Pole Tip Research Articles

Micromagnetic Studies of Spin-Torque Oscillator Adjacent to Soft Magnetic Thin Film

We studied the precession frequency and the corresponding Fourier component intensity of a spin-torque oscillator (STO), when adjacent to a soft magnetic film or a magnetic recording write-pole tip, for a drive current range of 10-400 MA/cm 2 . The soft film interferes with the precession of the magnetic moment in the field-generation layer (FGL) of the STO, enhancing the critical current density required to drive a stable, high-frequency precession with a large in-plane component (working mode). Vibration of the magnetization in the soft film occurs when the FGL precesses nearby. A saturated pole tip can generate a large but inhomogeneous field in the STO, and the STO can achieve a working mode under a large driven current density of around 250 MA/cm 2 . However, the precession frequency is limited to below 14 GHz, which does not meet the optimum recording condition in microwave-assisted magnetic recording.

Computerized 3D morphological analysis of glenoid orientation.

An accurate preoperative measurement of glenoid orientation is crucial for evaluating pathologies and successful total shoulder arthroplasty. Existing methods may be labor-intensive, observer-dependent, and sensitive to the misalignment between the scapula plane and CT scanning direction. In this study, we proposed a computation framework and performed an automated analysis of the glenoid orientation based on 3D surface data. Three-dimensional models of 12 scapulae were analyzed. The glenoid cavity and external anatomical features were automatically extracted from these 3D models. Glenoid version was calculated using the scapula plane and the fulcrum axis alternatively. Glenoid inclination was measured both relative to transverse axis of the scapula and the medial pole-inferior tip axis. The mean (±SD) of the fulcrum-based glenoid version was -0.55° (±4.17°), while the scapular-plane-based glenoid version was -5.05° (±3.50°). The mean (±SD) of glenoid inclinations based on the medial pole and inferior tip was 12.75° (±5.03°) while the mean (±SD) of the glenoid inclination based on the medial pole and glenoid center was 4.63° (±4.86°). Our computational framework was able to extract the reproducible morphological measures free of inter- and intra- observer variability. For the first time in 3D, we showed that the fulcrum axis was practically perpendicular to the glenoid plane normal (radial line), and thus extended the fulcrum-based glenoid version for quantifying 3D glenoid orientation.

An inspection of force reduction in high force electromagnetic tweezers made of FeCo-V foil by laser cutting

One of the main goals in the design of magnetic tweezers is to obtain a high force output. In general, the force can be enhanced by adopting materials with high saturation magnetization and by using small sharp structures as magnetic pole tips. However, the practically achieved saturation forces are usually lower than predicted values. In this article, we inspect this issue in detail both by experiments and simulations. Our results show that the observed force reduction can be ascribed to two factors: magnetic performance deterioration near the cutting edges of the tips and a 3D geometrical effect. The high power laser used in cutting causes segregation and morphological roughness near the cutting edge. Moreover, the geometry of the magnetic tips plays an important role regarding the force behavior. As a matter of fact, there is a trade-off among high force, maneuverability, throughput, and manufacturing issues in practical design of magnetic tweezers.

Incorporating Dynamics in a Mesh-Based Magnetic Equivalent Circuit Model of Synchronous Machines

A mesh-based magnetic equivalent circuit has been derived to model the dynamics of wound rotor synchronous machines (WRSMs). A particular focus has been placed on the derivation of flux tubes to model machines with an arbitrary number of damper bars placed at an arbitrary depth in the rotor pole tip. Faraday's Law is applied to establish a state model in which winding and damper bar flux linkages are selected as state variables. The resulting coupled magnetic equivalent circuit/state model is solved to predict machine dynamics. An important attribute of the model is that saturation is represented without the need for a relaxation factor, which enables its use as a practical tool in machine design. Data obtained from hardware experiment and a finite-element model are used to validate the proposed methods.

Generating Large Magnetic Field in a High Resolution Electron Beam Lithography

In this talk we will report about first results and measurements achieved with an electron beam lithographic system (EBL, eLINE Plus CAS) containing a unique in situ setup to not only fabricate but also characterize and measure GMR/TMR based devices. The eLINE Plus CAS is additionally equipped with the capability of generating a large lateral magnetic field, which is indispensible in measurement of spintronic devices or materials, besides of its original SEM and EBL functionalities. This unique EBL instrument, thus, could be not only used as a microscope or a lithography system but also as a analytical instrument for in-situ magnetoelectric transport measurements. Figure 1 shows the structure of the magnet in chamber whose pole tips are placed underneath pole piece of SEM column. The maximum lateral field realized at current stage is about 400 Oe as exciting current is elevated as 2.5 A. This large field has already been applied in in-situ measurement GMR or TMR devices. As shown in Figure 2, utilizing the magnet as well as the other two tungsten probers already installed in the chamber, we have measured magnetotransport properties of a TMR device whose TMR ratio is nearly the same as that measured outside chamber. We will further report about the characterization of the homogeneity of the magnetic field and next steps taken in this project.

Analysis, Optimization, and Prototyping of a Brushless DC Limited-Angle Torque-Motor With Segmented Rotor Pole Tip Structure

This paper presents the analysis and design of a two-pole dc brushless limited-angle torque-motor (LATM) with a toroidally wound armature using magnetic equivalent circuit modeling. A variety of design concepts is considered based on several defined criteria, and then, the most worthy alternative is chosen. To select the best design concept, an efficient decision-making tool called analytical hierarchy process is employed. A new segmented rotor pole tip structure of the LATM is introduced. The results obtained from finite-element simulation of the proposed machine conform well with the analysis. The selected design is prototyped, and the experimental results are presented.

The timing of anterior temporal lobe involvement in semantic processing.

Despite indications that regions within the anterior temporal lobe (ATL) might make a crucial contribution to pan-modal semantic representation, to date there have been no investigations of when during semantic processing the ATL plays a critical role. To test the timing of the ATL involvement in semantic processing, we studied the effect of double-pulse TMS on behavioral responses in semantic and difficulty-matched control tasks. Chronometric TMS was delivered over the left ATL (10 mm from the tip of the temporal pole along the middle temporal gyrus). During each trial, two pulses of TMS (40 msec apart) were delivered either at baseline (before stimulus presentation) or at one of the experimental time points 100, 250, 400, and 800 msec poststimulus onset. A significant disruption to performance was identified from 400 msec on the semantic task but not on the control assessment. Our results not only reinforce the key role of the left ATL in semantic representation but also indicate that its contribution is especially important around 400 msec poststimulus onset. Together, these facts suggest that the ATL may be one of the neural sources of the N400 ERP component.

Imaging the equilibrium state and magnetization dynamics of partially built hard disk write heads

Four different designs of partially built hard disk write heads with a yoke comprising four repeats of NiFe (1 nm)/CoFe (50 nm) were studied by both x-ray photoemission electron microscopy (XPEEM) and time-resolved scanning Kerr microscopy (TRSKM). These techniques were used to investigate the static equilibrium domain configuration and the magnetodynamic response across the entire structure, respectively. Simulations and previous TRSKM studies have made proposals for the equilibrium domain configuration of similar structures, but no direct observation of the equilibrium state of the writers has yet been made. In this study, static XPEEM images of the equilibrium state of writer structures were acquired using x-ray magnetic circular dichroism as the contrast mechanism. These images suggest that the crystalline anisotropy dominates the equilibrium state domain configuration, but competition with shape anisotropy ultimately determines the stability of the equilibrium state. Dynamic TRSKM images were acquired from nominally identical devices. These images suggest that a longer confluence region may hinder flux conduction from the yoke into the pole tip: the shorter confluence region exhibits clear flux beaming along the symmetry axis, whereas the longer confluence region causes flux to conduct along one edge of the writer. The observed variations in dynamic response agree well with the differences in the equilibrium magnetization configuration visible in the XPEEM images, confirming that minor variations in the geometric design of the writer structure can have significant effects on the process of flux beaming.

Status of CSNS H− ion source**Supported by NSFC (91126003)

A new H− ion source has been installed successfully and will be used to serve the China Spallation Neutron Source (CSNS). In this paper, we report various components of the ion source, including the discharge chamber, temperature, cooling system, extraction electrodes, analyzing magnet, remote control system and so on. Compared to the previous experimental ion source, some improvements have been made to make the ion source more compact and convenient. In the present arrangement, the Penning field is generated by a pair of pole tip extensions on the 90° analyzing magnet instead of by a separate circuit. For the remote control system, F3RP61-2L is applied to the accelerator online control system for the first time. In the running of the ion source, a stable pulse H− beam with a current of 50 mA at an energy of 50 keV is produced. The extraction frequency and pulse width is 25 Hz and 500 μs, respectively. Furthermore, an emittance scanner has been installed and measurements are in progress.

Magnetic tweezers with high permeability electromagnets for fast actuation of magnetic beads.

As a powerful and versatile scientific instrument, magnetic tweezers have been widely used in biophysical research areas, such as mechanical cell properties and single molecule manipulation. If one wants to steer bead position, the nonlinearity of magnetic properties and the strong position dependence of the magnetic field in most magnetic tweezers lead to quite a challenge in their control. In this article, we report multi-pole electromagnetic tweezers with high permeability cores yielding high force output, good maneuverability, and flexible design. For modeling, we adopted a piece-wise linear dependence of magnetization on field to characterize the magnetic beads. We implemented a bi-linear interpolation of magnetic field in the work space, based on a lookup table obtained from finite element simulation. The electronics and software were custom-made to achieve high performance. In addition, the effects of dimension and defect on structure of magnetic tips also were inspected. In a workspace with size of 0.1 × 0.1 mm(2), a force of up to 400 pN can be applied on a 2.8 μm superparamagnetic bead in any direction within the plane. Because the magnetic particle is always pulled towards a tip, the pulling forces from the pole tips have to be well balanced in order to achieve control of the particle's position. Active video tracking based feedback control is implemented, which is able to work at a speed of up to 1 kHz, yielding good maneuverability of the magnetic beads.

Excellent wear life of silicon nitride/tetrahedral amorphous carbon bilayer overcoat on functional tape heads

Developing ultrathin and highly wear-resistant overcoats for magnetic tape heads is one of the current research areas of interest, because of its potential to delay pole tip recession and increase the operational lifetime of high areal density tape drives. Using optimized process conditions and an appropriate overcoat design, we report on the development of a ∼20 nm thick silicon nitride/tetrahedral amorphous carbon (Si/SiNx/ta-C) bilayer overcoat, where the ta-C film was deposited by a filtered cathodic vacuum arc process. The bilayer overcoat deposited on a functional tape head survived 40–50 × 106 m of testing with commercial tape media under standard industrial testing conditions. The excellent wear resistance of the overcoat was attributed to the generation of high (∼72%) sp3 carbon content and the formation of strong interfacial bonds, such as Si-C, C=N, nitrile, and (Al, Ti)N at the interfaces, as confirmed by various spectroscopic techniques. This study demonstrates the pivotal role of high sp3 carbon bonding combined with enhanced interfacial bonding in developing an ultrathin yet durable protective overcoat for magnetic tape heads.

Optimization of the Sextupole Magnets With Trim Coils for the Collector Ring of the FAIR Project

The collector ring (CR) is a part of the challenging international project facility for antiproton and ion research started in Darmstadt, Germany. The ion optics of the CR includes sextupole magnets with embedded dipole correction coils. To fulfill the requirements of the field quality in the magnet aperture, it is necessary to find an optimal pole tip shape as well as an optimal configuration of the coil system. We used a specially developed optimization procedure for designing the magnet cross section. This procedure consists of two steps. The first one is based on a representation of the pole border line by a superposition of hyperbolic functions corresponding to different Fourier components of the magnetic field expansion. The optimization procedure based on the Newton–Raphson descend algorithm minimizes amplitudes of the undesired field harmonics. At the second step of the magnet design procedure, the parameters of the dipole winding are optimized to provide the required field characteristics in the magnet aperture.

Durable ultrathin silicon nitride/carbon bilayer overcoats for magnetic heads: The role of enhanced interfacial bonding

Pole tip recession (PTR) is one of the major issues faced in magnetic tape storage technology, which causes an increase in the magnetic spacing and hence signal loss during data readback. Despite efforts to reduce the magnetic spacing, PTR, and surface wear on the heads by using protective overcoats, most of them either employ complex fabrication processes and approaches do not provide adequate protection to the head or are too thick (∼10–20 nm), especially for future high density tape storage. In this work, we discuss an approach to reduce the PTR and surface wear at the head by developing an ultrathin ∼7 nm bilayer overcoat of silicon/silicon nitride (Si/SiNx) and carbon (C), which is totally fabricated by a cost-effective and industrial-friendly magnetron sputtering process. When compared with a monolithic C overcoat of similar thickness, the electrically insulating Si/SiNx/C bilayer overcoat was found to provide better wear protection for commercial tape heads, as demonstrated by Auger electron spectroscopic analyses after wear tests with commercial tape media. Although the microstructures of carbon in the monolithic and bilayer overcoats were similar, the improved wear durability of the bilayer overcoat was attributed to the creation of extensive interfacial bonding of Si and N with the C overcoat and the alumina-titanium carbide composite head substrate, as predicted by time-of-flight secondary ion mass spectrometry and confirmed by in-depth X-ray photoelectron spectroscopy analyses. This study highlights the pivotal role of enhanced interfaces and interfacial bonding in developing ultrathin yet wear-durable overcoats for tape heads.

Hybrid tool with both fixed-abrasive and loose-abrasive phases

The use of multiple pole tips in a Magnetic Abrasive Finishing (MAF) system allows for shorter processing times, but it can also exacerbate any difficulties encountered in abrasive insertion and distribution. This paper proposes a hybrid tool that initially works as fixed abrasive but works as loose abrasive once the abrasive binder dissolves into the lubricant. The fixed-abrasive configuration simplifies the abrasive insertion, and the loose-abrasive configuration ensures the abrasive uniform distribution along the workpiece surface. The internal finishing of needles demonstrated that the hybrid tool saves about 80% abrasive insertion time while achieving a smooth surface equivalent to the conventional MAF method.

Сравнение статических моделей явнополюсных двигателей с реактивным ротором

Two substitution schemes for one pole pitсh of stator of the motor with a reluctance rotor are considered in the report. The schemes are made in consideration of magnetic flux branching at the edges of the stator pole tips either in the radial direction or in the tangential direction, respectively. The comparison of calculated and experimental results of torque determination allows to make a conclusion about the greater reliability of the scheme with the tangential magnetic flux.

Deformation of Hard Disk Drive Media Caused by Thermal Stress and Induced Air Flow

The spindle motor in a hard disk drive spins at a high rotational speed. These rotations generate air flow and thermal stress. Air flow is induced by the surface roughness of the media that is moving at a high speed through air. This air passes over the surface of many parts in the drive, including the media. Thermal stress is generated by heat in the parts, e.g. voice coil motor, pre-amplifier, slider pole tips, which are heated by electric power and by the spinning of the spindle motor. The air flow and thermal stress cause a change in the media shape called deformation.Simulation results show the trend of deformation has more bending when the slider moves outward from the media axis. The pressure acted more on the underside than on the upper side which caused the media to bend up to the top cover side of hard disk drive. The maximum deformation, 15 μm, occurred at the rim of media while the distance between the media and the slider is 30 μm; thus they did not contact each other.

An Advanced Vision System for the Automatic Inspection of Corrosions on Pole Tips in Hard Disk Drives

Pole tips of hard disk drives are made of FeCo film, which is prone to corrosion due to the corrosive nature of the environment and wide range of pH experienced during the production. A machine vision system is utilized for the automatic auditing of corroded pole tips. This comprises three steps: 1) extraction of the top shield region; 2) fusion of extracted features; and 3) decision making. This model is capable of detecting corrosion in the critical part of the pole tips. The experiments show that the proposed system detects corrosion with high accuracy, whereas the overall processing time meets industrial requirements.

Head and Media Challenges for 3 Tb/in<inline-formula> <tex-math notation="TeX">\(^{\boldsymbol {2}}\) </tex-math></inline-formula> Microwave-Assisted Magnetic Recording

A specific design for microwave assisted magnetic recording (MAMR) at about 3 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (0.47 Tb/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> or 4.7 Pb/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) is discussed in detail to highlight the challenges of MAMR and to contrast its requirements with conventional perpendicular magnetic recording (PMR). In particular, it has been determined that MAMR-optimized media should have: higher damping than today's PMR media upon which ferromagnetic resonance measurements are reported, very low intergranular exchange coupling, and somewhat stronger layer to layer exchange coupling. It was found that with exchange-coupled composite type media (i.e., graded anisotropy with controlled exchange), a spin torque oscillator in the write gap of a wider shielded pole cannot adequately define the written track at high track density. Adequate lateral field gradient, however, is achieved by modifying the pole tip geometry. Other details of this conceptual design are also discussed.

Optimization of the Quadrupoles for the Collector Ring of FAIR

The Collector Ring (CR) is a part of the challenging international project Facility for Anti-proton and Ion Research (FAIR) started in Darmstadt, Germany. The ion optics of the CR includes quadrupole magnets with an elliptical aperture and embedded octupole correction coils. To fulfill the requirements of the field quality in the magnet aperture, it is necessary to find an optimal pole tip shape as well as an optimal configuration of the coil system. We used a specially developed optimization procedure for designing the magnet cross-section. This procedure is based on a representation of the pole border line by a superposition of hyperbolic functions corresponding to different Fourier components of the magnetic field expansion. The goal function of the optimization minimizes amplitudes of the undesired field harmonics, and the Newton-Raphson method is applied to find a solution of the problem. We combined the described optimization procedure with the finite element method of the magnetic field computation and applied the developed algorithm as well as the corresponding computer program for optimizing the pole tip shape and the configuration of the coil system in the quadrupole magnets of the FAIR CR.

Domain structures of a main pole tip in a hard disk drive observed by magnetic strain imaging

The detection and imaging of magnetic‐field‐induced strains via scanning probe microscopy enable us to observe magnetic domain structures. A perpendicular‐recording head in a hard disk drive has been observed using this method to investigate the causes of head‐induced data erasure. Domain structures of the main pole tip in a head that induces the erase‐after‐write phenomenon are observed, and we confirm the presence of remanent domains. The structures are not single domain but comprise multiple domains changing with the writing currents. The remanent magnetic fields causing the erase‐after‐write should be generated by the large magnetization near the domain walls. Copyright © 2014 John Wiley &amp; Sons, Ltd.