentricity of the rotating spindle, which increases the stiffness and damping effect of fluid dynamic bearings. The performance of the proposed design is verified by the experiment, as well as by the numerical simulation. The experiment shows that the proposed design decreases the nonrepeatable vibration by 10% when the eccentricity increases by 0.2 m in the given motor. Index Terms—Fluid dynamic bearings (FDBs), hard disk drive (HDD), nonrepeatable runout (NRRO), spindle motor.
  IEEE Transactions on Magnetics 01/2570; 43.
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  Available from: 140.120.11.121

  Article: Thermal magnetization noise, damping fundamentals, and mode analysis: Application to a thin film GMR sensor

  H N Bertram, V L Safonov, Z Jin
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  ABSTRACT: Magnetization dynamic equations with thermal fluc-tuations are derived fundamentally for small amplitude oscilla-tions. A tensor form of phenomenological damping occurs whose components reflect a general elliptical magnetization motion. This analysis implies that thermal energy and associated damping must be applied to independent normal modes of the system. As an ex-ample, this formulism is applied to a thin film magnetoresistance sensor to find the eigenmodes for general nonuniform magnetiza-tion variations. Examples are given of both the various modes and the noise power spectra.
  IEEE Transactions on Magnetics 01/2514; 38.
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  Available from: hanyang.ac.kr

  Article: Development of a New Magnetizing Fixture for the Permanent Magnet Brushless DC Motors to Reduce the Cogging Torque

  C J Lee, G H Jang
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  ABSTRACT: Excitation frequencies of cogging torque in a brushless dc motor are composed of the fundamental and its harmonics of the least common multiple of the poles and slots in ideal brushless dc motors, and the slot harmonics of the multiple of slot number due to the manufacturing errors. One of the main sources of the slot harmonics is the uneven magnetization of the permanent magnet. We investi-gated numerically and experimentally the uneven magnetization of permanent magnet due to the design of the magnetizing fixture and its effect on the harmonics of cogging torque. We proposed a new design of the magnetizing fixture to uniformly magnetize the permanent magnet in such a way to reduce both the fundamental harmonic and the slot harmonics of cogging torque. The slot harmonics and the fundamental harmonic of cogging torque of a brushless dc motor can be suppressed by including the back-yoke and the slots of the teeth in the magnetizing fixture, respectively. Index Terms—Back-yoke, cogging torque, magnetizing process, permanent magnet (PM), slot, uneven magnetization.
  IEEE Transactions on Magnetics 01/2410; 47.
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  Available from: psu.edu

  Article: Third-order Nédélec curl-conforming finite element

  L E García-Castillo, A J Ruiz-Genovés, I Gómez-Revuelto, M Salazar-Palma, T K Sarkar
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  ABSTRACT: The third-order version of Nédélec's first family of curl-conforming elements over simplices is presented. Following the definition of the element given by Nédélec, the third-order vector basis functions are deduced. The elements thus ob-tained exhibit some important differences with respect to other higher-order curl-conforming elements appeared in the literature. Among other features, the proposed third-order curl-conforming finite element leads to better conditioned finite-element method matrices. Index Terms—Curl-conforming element, finite element calcula-tions, Nédélec elements, third-order element.
  IEEE Transactions on Magnetics 01/2370; 38.
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  Available from: yu.ac.kr

  Article: High frequency characteristics of as-sputtered Co-Ni-Fe-N soft magnetic thin films

  Y M Kim, D Choi, K H Kim, J Kim, S H Han, H J Kim
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  ABSTRACT: Co–Ni–Fe–N soft magnetic thin films with excellent high frequency characteristics were fabricated by a N 2 reactive rf magnetron sputtering method. The nitrogen partial pressure (N2) was varied in the range of 0 10%. As N2 increases in this range, the saturation magnetization (s) linearly decreases from 19.8 kG to 14 kG and the electrical resistivity () increased from 27 cm to 155 cm. The coercivity () exhibits the minimum value at 4% N2 . The magnetic anisotropy field () are in the range of 20 40 Oe. High frequency characteristics of (Co 22 2 Ni 27 6 Fe 50 2) 100 N films are excellent in the range of 3 5% N2 . Especially, the effective permeability of the film fabricated at 4% N2 is about 800, which is maintained up to 600 MHz. This film exhibits of 17.5 kG, of 1.4 Oe, of 98 cm, and of about 25 Oe. The corrosion resistance of (Co 22 2 Ni 27 6 Fe 50 2) 100 N is improved with the increase of N concentration.
  IEEE Transactions on Magnetics 01/2288; 37.