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Magnetics, IEEE Transactions on

Issue 2 • Date Feb. 2013

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  • Front cover

    Page(s): C1
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  • IEEE Transactions on Magnetics publication information

    Page(s): C2
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  • Table of contents

    Page(s): 657 - 658
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  • XXIII Magnetic Recording Conference 2012 - Foreword

    Page(s): 659 - 661
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  • Nonbinary LDPC Coding and Iterative Decoding System With 2-D Equalizer for TDMR R/W Channel Using Discrete Voronoi Model

    Page(s): 662 - 667
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (2195 KB) |  | HTML iconHTML  

    A 2-D magnetic recording (TDMR) by shingled magnetic recording (SMR) is one of the most promising technologies for realizing ultra-high areal densities. We have developed the discretized granular medium model with nonmagnetic grain boundaries and the simple writing process considering intergranular exchange fields and magnetostatic interaction fields between grains on the discrete Voronoi model for TDMR. In this paper, the bit-error rate (BER) performance of the iterative decoding system using a nonbinary low-density parity-check (LDPC) code over Galois field GF(q) with the 2-D finite-impulse-response equalizer (2D-FIRE) is obtained via computer simulation using an R/W channel model employing the writing process under TDMR specifications of 4.12 Tb/in2, and it is compared to that with the 1-D FIRE (1D-FIRE). The results show that the BER performance of the nonbinary LDPC coding and iterative decoding system with the 2D-FIRE is better than that with the 1D-FIRE. View full abstract»

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  • Control of Microstructure and Magnetic Properties of FePt Films With TiN Intermediate Layer

    Page(s): 668 - 674
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    The effects of a TiN intermediate layer on the microstructure and magnetic properties of the FePt films were investigated. It was found that the TiN layer could effectively block the diffusion of Cr into the FePt film. The good epitaxial relationships among these layers were revealed from the transmission electron microscopy (TEM) results. With introducing TiN intermediate layer the chemical ordering and magnetic properties of FePt films significantly improved. The FePt film with 5 nm TiN exhibited a high perpendicular coercivity of 13.7 kOe and a low in-plane coercivity of 0.24 kOe, resulting from the combined contribution of TiN (200) orientation, TiN layer roughness and the effective block of Cr diffusion. Moreover, with doping C into the FePt-SiNx films, the out-of-plane coercivity increased due to the decrease of the exchange coupling, the grain size of FePt films decreased, and well-separated FePt grains and uniform size were formed. By optimizing the sputtering process, the [FePt (4 nm)-SiNx 40 vol·% ]- 20 vol·% C (001) film with coercivity higher than 21.5 kOe, a single layer structure, and small FePt grain size of 5.6 nm in average diameter was obtained, which are suitable for ultrahigh density perpendicular recording. View full abstract»

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  • Joint Message-Passing Decoding of LDPC Codes and 2-D ISI Channels

    Page(s): 675 - 681
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1444 KB) |  | HTML iconHTML  

    Two-dimensional (2-D) intersymbol-interference (ISI) channels represent an important class of next-generation data-storage systems. In order to mitigate the 2-D ISI and improve system performance, a joint message-passing decoding algorithm is proposed in this paper. The proposed algorithm conducts an iterative decoding of low-density parity-check code and a 2-D ISI channel, which can be implemented in a fully parallel manner. An extrinsic information transfer chart is also introduced to analyze the performance and convergence behavior of the proposed message-passing decoding system. Simulation results show that the proposed joint message-passing decoding algorithm outperforms some existing joint decoding algorithms. View full abstract»

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  • Tetragonal Heusler Compounds for Spintronics

    Page(s): 682 - 685
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (784 KB) |  | HTML iconHTML  

    With respect to the requirements of spin torque transfer (STT) materials, one the most promising materials families are the tunable tetragonal Heusler compounds based on Mn2YZ (Y=Co,Fe,Ni,Rh,...; Z=Al, Ga, Sn). They form the inverse cubic Heusler structure with three distinct magnetic sublattices, which allows a fine tuning of the magnetic properties. Starting with the stoichiometric Mn3Ga compound, we explored the complete phase diagram of Mn3-xYxZ (Y=Co, Fe, Ni and Z=Ga ). All series exhibit thermally stable magnetic properties. As we demonstrate, Mn3-xFexGa series, which are tetragonal over the whole range of compositions, are good as hard magnets, whereas magnetically more weak Mn3-xNixGa series exhibit the shape memory phenomenon. Even more surprising properties are found in the Co-based series. On the one hand, the Co-poor (x <; 0.5) Mn3-xCoxGa tetragonal alloys are well-suited for the STT. At the same time, the Co-rich cubic series (0.5 <; x <; 1) are half-metallic ferromagnets, following the Slater-Pauling rule, similar to Co2YZ cubic Heusler compounds. View full abstract»

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  • HAMR Recording Limitations and Extendibility

    Page(s): 686 - 692
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1360 KB) |  | HTML iconHTML  

    Heat-assisted magnetic recording (HAMR) limitations and extendibility are studied in light of the recent 1.0 Tb/in2 technology demonstration. The paper examines HAMR specific technology challenges, including switching field distributions at elevated temperature, saturation noise, and near-field transducer (NFT) thermal spot-size limits. While current HAMR recording density ( ~ 1 Tb/in2) is limited by switching field distribution and thermal spot size, ultimate HAMR density (up to 5 Tb/in2) is determined by achievable recording-layer magnetic anisotropy and grain size. View full abstract»

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  • 5 Tdots/in2 bit patterned media fabricated by a directed self-assembly mask

    Page(s): 693 - 698
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (2445 KB) |  | HTML iconHTML  

    FePt bit patterned media (BPM) was fabricated with a self-assembled polymer mask with a feature size of 12 nm pitch (equivalent to 5 Tdots/in2 ). A 3.5 nm FePt film with high c-axis crystal orientation was prepared for the magnetic recording layer. A solvent vapor annealing process was applied for obtaining uniform directed self-assembling of polystyrene (PS)-polydimethylsiloxane (PDMS) diblock copolymer pattern. Pattern transfer from a polymer mask to FePt layer was achieved by employing a carbon hard mask. In spite of excellent magnetic characteristics of FePt layer, the fabricated FePt BPM showed small coercivity (Hc) of 6 kOe and large switching field distribution (SFD) of 21%. These results are due to the etching damage of FePt dots. Disordering of FePt L10 phase by the etching damage reduced magnetic anisotropy energy (Ku). The damaged portion became a nucleus of the magnetization reversal and reduced Hc. Distribution of the damaged volume and the extent of the Ku reduction contributed to large SFD. This model is supported by the experimental data of magnetic field angle dependence of switching field. The result suggests the domain wall motion type of magnetization reversal mode, where the domain wall is created at the interface between the damaged portion and the internal high-Ku region. View full abstract»

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  • Thermal Asperity Sensor Application to Hard Disk Drive Operational Shock

    Page(s): 699 - 702
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1162 KB) |  | HTML iconHTML  

    Thermal asperity (TA) sensor is used to investigate head disk interface (HDI) dynamics during operational shock. It is revealed that the output of TA sensor as a function of shock acceleration and back-off level. Air bearing modes being excited during operational shock indicates head-disk contact happens. The mechanisms and exciting sources of head-disk contact were explored by analyzing the TA sensor signal. Both experiment and simulation results show that motor base mode and head stack assembly (HSA) bending mode are the two most important contributions to head-disk contact. From this study, TA sensor is found to be useful for in situ monitoring of operational shock dynamics and, thus providing the strategies to improve shock-resistant performance. View full abstract»

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  • Head-Stack Assembly Offtrack Dynamics Investigation via Slider Protrusion Touch Down

    Page(s): 703 - 706
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (799 KB) |  | HTML iconHTML  

    To further increase the areal density to 2 TB/in2, the head-disk spacing needs to be reduced to sub 1 nm by using a thermal flying-height control technique that can adjust slider pole tip protrusion dynamically. At such low spacing, the slider pole tip protrusion-disk contact is inevitable. This contact will lead to slider vibration and head-stack assembly (HSA) going offtrack. This paper focuses on HSA offtrack dynamics induced by slider protrusion-disk contact, which was observed to have significantly different characteristics with traditional HSA dynamics excited by a voice-coil motor (VCM). With VCM excitation, the most dominant modes are from the entire HSA structure. However, with slider protrusion touch down (SPTD) excitation, head gimbal assembly (HGA) local modes are dominant. Both numerical analysis and experimental verification were used to explore the major factors influencing this characteristic. It is found that the gain of HGA sway is very sensitive to bending location when slider is excited by SPTD. Moreover, if the HGA local modes are close to HSA modes, they are easily coupled, which makes the offtrack vibration increase significantly. These findings can be used as references for optimizing the design of the HGA and HSA in order to improve the offtrack dynamics of HSA. View full abstract»

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  • Fabrication and Characterization of FePt Exchange Coupled Composite and Graded Bit Patterned Media

    Page(s): 707 - 712
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1137 KB) |  | HTML iconHTML  

    Three methods to fabricate continuous FePt films with graded magnetic anisotropy for bit patterned media (BPM) were evaluated. Continuous FePt films with surface roughness of less than 0.3 nm were achieved in continuous FePt hard magnetic films, FePt/Fe exchange coupled composite (ECC) films and FePt/Fe based graded films. Depositing an Fe-rich film on FePt at high temperature was found to form large grains and cause the film surface very rough for BPM fabrication. Depositing Fe on FePt at room temperature and then annealing it to create graded anisotropy through the layer interdiffusion process was demonstrated to fabricate FePt/Fe based graded BPM. The continuous FePt films with hard layer only, ECC structure and graded magnetic anisotropy were patterned using a di-block copolymer self-assemble hard mask method with 25 nm dot size over 2-inch substrate. The switching field distribution (SFD) broadening and degradation of FePt BPM was studied. The reduction of SFD was achieved using a postannealing process. It was confirmed that the patterned graded BPM sample has smaller switching field and larger thermal energy barrier than the ECC sample. View full abstract»

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  • Scalability of Spin Accumulation Sensor

    Page(s): 713 - 717
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1445 KB) |  | HTML iconHTML  

    In this work, we investigated a scalability of spin accumulation signal for various device sizes. We found that the spin accumulation signal is enhanced by shrinking Cu wire width and thickness. Moreover we estimate the signal to noise ratio (SNR) of a spin accumulation head structure by using the size dependence of spin accumulation signal. The SNR increases with reducing nonmagnetic wire width and thickness, which are corresponding to a track width and a gap width, respectively. And it has the maximum value around 2 ~ 3 Tbit/inch2 resolution. From these results, it has a possibility that spin accumulation effect is applicable to a new read head for high density hard disk drive. View full abstract»

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  • L1 _{0} -Ordered FePt-Based Perpendicular Magnetic Recording Media for Heat-Assisted Magnetic Recording

    Page(s): 718 - 722
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (2683 KB) |  | HTML iconHTML  

    We update our continuous effort to optimize the microstructures and magnetic properties of FePt-X granular films to achieve an ideal media structure on glass substrates for heat-assisted magnetic recording. For segregant X, we investigated C, SiO2 , TiO2 and their mixtures. While FePt-C granular films show excellent inplane granular structure for the thickness (t) smaller than 6 nm, a second layer appears for t >; 6 nm. On the other hand, FePt-TiO2 granular film shows a columnar structure with a smooth surface, but the inplane morphology is interconnected. To enhance the phase separation and realize the laterally isolated columnar structure, we mixed the segregant materials of SiO2 or TiO2 with C. We also used the thin FePt-C films as templates for FePt-X(X=SiO2 and TiO2) since the FePt-C showed good particle separation with the fine particle size. Based on these experimental results, we discuss how to attain the ideal media structure for heat-assisted magnetic recording. View full abstract»

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  • Channel Characterization and Performance Evaluation of Bit-Patterned Media

    Page(s): 723 - 729
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    Bit-patterned media (BPM) is a promising approach to push back the onset of the superparamagnetic limit faced by conventional continuous granular media. Today, BPM islands can be fabricated at densities higher than can be characterized by existing methods and full bit-patterned media recording (BPMR) is still a long way off. In this work, we rely on simulations to predict how such islands would perform in a real recording scenario. The grain flipping probability (GFP) model is trained via micromagnetic simulations and reproduces the magnetic profiles used to generate readback signals for channel simulations. The geometrical parameters to the micromagnetic simulations, such as the island size variations and island position jitter are characterized from measurements of islands fabricated via e-beam at various channel densities. View full abstract»

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  • HAMR Drive Performance and Integration Challenges

    Page(s): 730 - 733
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (842 KB) |  | HTML iconHTML  

    The commercialization of heat-assisted magnetic recording (HAMR) presents some significant technical challenges that need to be resolved before the widespread adoption of the technology can begin. In this paper, we present some HAMR data from prototype drives and discuss some of the challenges related to protrusion management, recording performance optimization, and drive power requirements within the drive. View full abstract»

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  • Signal Model for Shingled Magnetic Recording Based on Data Dependent Erase Band Analysis Under Track Squeeze

    Page(s): 734 - 738
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1255 KB) |  | HTML iconHTML  

    In this study, a new signal model for shingled magnetic recording (SMR) is developed based on the drive waveform analysis with squeezed side tracks. Shingled recording effects are modeled by the data dependent effective write width or erase band, and the readback signals are represented by the weighted sum of the individual target and side track signals without intertrack interference (ITI). The weights of the target and side track signals, denoted by the ITI coefficients, are estimated from the drive waveforms collected from the track squeeze tests. Near the track edge location, the ITI coefficients are reduced by the erase band and the data-independent electronic noise is effectively increased. In addition, the ITI coefficients also depend on the recorded data patterns due to such effect as track width broadening by low frequency data patterns. Cross-track ITI coefficient profiles are modeled by Gaussian read sensitivity and rectangular magnetization functions and applied for various SMR conditions. Two data sets of 50% and 44% bit transition rates are investigated to take into account data dependent erase band. Based on the proposed signal model, the cross-track bit error rate (BER) performance of the SMR are evaluated with the readback signals, synthesized for various read offsets. The results show that modulating the data bits to have fewer bit transitions may improve the on-track BER performance, while its off-track performance can be limited by the increased ITI due to the low frequency data patterns. In addition, BER performance of SMR can be improved by the ITI cancellation (ITIC) scheme. View full abstract»

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  • Pulsed Thermally Assisted Magnetic Recording

    Page(s): 739 - 743
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1046 KB) |  | HTML iconHTML  

    In thermally assisted magnetic recording, one of the most important issues is the reliability of the near-field transducer and the magnetic recording head. When the near-field transducer transfers the optical energy to the media through coupling from the waveguide, it inevitably heats itself and its surrounding area, such as the magnetic recording head, which tends to cause long-term performance degradation. To mitigate the problem, one possible solution is to apply a pulsed laser beam instead of a constant one. In this paper, the performance characteristics of the pulsed thermally assisted magnetic recording are analyzed by modeling. Simulation results show that similar recording performance can be achieved through a much reduced laser beam duty cycle (~ 30%). View full abstract»

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  • Characterization of Adjacent Track Erasure in Perpendicular Recording by a Stationary Footprint Technique

    Page(s): 744 - 750
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1653 KB) |  | HTML iconHTML  

    As the perpendicular recording writer keeps scaling down to improve recording areal density and data rate throughput, the off-track performance of the writer, including both adjacent track erasure and wide area adjacent track erasure, becomes one of the key challenges in hard disk drives. Understanding writer erasure locations and mechanisms is critical. In this paper, stationary footprint technique is used to study various perpendicular writer designs. The asymmetric responses of two side shields in wrapped-around shielded writer are observed, which agree well with micromagnetic modeling results. The impacts of asymmetric side shields on cross track gradient and adjacent track erasure were qualitatively verified. The hot spot locations and mechanisms of wide area track erasure have been investigated. The impacts of recording media and recording reader in stationary footprint measurement are also discussed. View full abstract»

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  • Simulation of Heat-Assisted Magnetic Recording Using Renormalized Media Cells

    Page(s): 751 - 757
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1301 KB) |  | HTML iconHTML  

    A new scheme for the simulation of heat-assisted magnetic recording (HAMR) that systematically includes fluctuating material properties above a predefined length scale, while retaining magnetostatic interactions, is introduced. Renormalized media parameters Ms, Ku, Aex, and αdamp, suitable for useful length scales, are found numerically. These renormalized parameters are then used to model the Voronoi-cell-composed medium in the HAMR simulation. Transition jitters are obtained under various conditions. The results show that moderate maximum temperature of the heat spot, intergranular exchange coupling, media thickness of at least 10 nm, nonzero canting angle of the head field, relatively low head velocity, and large head-field strength are helpful for a successful recording. View full abstract»

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  • Writability Improvement in Perpendicular Recording Media Using Crystalline Soft Underlayer Materials

    Page(s): 758 - 764
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    Perpendicular recording media technology faces limitation in the writability of media with high anisotropy. Therefore, alternative methods are sought to overcome this problem. Heat-assisted magnetic recording may give rise to high areal densities, but the technological process will take time. We have studied methods to improve the writability in perpendicular recording media using crystalline soft underlayer (C-SUL) materials with fcc(111) texture. One of the recording media designs consists of a simple insertion of C-SUL material between the nonmagnetic intermediate layers, thus providing a path for magnetic flux during the writing. In the alternative methods, part of the nonmagnetic intermediate layer material was replaced with C-SUL material. Overwrite improvement was observed in both the cases, indicating that the C-SUL material will play a major role in improving writability in recording media. View full abstract»

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  • Understanding Signal and Noise in Heat Assisted Magnetic Recording

    Page(s): 765 - 772
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (2234 KB) |  | HTML iconHTML  

    In this paper, we report a micromagnetic modeling study on the recording processes in heat-assisted magnetic recording. By solving coupled Landau-Lifshitz-Bloch equations, recording simulations are performed for granular FePt-L10 thin film media. The calculated signal-to-noise ratio shows strong dependence on recording field amplitude, especially for media of small size grains. It is found that low field amplitude yields unsaturated recording whereas high field amplitude causes transitions to be broadened immediately after writing. High thermal gradient will alleviate the transition broadening, allowing high field amplitude to be employed so that completed magnetization in the recorded bits can be achieved in small grin size media. The study concludes that the ability in obtaining the expected signal-to-noise ratio performance at very small grain pitches would critically rely on whether sufficiently high thermal gradient can be achieved in the media. View full abstract»

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  • Bit Patterned Media at 1 Tdot/in2 and Beyond

    Page(s): 773 - 778
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (2485 KB) |  | HTML iconHTML  

    Bit patterned media (BPM) provide an alternative to conventional granular thin film recording media, circumventing the challenges of managing grain size and its associated noise and thermal stability issues in granular media. A viable fabrication strategy involves creation of a master pattern by rotary-stage e-beam lithography and directed self-assembly of block copolymers, followed by pattern replication via UV-cure nanoimprint lithography and pattern transfer to a magnetic thin film by ion beam etching. These steps have been demonstrated for 150 Gdot/cm2 (1 Tdot/in2) hcp patterns, achieving a dot placement tolerance of 1.2 nm 1σ and a defect rate of <; 10-3. Media samples fabricated in this manner from continuous CoCrPt alloy films have achieved a 1σ switching field distribution of 4% of Hc. A 2T medium SNR of nearly 14 dB and a write bit error rate of 2 × 10-3 have been shown using a static tester with a conventional product read/write head. Modeling and experiment suggest that higher recording density can be achieved using BPM with a bit aspect ratio (BAR) >; 1. A master pattern generation generation strategy for BAR>; 1 with rectangular islands is shown using intersecting lines generated by directed self-assembly of lamellar block copolymers in combination with spacer-defined line doubling. View full abstract»

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  • HAMR Areal Density Demonstration of 1+ Tbpsi on Spinstand

    Page(s): 779 - 782
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (561 KB) |  | HTML iconHTML  

    Heat-assisted magnetic recording (HAMR) is being developed as the next-generation magnetic recording technology. Critical aspects of this technology, such as plasmonic near-field transducer (NFT) and high anisotropy granular FePt media, have been demonstrated and reported. However, progress with areal density was limited until recently. In this paper, we report a basic technology demonstration (BTD) of HAMR, at 1.007 Tbpsi with a linear density of 1975 kBPI and track density of 510 kTPI, resulting from advances in magnetic recording heads with NFT and FePtX media. This demonstration not only shows significant areal density improvement over previously reported HAMR demos, more significantly, it shows HAMR recording at a much higher linear density compared to previous reports. It is an important milestone for the development of such a new technology. Many challenges still remain to bring this technology to market, such as system reliability and further advancement of areal density. View full abstract»

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Aims & Scope

IEEE Transactions on Magnetics publishes research in science and technology related to the basic physics and engineering of magnetism, magnetic materials, applied magnetics, magnetic devices, and magnetic data storage.

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Meet Our Editors

Editor-in-Chief
Pavel Kabos
National Institute of Standards and Technology