bluuebunny/crossref_metadata_2025 huggingface dataset:
Polarized light transmission of smooth muscle cells during magnetic field exposures
Authors: Iwasaka M. , Ueno S.
Published: May 2003

Strong static magnetic fields in the Tesla range have a diamagnetic force on cell components. Within an exposure period of 3 d, such magnetic fields generate a clear alignment of the smooth muscle cell assembly, which is parallel to the direction of the fields. In the present study, behavioral changes in intracellular components were suggested by changes in optical transmission under the influence of magnetic fields of 8 T. The magnetically oriented cells were exposed both in parallel and perpendicularly to the 8 T magnetic fields, and the time course of the polarized light transmission at 500 nm was measured in real time. The perpendicularly exposed cells had a presteady state, which was slow to increase. However, the total change in the intensity of the polarized light was larger than that of the cells exposed in parallel. We observed that no distinct changes in cell morphology including cell membrane components occurred during the 3 h of exposure to the magnetic fields. Based on the results, we concluded that intracellular component motion had a major influence on polarized light transmission under 8 T magnetic fields. We speculated that unconstrained intracellular macromolecules, such as actin fibers and microtubules, rotated due to diamagnetic torque forces during the 3 h of exposure to 8 T magnetic fields.
Magnetic micromachines prepared by ferrite plating technique
Authors: Nishimura K. , Uchida H. , Inoue M. , Sendoh M. , Ishiyama K. , Arai K. I.
Published: May 2003

By the sterolithography technique and the ferrite plating, we fabricated three types of magnetic micromachines which coated Fe3O4, Co0.39Fe2.61O4, and Co0.45Fe2.55O4, respectively. The stereolithography technique enables one to form the spiral-shape resinous templates and the ferrite plating enables one to coat the ferrites uniformly onto these templates from an aqueous solution. The total machine weight is light because a resin of the template body is almost the same density as water. We verified that the machines swim freely and wirelessly in water by applying a rotational magnetic field. As these machines coated with ferrites have the biocompatibility, there is the possibility of medical microrobots which swim in the human body for medical operations.
Magnetosensitive enzyme electrode for hydrogen peroxide sensing
Authors: Yaoita M. , Iwasaka M. , Ueno S.
Published: May 2003

Peroxidase is one of the magnetosensitive enzymes, and has an important role in scavenging reactive oxygen species. In the present study, a surface of platinum black electrode was laminated by peroxidase molecules, and H2O2 decomposing processes by peroxidase and platinum black were monitored with and without magnetic fields of up to 14 T. An increase in the current reflected a decrease in the activity of peroxidase molecules. The current in the electrode with peroxidase increased significantly depending on the applied magnetic flux intensity. The increases of current during the magnetic field exposures were observed consistently both when the currents were transiently decreasing and at a constant level. It is suggested that the layers of peroxidase molecules on the platinum black cause a distortion by diamagnetic forces acting on the layers, and the distortion is concentrated on a catalytic part in the peroxidase resulting in the observed activity decreases.
Magnetoresistance study in NiFe semicircle-ring patterned wires
Authors: Yu C. , Lee S. F. , Yao Y. D. , Ma Y. R. , Huang E. W. , Tsai J. L. , Chen T. Y. , Chang C. R.
Published: May 2003

Micron size NiFe wire having a patterned shape of semicircle in series was fabricated. Magnetoresistance of the wire has been studied from 10 to 300 K. Domain wall resistivity is nearly independent of the temperature; however, the domain wall switching field decreases relatively rapidly with increasing temperature. We have observed experimentally two distinct domain structures at the corners of the NiFe semicircle-ring patterned wire at remanence after longitudinal and transverse saturation fields. We can explain successfully the magnetization reversal process on the magnetoresistance loops by the contribution from anisotropic magnetoresistance and magnetic domain configuration.
Charge localization in nanometric La2/3Ca1/3MnO3 thin films grown on nearly matching substrates
Authors: Herranz G. , Berkowski M. , Jedryka E. , Wojcik M. , Sánchez F. , Bibes M. , Fontcuberta J.
Published: May 2003

By using magnetotransport measurements as well as nuclear magnetic resonance as local magnetic probe, we conclusively show that, irrespective of the film/substrate structural mismatch and film thickness, there are nonconductive regions at the film/substrate interface. We thus discard dimensional effects and structural mismatch as the main reasons for the suppression of conductivity and the weakening of the ferromagnetic coupling in nanometric thin films of manganites. We shall argue that this behavior is related to a charge localization which also leads to a weakening of the magnetic interactions and a decrease of the Curie temperature.
Detection of flaws in ferromagnetic samples based on low frequency eddy current imaging
Authors: Fujita Y. , Sasada I.
Published: May 2003

We report a nondestructive method to detect flaws in a ferromagnetic plate based on eddy current imaging. The inductive pickup in our previous Dahle-type probe is replaced by a thin orthogonal flux gate driven by a 1 MHz ac current. Samples are 2-mm-thick steel plates (SS400) having a 0.4-mm-wide EDM slot on the backside with various slot depths (0.4, 1.0, 1.2, and 1.6 mm). These samples were line scanned by the probe with the lift-off about 0.01 mm. The excitation field inducing the eddy current in the sample was 40 Hz–2 kHz. An interesting thing to note is the relationship between the slot depth and the excitation frequency at which peaks in the phase response locate, that is, the deeper the flaw position is, the lower the excitation frequency of peak locates. The resolution in depth profiling is evaluated to be better than 0.2 mm.
In-plane and out-of-plane uniaxial anisotropies in rectangular arrays of circular dots studied by ferromagnetic resonance
Authors: Kakazei G. N. , Wigen P. E. , Guslienko K. Yu. , Chantrell R. W. , Lesnik N. A. , Metlushko V. , Shima H. , Fukamichi K. , Otani Y. , Novosad V.
Published: May 2003

Ferromagnetic resonance at 9.2 GHz (X band) was used to characterize the uniaxial magnetic anisotropies in rectangular arrays of submicron circular Ni dots. The in-plane anisotropy, originated from interdot interactions in the rectangular lattice, and the perpendicular anisotropy, due to individual dot shape and magnetostriction, were explored. For in-plane dependencies of the resonance field (Hr), the main resonance mode angular dependence was well described by the standard Kittel formula. As the interdot distances decreased from 800 to 50 nm, the in-plane uniaxial anisotropy field changed from 5 to 130 Oe, in reasonable agreement with calculations. Simultaneously, the position of perpendicular Hr increased from 6.38 to 6.83 kOe, also following Kittel’s formula.
Magnetoresistance study in NiFe–Al–NiFe single-electron tunneling devices
Authors: Shyu J. H. , Yao Y. D. , Chen C. D. , Lee S. F.
Published: May 2003

Magnetoresistance in NiFe–Al–NiFe single-electron tunneling device has been studied at temperatures between 0.066 and 0.8 K and in magnetic fields up to 3 T. The competition effect among the superconducting, the Coulomb blockade, and the magnetic tunneling has been experimentally investigated. An enhancement effect of the tunneling magnetoresistance due to the superconductivity of the Al island has been observed in the nonlinear range of the current–voltage I–V characteristics. The superconducting critical magnetic fields obtained from the magnetoresistance curves are roughly decreased from 1.5±0.1 to 1.3±0.1 T, and 1.2±0.1 T for temperature increasing from 66 to 400 mK, and 800 mK, respectively. For Al island in its normal state, the resistance of the NiFe–Al–NiFe single-electron tunneling device is roughly 40 kΩ and is insensitive to the current variation. However, in general, the resistance increases with decreasing the current for the central Al island in its superconducting state.
Dynamics of a magnetic domain wall in magnetic wires with an artificial neck
Authors: Himeno A. , Ono T. , Nasu S. , Shigeto K. , Mibu K. , Shinjo T.
Published: May 2003

Magnetization reversal in submicron magnetic wires consisting of a NiFe/Cu/NiFe trilayer with an artificial neck was investigated by utilizing the giant magnetoresistance effect. A magnetic domain wall was injected into the wire by a local magnetic field applied at the end of the wire. Pinning and depinning of the magnetic domain wall were detected as sharp changes in resistance. It was found that the neck works as a pinning site of a domain wall and that the depinning field increases with a decrease of the neck width.
Random telegraph noise in a nickel nanoconstriction
Authors: Céspedes O. , Jan G. , Viret M. , Bari M. , Coey J. M. D.
Published: May 2003

Nickel nanoconstrictions about 20 nm wide produced by electron beam lithography in a 60 nm nickel film exhibit resistivities in the kΩ range with a nonlinear and asymmetric I–V characteristic. Noise spectra of the contacts sometimes deviate from 1/f behavior due to random telegraph fluctuations at room temperature with a frequency in the 10 Hz range. The resistance fluctuations between the two states are about 0.1%. The time spent in the high resistance state increases as we increase the temperature, and the discrete fluctuations eventually disappear with an increase of the temperature of more than about 15 °C. An explanation is proposed in terms of electron-wind electromigration which interacts with the narrow domain wall formed at the nanocontact.
Spin dependent tunneling junctions with reduced Neel coupling
Authors: Wang Dexin , Daughton James M. , Qian Zhenghong , Nordman Cathy , Tondra Mark , Pohm Art
Published: May 2003

A new structure of spin dependent tunneling (SDT) junctions has been demonstrated to have a much reduced Neel coupling field between the free and pinned ferromagnetic layers comparing with conventional SDT structures. The new structure consists of a modified synthetic-antiferromagnetic composite layer as the pinned layer with two Ru spacer layers and three ferromagnetic layers. The Neel coupling field is much reduced for both top- and bottom-pinned SDT structures using this new composite pinned layer. Furthermore, the net magnetic moment is kept at zero for the composite pinned layer to minimize the fringe field after patterning. The coupling reduction can be understood by considering the additive contribution from the first two interfaces with Ru in the composite pinned layer, which cancels that from the pinned layer interface with the barrier. By properly spacing these three most important interfaces, reducing the coupling to basically zero is realized. The coupling reduction allows the elimination of an on-chip bias coil used to correct the coupling, therefore simplifying the electronics and reducing the power to operate the SDT sensors. The new SDT structure has potential impacts on many SDT and spin valve devices such as magnetoresistive sensors, galvanic isolators, magnetic logic, and MRAM devices.
Modular permanent magnet brushless servo motors
Authors: Ede J. D. , Atallah K. , Howe D.
Published: May 2003

This article discusses the suitability of modular permanent magnet brushless machines for applications where servo characteristics are key performance requirements. It is shown that among the large number of feasible combinations of slot number and pole number, optimum combinations exist, which exhibit good servo characteristics.
Magnetic force characteristics according to the variation of asymmetric overhang ratio in brushless direct current motor
Authors: Chun Yon Do , Wakao Shinji , Lee Ju
Published: May 2003

As usual, brushless dc motor (BLDCM) has the overhang structure of which permanent magnet (PM) length is longer than the stator’s stack length. Specially, in case of asymmetric overhang, we have to pay attention to the choice of suitable overhang ratio of the lower to the upper overhang length because of the effect of the ratio on the magnetic forces. In this article, the magnetic force characteristics of BLDCM with the asymmetric overhang and the cap of the rotor are analyzed. A three-dimensional equivalent magnetic circuit network (EMCN) method is used for the efficient analysis of the magnetic field. The magnetic forces and torque are calculated by Maxwell’s stress tensor. The variation of magnetic force characteristics is also analyzed according to the overhang ratio. From the results, when the overhang ratio is 0.38, the axial magnetic force characteristic could be improved remarkably. The validity of the analysis results is also clarified by comparison between calculated results and measured ones.
Performance analysis of a brushless dc motor due to magnetization distribution in a continuous ring magnet
Authors: Hur Jin , Jung In-Soung , Sung Ha-Gyeong , Park Soon-Sup
Published: May 2003

This paper represents the force performance of a brushless dc motor with a continuous ring-type permanent magnet (PM), considering its magnetization patterns: trapezoidal, trapezoidal with dead zone, and unbalanced trapezoidal magnetization with dead zone. The radial force density in PM motor causes vibration, because vibration is induced the traveling force from the rotating PM acting on the stator. Magnetization distribution of the PM as well as the shape of the teeth determines the distribution of force density. In particular, the distribution has a three-dimensional (3-D) pattern because of overhang, that is, it is not uniform in axial direction. Thus, the analysis of radial force density required dynamic analysis considering the 3-D shape of the teeth and overhang. The results show that the force density as a source of vibration varies considerably depending on the overhang and magnetization distribution patterns. In addition, the validity of the developed method, coupled 3-D equivalent magnetic circuit network method, with driving circuit and motion equation, is confirmed by comparison of conventional method using 3D finite element method.
Modeling of switched reluctance motor using Fourier series for performance analysis
Authors: Hur Jin , Kim Chong-Chul , Hyun Dong-Seok
Published: May 2003

A computational method for a performance analysis of the switched reluctance motor has been developed. Most of the papers present static characteristics, such as static torque based on a finite element method analysis. However, this method is based on a Fourier series expansion with step-by-step, current-dependent, adjustable coefficients. Analytical expression for the calculation of instantaneous phase inductance, flux linkage, co-energy, and electromagnetic torque as a function of rotor position and winding current are derived. Finally, the strategy to represent the winding inductance variation with position is made by approximating the actual inductance profile.
Detent torque from the soft magnetic stator stack of a hybrid stepper motor
Authors: Rajagopal K. R. , Singh Bhim , Singh B. P.
Published: May 2003

The residual magnetism present in the stator stack of a hybrid stepper motor, which will be of the last excited polarity, will develop a detent torque, similar to the static torque of the last excited phase, but with reduced excitation. The predominant component is fundamental. Even though this torque is the result of the stator excitation, it can be considered as a detent torque as it is available when the stator excitation is withdrawn. This detent torque is in addition to the inherent one due to the rotor permanent magnet, and it will be present until the next phase is switched ON. Once the excitation is removed from the second phase, because of the hysteresis the detent torque will be available, similar to the static torque profile for phase 2, but with less excitation. This means because of the hysteresis the detent torque follows the static torque of the phases. The magnitude of this detent torque will depend upon the level to which the phase was excited. Experiments have been carried out on a 0.5° hybrid stepper motor to study the effects of the excitation on the detent torque profiles. Harmonic analyses of the measured detent torque curves had been carried out and it is seen that the fundamental detent torque increases with the excitation and also follows the respective phases. Therefore, the residual flux density and the coercive force of the soft magnetic material used for the stator stack have to be as small as possible to get rid of this effect. Controlling a hybrid stepper motor by a suitable switching scheme for a given application requires the measured static torque profiles of the motor for all the phases at the exact working voltage (or voltages in case of a variable voltage supply) and detent torque profiles both before and after excitation. Harmonic spectrum of the measured static and detent torque profiles have to be accounted for while simulating stepping behavior of the motor, from which an appropriate switching scheme can be arrived at.
Effects of in situ magnetic field application and postdeposition magnetic annealing on sputtered Ni80Fe20/Fe50Mn50/Ni80Fe20 trilayers
Authors: Chen F. H. , Ng V. , Adeyeye A. O.
Published: May 2003

The effects of in situ magnetic field deposition and postdeposition magnetic annealing on the exchange field and coercivity of Ni80Fe20/Fe50Mn50/Ni80Fe20 trilayers were investigated. Hysteresis loops reveal a one-sided plateau for unannealed samples, A clean loop without plateau was observed for samples annealed at 230 °C for 15 min in vacuum with an external magnetic field. An increase in coercivity was also observed. This can be attributed to the formation of NiFeMn alloy at the interfaces caused by annealing.
Geometric integration of the Gilbert equation
Authors: Spargo A. W. , Ridley P. H. W. , Roberts G. W.
Published: May 2003

Geometric integration refers to numerical methods which aim to preserve the qualitative and geometric features of a differential equation after discretization. In micromagnetics, the magnetization vector M represents a statistical average of magnetic moments, the magnitude of which should be conserved in time. In general, the midpoint scheme preserves the modulus of solutions on the sphere due to intrinsic quadratic invariance. This method has previously been implemented in various forms to solve the Landau–Lifshitz and Landau–Lifshitz–Gilbert equations within finite-difference formulations. In this article, it is shown that an explicit Euler method will overestimate |M| while an implicit Euler method will make an underestimate, whereas the midpoint method conserves |M| up to round-off error. The midpoint scheme is then utilized within a variational finite-element formulation of the Gilbert equation. Numerical stability and error control are discussed using the reversal of a cobalt nanoparticle as an example calculation.
Analytical and experimental study of identification of Preisach–Néel-type models for magnetic nanoparticle systems
Authors: Borcia I. D. , Cerchez M. , Stancu Al. , Tung L. D. , Spinu L.
Published: May 2003

This article describes an identification methodology for a Preisach–Néel model based on a nonlinear Stoner–Wohlfarth energy barrier hypothesis. In the identification, one uses data from a set of remanent curves, the major hysteresis loop at two different temperatures below the blocking temperature and the zero-field-cooled and field-cooled magnetization curves. This identification scheme was experimentally applied to a system of Co ferrite nanoparticles with an average diameter of 3 nm. The quality of the identification procedure is analyzed and the differences between the experimental and computational results are discussed.
Setup to test biaxial stress effect on magnetomechanic coupling
Authors: Maurel Vincent , Ossart Florence , Marco Yann , Billardon René
Published: May 2003

Many authors provide a self-method to take into account a multiaxial stress state for magnetic coupling. By reviewing those criteria we try to point out the most general one. We also propose an original setup validated by FE calculation.
Theoretical and experimental analysis of magnetic inductive heating in ferrite materials
Authors: Zhang X. K. , Li Y. F. , Xiao John Q. , Wetzel E. D.
Published: May 2003

By incorporating magnetic particles into elevated temperature-curing adhesives, or hot-meltable thermoplastics, induction fields can be used to heat bondlines and join composite adherends. In this article we investigate the fundamental heating behaviors of such magnetic particles in order to aid in the design and selection of improved magnetic materials. Magnetic inductive heating tests were performed on two types of magnetic materials: Co2Ba2Fe12O22 semihard ferrites and NiFe2O4 soft ferrites. The Curie temperatures of these ferrites were also varied through Zn substitution. All of the samples clearly demonstrated Curie temperature-controlled final heating behavior. The magnitude of heat generation has been related to dc magnetic hysteresis measurements, with reasonable agreement. Deviations are theorized to be due to domain wall resonance effects.
Transition from parallel to antiparallel magnetic state at the interface of Fe3O4/Mn3O4 superlattices (invited)
Authors: Chern G. , Horng Lance , Wang Y. C.
Published: May 2003

A series of ferrimagnetic/ferrimagnetic Fe3O4/Mn3O4 superlattices are grown on MgO(001) and (011) substrates by plasma-oxygen-assisted molecular beam epitaxy. The thickness of the Fe3O4 layer is 4 nm and the thickness of Mn3O4 is varied from 1 to 16 nm and the total thickness of these films is between 100 and 200 nm. The magnetization as a function of temperature and external field (parallel to the film surface) is measured in the range of 4–300 K and −50–50 kOe. The main magnetic response of the superlattices with thicker Mn3O4 layers show compensation points (∼39 K) and spin-flop phases at a high external field (>15 kOe) indicating the net moments in Fe3O4 and Mn3O4 are antiparallel at the interface. The magnetic response of the superlattices with thinner Mn3O4 layers show regular ferromagnetic hysteresis and temperature curves indicating the net moments in Fe3O4 and Mn3O4 are parallel at the interface. A transition of the magnetic coupling between Fe3O4 and Mn3O4 from antiparallel to parallel is clearly observed when the Mn3O4 layer thickness is less than 4 nm. A domain-wall-like configuration is believed to form at the interface to maintain the antiparallel state of Fe3O4/Mn3O4. When the layer thickness reduces below certain thickness the domain-wall-like configuration no longer exists and a parallel state is preferred. A model of the domain-wall-like configuration between Fe3O4 and Mn3O4 is discussed.
Robust exchange coupling in bilayer exchange-spring thin films
Authors: Crew D. C. , Kim J. , Barmak K. , Lewis L. H.
Published: May 2003

Exchange coupling across the interface in polycrystalline exchange-spring thin films of L10 CoPt and hcp Co has been investigated with magnetic measurements and micromagnetic modeling. For thick enough Co films, the reversal in the second quadrant of the hysteresis loop is multiphase, with full reversal of the soft Co phase accomplished before reversal of the hard CoPt phase. Therefore the exchange field acting on the soft phase is determined as the field of maximum susceptibility in the soft-phase reversal portion of the second quadrant. Experimentally, it is found that the exchange field is a linear function of the inverse thickness of the soft phase. This result is in agreement with the exchange field calculated from the associated micromagnetic modeling only under the condition that the magnitude of the exchange coupling constant between spins across the CoPt-Co interface is similar to that characterizing the bulk CoPt. Experimental and modeling-derived results do not agree for decreased interfacial exchange coupling. This result is surprising, because it was expected that the condition of the interface between the exchange-coupled film layers would have a dominant effect on the magnetic reversal of the system; the presence of expected atomic disorder or chemical impurity at the interface should severely degrade the exchange coupling.
Recording properties of CoCrPt tape media sputter-deposited at room temperature on polymeric substrates
Authors: Lee Hwan-Soo , Laughlin David E. , Bain James A.
Published: May 2003

Recording properties of CoCrPt thin films sputter-deposited on polymeric substrates at room temperature were investigated. Typical frequency spectra of the readback signal from sputtered tapes at various recording densities revealed an appreciable amount of modulation noise, despite the fact that the sputtered tape is expected to be smooth and conformal with the substrate. Atomic force microscopy images suggest the modulation noise, present as side bands about the signal peak, is primarily due to the roughened polymeric substrate. The nonlinear partial erasure method was used to measure the sharpness of recorded transitions via the frequency domain one-dimensional/three-dimensional method. Intertransition interactions occurred at a linear density of 130 kfci, which corresponds to a transition length parameter (πa) of 195 nm. This rather large value of transition length is attributed to exchange coupling between grains in the media, suggesting inadequate Cr segregation due to the low-temperature deposition needed to avoid substrate damage.
High-density laser-assisted magnetic recording on TbFeCo media with an Al underlayer
Authors: Miyanishi S. , Kojima K. , Sato J. , Takayama K. , Fuji H. , Takahashi A. , Ohta K. , Katayama H.
Published: May 2003

The effects of an Al underlayer with a nanoscaled rugged structure are studied on the laser-assisted magnetic recording of TbFeCo media. The performance of laser-assisted recording is dramatically improved by employing the Al underlayer. The reproducing signal is observed at the linear density of 450 kilo-flux change per inch. The relationship between the surface morphology and the magnetic properties of TbFeCo media proves that the nanoscaled rugged structure of Al underlayers influences the process of the magnetization reversal; in particular, the coercivity and the exchange interaction are significantly changed in the TbFeCo media. The authors conclude that the nanoscaled rugged structure of the Al underlayer leads to magnetic pinning sites, which impede the motion of a magnetic domain wall, and thereby the laser-assisted recording performance is improved.
Increase of magnetic damping in thin polycrystalline Fe films induced by Cu/Fe overlayers
Authors: Lubitz P. , Cheng Shu Fan , Rachford F. J.
Published: May 2003

The ferromagnetic resonance properties of thin polycrystalline layers in the sequence Cu/Fe/Cu/Fe/Cu, grown on Si wafers, were studied. Fe grown on Cu can have a very narrow ferromagnetic resonance (FMR) linewidth. Similar structures are of interest for spin transport studies and for giant magnetoresonance applications. The thinner Fe underlayer ranged from 2 to 5 nm, the intermediate Cu spacer thickness from 2 to 10 nm, and the Fe outer layer was 20 nm thick. The increased damping of the thinner Fe layer in this structure, as reflected in the FMR linewidths observed at 9.46 and 33.5 GHz, is similar to that predicted by Berger and recently observed experimentally using epitaxial single crystal Fe films. In addition to confirming the 1/(Fe layer thickness) dependence of the additional damping, the present measurements show a decrease of damping with increasing Cu spacer thickness, implying a short spin transport decay length in our Cu of about 3.5 nm. The linewidth in isolated Cu/Fe/Cu layers also increased with decreasing Fe thickness, as predicted and observed in some other cases. Magnetization data indicate negligible magnetic coupling of the Fe layers except for the smallest interlayer Cu thicknesses used. The linewidths studied increased moderately with cooling to 77 K.
Quantum conductance in electrodeposited nanocontacts and magnetoresistance measurements
Authors: Elhoussine F. , Encinas A. , Mátéfi-Tempfli S. , Piraux L.
Published: May 2003

We present conductance and magnetoresistance measurements in magnetic Ni–Ni and Co–Ni nanocontacts prepared by electrodeposition within the pores of a track-etched polymer membrane. At room temperature, Ni–Ni constrictions show broad quantization plateaus of conductance during their dissolution into units of e2/h, as expected for ferromagnetic ballistic nanocontacts. Additionally, positive and negative magnetoresistance has been measured in Co–Ni nanocontacts.
Understanding field rise time and magnetic damping in thin film recording heads
Authors: Zhu Jian-Gang , Bai Daniel Z.
Published: May 2003

Magnetization dynamics during field reversal in thin film recording heads are studied via micromagnetic modeling. It is found that head field reversal under the current field can be characterized by two stages: an initial stage that is mainly governed by the gyromagnetic motion and virtually independent of energy damping, followed by a damping stage that strongly depends on the energy damping constant. Due to the finite energy damping in practical recording heads, the head field amplitude will roll off at high recording frequencies. Simply decreasing the current rise time or increasing current amplitude will not alleviate the situation, but rather worsen it. There exists an optimum current rise time at which the head field rise time is minimized and the roll-off frequency is maximized.
Writing at high data rates
Authors: Klaassen K. B. , van Peppen J. C. L.
Published: May 2003

The physical restrictions on the write signal path in a hard disk drive are such that, at high data rates, the write electronics typically need a large voltage supply and generates much heat. In addition, the electronics produce inadequate write current wave forms that adversely affect the magnetic patterns to be recorded. After a brief overview of what has been proposed to alleviate this problem, a solution is presented wherein a high inductance head (56 nH) can be driven up to 1 Gb/s. This is made possible by compensating the head inductance into (nearly pure) resistance and matching this resistance to the characteristic impedance (75 Ω) of the 5 cm long driver-to-head interconnect. This compensation/matching only requires small, lossy capacitors, which can be implemented in the head-processing phase onto the head wafers as an integral part of head processing.
Nitrogen-induced local magnetic and structural properties of sputtered FeAlN thin films
Authors: Liu Y.-K. , Kryder M. H. , Ryan D. H. , Altounian Z.
Published: May 2003

Conversion electron Mössbauer spectroscopy (CEMS) and x-ray diffraction were used to study a series of rf-reactively sputtered FeAlN thin films with the goal of gaining insight into the origins of their uniaxial magnetic anisotropy (UMA) and the effect of sputtering conditions on its thermal stability. At N/Fe=4.6%(±0.5%), N goes into interstitial vacancies without significant reductions in the hyperfine field (Bhf) of the Fe but causing lattice expansions, leading to an increased linewidth in the CEMS spectra of the film while the sample has an in-plane UMA. Reducing the target-to-substrate spacing used in the deposition process does not affect the nitrogen content of the films but yields an increased CEMS linewidth reflecting larger disorder in the local Fe environment. Heavy N doping causes the phase transform into γ′-Fe4N, ε-Fe3N, and ζ-Fe2N phases. A well defined UMA exists in mildly doped FeAlN films when doped N atoms mostly occupy the interstitial sites while FeAl and heavily doped FeAlN films are either isotropic or weakly anisotropic. UMA and its thermal stability are therefore concluded to have a correlation with the local environment of the Fe structure induced by N doping.
Time-resolved observation of Barkhausen avalanche in Co thin films using magneto-optical microscope magnetometer
Authors: Kim Dong-Hyun , Choe Sug-Bong , Shin Sung-Chul
Published: May 2003

We develop an experimental technique for direct, full-field, time-resolved observation of the Barkhausen avalanche in a two-dimensional thin-film system, using a magneto-optical microscope magnetometer (MOMM). Real-time visualization capability of the MOMM enables us to microscopically observe all the details of the Barkhausen avalanche in Co thin films, which is not feasible using other indirect experimental techniques adopted so far. We find that there exist fluctuating flexible domain walls deformed by defects and that, interestingly enough, the domain wall exhibits still-detectable fluctuation even around a strong linear defect as well as a strong point-like defect, from which we conclude that a critical avalanche continues to exist even in strong pinning cases.
External field effects on the resonant frequency of magnetically capped oscillators for magnetic resonance force microscopy
Authors: Miller Casey W. , Mirsaidov Utkur M. , Messina Troy C. , Lee Yong J. , Markert John T.
Published: May 2003

We study the resonant frequency shift of CoPt-capped single-crystal-silicon micro-oscillators when a magnetic field is applied perpendicular to the magnetic film, as required for application to nuclear magnetic resonance force microscopy. The oscillator resonant frequencies show two distinct regimes of behavior. At low fields, when the magnetic moment is nearly perpendicular to the external field, the frequency decreases sharply with field, while at high fields, when the moment and field are nearly aligned, the frequency increases. We present models that accurately describe both behaviors. The transition point between these two regimes scales with the volume of the micromagnets.
High-speed massive imaging of hard disk data by using the spin-stand imaging technique
Authors: Tse C. , Mayergoyz I. D. , Mircea D. I. , Krafft C.
Published: May 2003

High-speed massive imaging of hard disk data by using the spin-stand imaging technique [I. D. Mayergoyz, C. Serpico, C. Krafft, and C. Tse, J. Appl. Phys. 87, 6824 (2000) and I. D. Mayergoyz, C. Tse, C. Krafft, and R. D. Gomez, J. Appl. Phys. 89, 6772 (2001)] is reported. In order to obtain these large-scale images of hard disk data, disks from commercial hard drives were scanned by a giant magnetoresistive head in the along- and cross-track directions. A special method of triggering has been devised to capture the data nondestructively. Challenges related to the eccentricity of the disk and the instability of the trigger have also been addressed. By using this massive imaging technique, we were able to image disk data with track densities as high as 60 000 tracks per inch. With a specially designed automated algorithm, the developed technique can be programmed to image the drive data of the whole disk surface with high resolution and speed.
Reversal-field memory in magnetic hysteresis
Authors: Katzgraber H. G. , Pázmándi F. , Pike C. R. , Liu Kai , Scalettar R. T. , Verosub K. L. , Zimányi G. T.
Published: May 2003

We report results demonstrating a singularity in the hysteresis of magnetic materials, the reversal-field memory effect. This effect creates a nonanalyticity in the magnetization curves at a particular point related to the history of the sample. The microscopic origin of the effect is associated with a local spin-reversal symmetry of the underlying Hamiltonian. We show that the presence or absence of reversal-field memory distinguishes two widely studied models of spin glasses (random magnets).
Rotational magnetization losses in vector models
Authors: Della Torre Edward
Published: May 2003

Unlike the case of an oscillating magnetic field, power losses in a magnetic material eventually start decreasing as the magnitude of a rotating field increases. In some hysteresis models, the loss increases monotonically as the magnitude of a rotating field increases. The simplified vector Preisach model and the reduced vector Preisach model do not suffer from this error. For magnetization-dependent reversible models, such as the Della Torre-Oti-Kadar, and for state-dependent models such as the complete moving model, the loss per cycle has two components. The first is computed from the Preisach integrals, and the second is computed from the change in the stored energy in the reversible component. It is noted that the latter loss can occur even if there are no Barkhausen jumps. This article discusses the details of how the model accomplishes this.
Design of high-magnetic field gradient sources for controlling magnetically induced flow of ferrofluids in microfluidic systems
Authors: He W. , Lee S. J. , Jiles D. C. , Schmidt D. H. , Porter M. D. , Shinar R.
Published: May 2003

The use of miniature electromagnets for ferrofluid-actuated liquid dispensing into microfluidic channels has been investigated by numerical simulations using the finite element method and measurements of fluid displacement and flow rate. The simulations illustrate the effect of structural and geometrical parameters of single and paired solenoid coils on the magnetic force experienced by the ferrofluid. Dual solenoids were used for extended fluid displacement. Ferrofluid positioning and flow rates were controlled also by using a solenoid with an iron core. The experimental measurements of fluid flow in capillaries were used to validate the modeling calculations. The results can be used as a basis for the development of on-chip ferrofluid-based devices integrated with microfluidic architectures.
Transport modeling of Py film with antidot array
Authors: Guo J. , Jalil M. B. A.
Published: May 2003

We present a model to investigate the lateral transport and anisotropic magnetoresistance (AMR) of a permalloy film, with a two-dimensional array of unfilled holes (antidots). The current density distribution j is first obtained numerically by solving the Poisson equation in the absence of B field, while the magnetization M of the film is calculated via micromagnetics. The M and j distributions are linearly interpolated through the sample, and then combined in a self-consistent scheme to obtain the galvanomagnetic effects (normal and planar Hall effects), which affect the current direction. The overall AMR is evaluated from the parallel contributions of the resulting current paths. Modifying the film geometry by reducing the antidot spacing results in an increase in the peak AMR ratio, which may be explained by the inhomogeneous current density and magnetization alignment induced by the antidots.
New early failure phenomenon in electrostatic discharge damaged giant magnetoresistive recording heads
Authors: Wallash Albert
Published: May 2003

The magnetic behavior of partially electrostatic discharge (ESD) damaged giant magnetoresistive (GMR) heads during thermal and bias current stress is studied. Heads were ESD damaged to a level that would still pass quasistatic specifications and then monitored while at a higher temperature (100 °C or 150 °C) and bias current (5 to 6 mA) for up to 40 h. While controls showed no early changes, significant changes in amplitude, asymmetry, and Barkhausen jump were seen within the first hour of stressing in 10% of the partially ESD damaged heads. Heads that experienced pinned layer reversal or developed Barkhausen jumps were especially prone to large changes in magnetic response during stressing. It is concluded that some partially ESD damaged GMR heads can exhibit spontaneous and large degradation in amplitude and magnetic stability during high thermal and bias current stressing.
Magnetic and thermal properties of PtMn giant magnetoresistive sensors
Authors: Khapikov Andrei , Simion Bogdan , Lederman Marcos
Published: May 2003

The thermal stability of PtMn exchange-biased magnetoresistive heads has been studied. Constant temperature and bias current have been applied, and the amplitude has been measured as a function of time to determine the decay rate and lifetimes. An approach to lifetime data analysis has been developed to estimate the intrinsic sensor temperature and activation energy of the decay mechanism in a self-consistent manner. It has been shown that the internal sensor temperature is higher than the average device temperature determined from the traditional resistance measurements. The method of the lifetime data analysis has allowed the comparison of the activation energy and thermal properties for different types of heads.
Read performance of GMR heads with in-stack longitudinal bias layer
Authors: Hatatani Masahiko , Hoshino Katsumi , Hoshiya Hiroyuki , Shintani Taku , Watanabe Katsuro , Nakamoto Kazuhiro , Tanaka Haruko , Ide Hiroshi
Published: May 2003

Shielded GMR heads with an in-stack bias layer were designed, fabricated, and their read performance was measured. Since the bias layer and the free layer antiferromagnetically couple at the edge of the sensor, a closed magnetic flux structure (CFS) is formed. The results of micromagnetic simulation revealed that when an MnIr antiferromagnetic layer is used to pin the magnetization of the bias layer, the Ms⋅t of the bias layer normalized by that of the free layer should be between 1.2 and 1.5. Here Ms and t are the saturation magnetization and the thickness of the layer. The read waveform of the fabricated head was noise free and well biased. The obtained read sensitivity of the CFS head was four times higher than that of a conventional abutted junction head. Calculation revealed that the read sensitivity of the CFS head was almost five times higher than that of conventional heads when the magnetic track width was reduced to 100 nm. Thus, CFS heads show potential for high track density recording.
Design of Curie point written magnetoresistance random access memory cells
Authors: Daughton J. M. , Pohm A. V.
Published: May 2003

Very high density magnetoresistance random access memory (MRAM) cells may be subject to thermal upset. This article describes designs that enhance thermal stability and increase ultimate density by using the combination of heat and magnetic field for writing data. The basic storage mechanism can be shape anisotropy, the coupling between an antiferromagnetic layer and a ferromagnetic layer, or a combination of the two. Two designs are described in this article. The first uses a low Curie point material with high shape anisotropy at room temperature. These cells use active semiconductor devices to restrict heating current to only one cell in an array. The second approach employs the interface coupling between a ferromagnetic film and an antiferromagnetic film as the storage mechansism. A cell may be written by heating above the Néel temperature and cooling the interface in a magnetic field by using orthogonal lines for heating and magnetic field. Heating and cooling times are a few nanoseconds. These design approaches could lead to stable MRAM cells with diameters less than 0.1 μm and requiring lower drive currents.
Size dependence of switching current and energy barrier in the magnetization reversal of rectangular magnetic random access memory cell
Authors: Nozaki Y. , Matsuyama K.
Published: May 2003

Write operation in magnetic random access memory (MRAM) with bit density of Gbit/cm2 order has been numerically simulated for low submicron scale magnetic cells. The amplitude of both the switching current Iw and the energy barrier ΔE of the magnetic cell show strong dependence on the width w and the thickness t of the cell. The calculated results show that the amplitude of Iw is proportional to t/w, while that of ΔE is proportional to wt2. To obtain the sufficient energy barrier against a thermal stability (ΔE>80kBT, at 300 K), the thicker magnetic cell is required as the cell is downsized for high-density memory applications, although the thinner one is preferred to lower the switching current. From the calculated results, it appears that the margin of the switching current for selective write operation is decreased with decreasing the lateral aspect ratio of the cell. The addressability for the write operation is also degraded by the formation of structural defect in the cell. In this article, optimum configurations of the magnetic cell applicable for Gbit MRAM are discussed.
Precessional switching of the magnetization in microscopic magnetic tunnel junctions (invited)
Authors: Schumacher H. W. , Chappert C. , Sousa R. C. , Freitas P. P. , Miltat J. , Ferré J.
Published: May 2003

We study the precessional switching of the magnetization in a microscopic magnetic tunnel junction cell as used in magnetic random access memories. By measuring the tunneling magnetoresistance versus time we follow the dynamical response of the cell’s free layer magnetization to ultrashort field pulses applied along the in-plane magnetic hard axis. In the presence of a strong easy axis bias field a pronounced precession of the magnetization with damping times of the order of 2 ns is observed. At lower bias fields the large angle precession induced by pulses as short as 170 ps can switch large domains of the free layer magnetization. Multiple application of identical pulses reversibly toggles the magnetization between the two easy directions. For longer pulses coherent higher order switching is observed in full agreement with theory.
Control of domain patterns in square shaped nickel rings
Authors: Zhu Xiaobin , Grütter P. , Metlushko V. , Ilic B.
Published: May 2003

Magnetic rings in a square shape are studied by magnetic force microscopy with in situ in plane magnetic fields. Well defined domain structures are accessible by changing the orientation of the magnetic field. Magnetic domain wall can easily be trapped at corners. The domain patterns can be controlled by the magnetic field strength and field direction.
Magnetization reversal in arrays of individual and coupled Co-rings
Authors: Welp U. , Vlasko-Vlasov V. K. , Crabtree G. W. , Hiller J. , Zaluzec N. , Metlushko V. , Ilic B.
Published: May 2003

The magnetization behavior of arrays on 10 μm Co rings has been studied using magnetometry, magneto-optical imaging, and Lorentz microscopy. Square arrays of individual rings and arrays of chains of interacting, touching rings have been investigated. In fields transverse to the chains the switching of the rings occurs always in pairs. This coupling introduces a broad distribution of switching fields and correspondingly a broad magnetization loop. Lorentz microscopy reveals that the switching for both, the isolated and the coupled rings, occurs through the formation of a buckled state, and the nucleation and propagation of a vortex domain wall.
Bit isolation in periodic antidot arrays using transverse applied fields
Authors: Jalil M. B. A.
Published: May 2003

Ferromagnetic films etched with a periodic array of holes (antidots) may be used as high-density storage media, where memory bits are stabilized by shape fields Hs near the edges of the antidots. A micromagnetic simulation is performed to study the conditions for well-defined bits at remanent state. The parameter under consideration is the bit signal-to-noise ratio (SNR), which is defined w.r.t. the ideal magnetization alignment, and is calculated for different values of intrinsic anisotropy Ku and transverse applied field Hy. Unlike previously thought, a transverse Ku hardly improves the SNR (<10%) due to its sign independence, which leads to vortex formation around the antidots and, hence, increased noise in the interbit regions. By contrast, a relatively weak Hy field of 50 to 100 Oe can effectively separate neighboring bits by aligning the interbit regions, leading to a 250%–400% improvement in SNR. Further improvement in SNR is achieved by increasing the bit-aspect ratio. The SNR however, degrades sharply when the anisotropy Hk and transverse Hy fields approach the shape field value obtained via an analytical model. The model used is corroborated by the coercivity trend of different antidot size, obtained by micromagnetics.