Daniel Z. Bai

Western Digital Corporation, Irvine, California, United States

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Publications (28)40.84 Total impact

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    ABSTRACT: A micromagnetic simulation is conducted to study the write field dynamics in the presence of antiferromagnetic exchange coupling (AFC) of writer pole for a perpendicular magnetic recording (PMR) writer head. It is found that, depending on the actual configuration of the AFC coupling in the yoke and the pole tip, there could be an intermediate transient state near zero write field during the magnetization reversal, compared to the normal writer design without AFC coupling. This transient state may cause recording performance degradation. We further investigated the AFC coupling effect on the remanent field in the degauss process. For different performance aspects, i.e., field dynamics, maximum field, and remanent field, the AFC in the yoke and in the pole tip plays different roles.
    IEEE Transactions on Magnetics 01/2013; 49(7):3725-3728. · 1.42 Impact Factor
  • Zhanjie Li, Daniel Z. Bai
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    ABSTRACT: The characteristics of spatial and temporal field distributions are presented for wrap-around-shielded perpendicular magnetic recording writers. Micromagnetic modeling simulations are conducted to study the effects of shield thickness and the magnetization on the shield-related side field. An improvement of side field is found with the thicker shield thickness and higher saturation magnetization. The maximum side field is spatially and time dependent during the dynamic writing process. The side field reaches the maximum around the transitions, where the write field from the main pole is at the minimum. In addition, the write current rise-time effect on the side field is investigated.
    IEEE Transactions on Magnetics 11/2012; 48(11):3915-3918. · 1.42 Impact Factor
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    ABSTRACT: High resolution magnetic force microscopy (MFM) imaging with high coercivity probes on perpendicular magnetic recording (PMR) writers directly characterizes magnetic field contour for the writer main pole as well as its shields’ magnetic state. Evolution of write bubble and return field was analyzed by MFM imaging in dynamic phase detection scheme. Different write field components and their out of plane second order derivatives were calculated via finite element modeling. The MFM imaged write field distribution correlates well with the PMR writer out of plane write field component. Magnetic responses of the PMR writer main pole, trailing and side shields are quantified. The trailing and side shields exhibit complicated magnetic saturation behaviors comparing with the PMR writer pole. The side shield’s magnetic response is dependent upon its initial equilibrium state.
    Journal of Applied Physics 03/2012; 111(7). · 2.21 Impact Factor
  • Zhanjie Li, Daniel Z. Bai, Ed Lin, Sining Mao
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    ABSTRACT: We present a systematic study of write field asymmetry by using micromagnetic modeling for a perpendicular magnetic recording (PMR) writer structure. Parameters investigated include initial magnetization condition, write current amplitude, write current frequency, and initial write current polarity. It is found that the write current amplitude and frequency (data rate) are the dominant factors that impact the field asymmetry. Lower write current amplitude and higher write current frequency will deteriorate the write field asymmetry, causing recording performance (such as bit error rate) degradation.
    Journal of Applied Physics 03/2012; 111(7). · 2.21 Impact Factor
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    ABSTRACT: A computational study of the head field was presented in a specific head/media configuration, where the intermediate layer (IL) consists of the periodic magnetic columns and Ru columns. It was found that the head field increases with the thicker and wider magnetic columns. Micromagnetic simulation shows that this approach is to achieve a lower effective IL thickness for enhancing the write field by replacing some Ru grains with the columnar magnetic material without reducing the IL thickness.
    Journal of Applied Physics 03/2012; 111(7). · 2.21 Impact Factor
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    ABSTRACT: A systematic experimental study of the reverse overwrite (ReOVW) process in the shingled recording scheme has been conducted in conjunction with characterization of corresponding recording performances from recording heads with different geometries. It was found that there is no ReOVW reduction as the track density increases in a strict shingled recording fashion. Nonetheless, ReOVW is indeed slightly decreased from 300 to 700 kpi in a so-called one write shingled recording process. Overall our obtained data suggest that conventional magnetic recording technology might be able to extend all the way beyond an areal density of one Tbit/in2 by using the shingled recording scheme.
    Journal of Magnetism and Magnetic Materials 01/2012; 324:252-259. · 1.83 Impact Factor
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    ABSTRACT: This article introduces a four track erase band width (EBW) measurement derived from triple track test. The method writes the first center track to adaptively control the track pitch (TP) and the final center track to minimize the impact from system's positioning error during EBW measurement. It uses one-way dual-frequency scan on the neighboring magnetic write width (MWW) edges to further minimize the impact from system's positioning error. Experiment shows that measured EBW decreases with TP decreasing due to the side reading, cross-track field interaction, and edge noise suppression in perpendicular recording. Setting TP at 100% MWW is proposed for measuring “intrinsic” erase band (EB). The new method demonstrates significant improvement on test accuracy over the conventional methods, and can control EBW measurement variation range within 1 nm. With the new method, the relations of EBW with write revolutions, write current, write frequency, and adjacent track interference (ATI) were experimented and analyzed. The experiment result further validates the new method.
    IEEE Transactions on Magnetics 11/2011; · 1.42 Impact Factor
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    ABSTRACT: Written transition curvature in perpendicular magnetic recording is generally understood to result in increased transition jitter noise and degraded signal to noise ratio or byte error rate (BER) performance. For the shingle writing scheme, asymmetry or curvature in written tracks is considered inherent due to the erasure and track edge writing characteristics. It is proposed that such a track asymmetry is more prominent at high track density/smaller track pitch recording conditions. In this report we present spin stand experimental results to study the effect of the possible track asymmetry or curvature by shingle writing and reading back in different skews. By comparing shingle writing BER bathtub profiles in different writing skew conditions 0°, +/−2°, +/−4°, +/−6°, the effect of varying shingle track asymmetry and curvature is analyzed via subsequent skewed reading process. The shingle writing BER bathtub profiles as well as the read back amplitude cross track profile are generally symmetric upon one sided erasure at different track pitches. We found that the 0° skew writing and reading process provides both the maximum BER and amplitude.
    Journal of Applied Physics 03/2011; 109(7):07B740-07B740-3. · 2.21 Impact Factor
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    ABSTRACT: This paper presents a study of the degauss process of a perpendicular magnetic writer pole using micromagnetic modeling as a tool. The dynamics of the yoke and the pole-tip magnetization relaxation during degauss are studied in detail, and the remanent field of the pole tip is used as a metric to measure the effectiveness of each of the degauss methods. Various aspects of the degauss current waveforms are investigated, such as frequency, duration, initial current amplitude, amplitude decay envelope, and overshoot. Some alternative waveforms, i.e., unipolar reverse pulse degaussing, have also been examined. It has been found for many of the parameters to have an optimal range outside of which the degauss effectiveness and efficiency are degraded. Index Terms—Degauss, micromagnetic modeling, perpendicular magnetic writer, pole erasure, pole-tip remanence. VER since its introduction, perpendicular magnetic recording (PMR) has had an intrinsic characteristic—remanent erasure (also known as pole erasure, erase after write, or domain lockup) from the writer pole tip, which is caused by the remanentfield from the writer pole tip after the write current is turned OFF. This could potentially cause catastrophic hard disk drive failure due to damage to the servo or data signal. The root causes of remanent erasure have been studied extensively and various fixes have been proposed and implemented in the writer design, materials selection, and manufacturing processes in order to mitigate this issue [1]–[8]. In addition to all these efforts, one feature in the write preamplifier has played a crucial role in further enhancing the robustness of remanent erasure-free operation in the drive. The feature is known as degauss, which means applying a specially designed driving current waveform at the end of each writing operation to bring the writer pole into a demagnetized state to minimize the remanent field. Traditional sense of degauss is achieved by applying an ac field with a decaying amplitude envelope, such that the magnetization will follow a series of minor hysteresis loops and eventually reach a zero magnetization. In the case of degaussing a PMR writer, although the micromagnetic processes of each head are not necessarily identical to the macroscale hysteresis-loop behavior, the fundamental principles remain the same. Furthermore, the primary forms of the degauss current waveform used in modern PMR writer preamplifier highly resemble the simple ac ringdown shape in the traditional sense degauss process. As the writer pole’s dimension continues to shrink, the requirements on the effectiveness and efficiency of degauss also become more demanding. It is, therefore, important to understand the degauss processes of the writer pole and its interaction with various forms of degauss waveforms. In this paper, we
    IEEE Transactions on Magnetics 01/2011; 47(10):3407-3410. · 1.42 Impact Factor
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    ABSTRACT: An effort has been made on evaluating magnetic and electrical performances of perpendicular recording (PMR) components at high recording data rates (2.0-3.0 Gbit/s). In general, a fully functional PMR recording subsystem up to 3.0 Gbit/s has been demonstrated experimentally in spin-stand testing by using product recording heads, media, preamps, and channels. Particularly, we studied the influence of the novel pole structures of recording heads on the high data rate performances. The obtained experimental results show that the novel pole structure design of recording write head has some significant impact on the performance of the bit error rate, write ability, and other parametric data at high data rate situations.
    IEEE Transactions on Magnetics 01/2011; 47(10):2996-2999. · 1.42 Impact Factor
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    ABSTRACT: The response of a trailing shielded perpendicular magnetic recording (PMR) write head with a three-turn two-layer coils is calculated using a full micromagnetic large scale model including return pole and soft underlayer. We study the effect of PMR yoke length and write current overshoot on the dynamic response of the head at high frequency. In particular, recording performance parameter, nonlinear transition shift (NLTS) is simulated using micromagnetic media model. Shorter yoke length head improves NLTS as compared with longer yoke length. Measured NLTS has qualitative agreement with micromagnetic model results.
    IEEE Transactions on Magnetics 12/2008; · 1.42 Impact Factor
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    ABSTRACT: This paper presents finite-element analysis of the stress-induced magnetic anisotropy in perpendicular magnetic recording (PMR) heads developed during fabrication. The intrinsic stresses of various thin films used in the heads were measured using wafer curvature, while the stress in the yoke region was measured using X-ray diffraction. The measured stresses are provided. The initial strains due to the intrinsic stresses in the films were modeled using equivalent thermal strains and the calculations were verified by correlations with the stress measurements. Finite-element simulations were performed for calculating the stresses in full PMR heads. Detailed simulation procedures are described. Computed results are presented on how sensitive the magnetic anisotropy in the write pole are to the pole tip length and the slider lapping and the compressive stress in overcoat as well as the ambient temperature
    IEEE Transactions on Magnetics 07/2007; · 1.42 Impact Factor
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    ABSTRACT: Commercial hard-drive products utilizing perpendicular magnetic recording technology have recently been announced and introduced. In this paper, we review key magnetic materials characteristics and wafer process attributes in fabricating perpendicular write heads. It becomes increasingly important for write-head materials to possess not only high magnetic moment, but also optimal coercivity, remanence, anisotropy Hk, magnetostriction, and stress in order to meet head performance and reliability requirements. Advanced materials and film architectures discussed in this paper resulted in a significantly improved performance margin, including reduced pole erasure; hence enabling higher recording densities. Novel wafer-processing techniques are required for fabrication of 3-D pole features with controlled shape, and with critical dimensions of less than 150 nm. The advance in wafer process has been driven by rapidly decreasing trackwidth, as well as by the evolving head architecture from unshielded rectangular pole to shielded trapezoidal pole
    IEEE Transactions on Magnetics 03/2007; · 1.42 Impact Factor
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    ABSTRACT: Return field-induced partial erasure (RFPE) in trailing-edge shielded perpendicular writers has been studied, both by modeling and by experiments. For a given head-media combination, the return field underneath the trailing shield increases with increasing write current. Once exceeding a certain threshold, it will cause partial erasure of the bits that have just been written by the main pole. Recording performance, such as reverse overwrite, spectral signal-to-noise ratio, and bit-error rate are all found degraded at high write currents, due to RFPE. Design optimization of both head and media together is needed, in order to maximize the advantage of a trailing-edge shielded pole head and minimize the impact of RFPE
    IEEE Transactions on Magnetics 03/2007; · 1.42 Impact Factor
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    ABSTRACT: Micromagnetic modeling of adjacent track squeeze in perpendicular magnetic recording, and its effect on track density was studied as a function of skew angle. Based on amplitude 747 curve of track squeeze and reader off track capability of 10%, a loss of 6.5% of track density was predicted by micromagnetic model for head of 150 kTPI track density. The analysis also shows that erase width is smaller than magnetic write width and erase width formula based on pole geometry overestimates the effect of track density loss due to skew angle
    IEEE Transactions on Magnetics 11/2006; · 1.42 Impact Factor
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    ABSTRACT: Trailing edge shielded pole head is the primary writer design for perpendicular recording (PMR) with soft under layer (SUL), due to its higher field gradient compared to a single pole head. In this paper, both by modeling and by experiments, with certain combinations of head, media, SUL, and write current, the return field induced partial erasure (RFPE) will occur, causing recording performance degradation was demonstrated. Therefore, head, media and system optimizations are necessary in order to avoid this issue. The finite element modeling results of the on-track Stoner-Wohlfarth field for three write currents are obtained. It can be seen that both the recording field from the main pole (MP) and the return field increase with increasing write current. Micromagnetic write modeling has also been done, which showed written magnetization patterns consistent with the experimental data.
    Magnetics Conference, 2006. INTERMAG 2006. IEEE International; 06/2006
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    ABSTRACT: We studied the remanent field from perpendicular writer pole tips by micromagnetic modeling. The pole tip remanence has two causes: 1) the residual flux from the yoke due to undesirable material properties and/or yoke design and 2) the geometry of the pole tip. An optimal yoke design for low remanence includes a wide and short yoke with relatively shallow yoke flare angle around 30° that favors transverse major domains. Horizontal anisotropy in the yoke and the pole tip also helps suppress the vertical magnetization, mainly in the yoke, which yields less residual flux into the pole tip, hence lower remanence. The remanent field is also a strong function of the throat height, i.e., the pole tip length, due to the shape anisotropy effect. Micromagnetic modeling shows that multilayer lamination, both in the yoke and in the pole tip, is an effective way to achieve low remanence. It also eliminates the sensitivity of remanence to the throat height. To improve the robustness of the remanence against the stray field, an antiferromagnetic coupling (AFC) between the lamination layers may also be necessary. Our results apply to both single pole heads and shielded pole heads.
    IEEE Transactions on Magnetics 04/2006; · 1.42 Impact Factor
  • Jian-Gang Zhu, D. Bai
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    ABSTRACT: A novel perpendicular write head design is proposed. In this design, an active stress is applied in synchronization with the write current waveform by applying voltage across electrostrictive material adjacent to the head yoke and pole tip. With sufficient stress magnitude, a planar magnetic anisotropy can be induced perpendicular to the head air-bearing surface via the natural magnetostrictive effect in high moment magnetic materials. The dynamically induced magnetic anisotropy, synchronized to the write current transition, can significantly enhance the write efficiency and, more importantly, improve cross-track write field gradient and ensure zero head remanence.
    IEEE Transactions on Magnetics 08/2004; · 1.42 Impact Factor
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    ABSTRACT: Using a dynamic micromagnetic write model, we have studied the spectral signal-to-noise ratio (SNR) performance of perpendicular media using a ring head without soft underlayer (SUL) and compared it with that of longitudinal media. With optimized exchange coupling of the media, the ring head shows good writability on the perpendicular media without SUL. At a linear density of 600 kfci, the ring head/perpendicular media essentially has no SNR gain over the longitudinal media, whereas going to 1000 kfci, the perpendicular media has an advantage of about 4 dB over the longitudinal. Comparison of the ring head (RH) without SUL was also made with the monopole head (MPH) and shielded pole head (SPH), both with SUL. RH and MPH both have about the same Stoner-Wohlfarth field gradient and SNR performance, whereas at the SNR plateau, SPH yields an SNR about 4-5 dB higher than RH and MPH, thanks to the high field gradient.
    IEEE Transactions on Magnetics 08/2004; · 1.42 Impact Factor

Publication Stats

92 Citations
40.84 Total Impact Points

Institutions

  • 2008–2011
    • Western Digital Corporation
      Irvine, California, United States
  • 2002–2006
    • Carnegie Mellon University
      • Department of Electrical and Computer Engineering
      Pittsburgh, PA, United States