H. Do

HGST, A Western Digital Company, San Jose, California, United States

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Publications (32)52.66 Total impact

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    ABSTRACT: In this paper, some issues concerning the reliability of heat-assisted magnetic recording (HAMR) media are highlighted. The large roughness of the grain structure originates from a surface energy mismatch between the FePt grains and the graphene-like segregant material. A simple roughness model, based on interfacial energies, is proposed that can quantitatively predict media grain structure and roughness. The thermal behavior of the disk lubricant is reviewed both experimentally as well as theoretically using molecular dynamics (MD) and density functional theory (DFT). The lubricant film can be subjected to evaporation and oxidation, both of which follow an Arrhenius reaction rate. MD also predicts that the disk carbon overcoat can undergo structural changes under thermal transient exposure in the nanosecond time frame, and Raman imaging performed on a disk zone that was HAMR written shows small but unequivocal changes, consistent with an increase in carbon ${rm sp}^{2}$ cluster size.
    IEEE Transactions on Magnetics 03/2014; 50(3):137-143. · 1.21 Impact Factor
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    ABSTRACT: We analyze the magnetic design for different generations of perpendicular magnetic recording (PMR) media using resonant soft x-ray small angle x-ray scattering. This technique allows us to simultaneously extract in a single experiment the key structural and magnetic parameters, i.e., lateral structural grain and magnetic cluster sizes as well as their distributions. We find that earlier PMR media generations relied on an initial reduction in the magnetic cluster size down to the grain level of the high anisotropy granular base layer, while very recent media designs introduce more exchange decoupling also within the softer laterally continuous cap layer. We highlight that this recent development allows optimizing magnetic cluster size and magnetic cluster size distribution within the composite media system for maximum achievable area density, while keeping the structural grain size roughly constant.
    Applied Physics Letters 01/2013; 103(11):112403-112403-4. · 3.52 Impact Factor
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    ABSTRACT: By analyzing the magnetic cluster size of various media, we have investigated the variation of inter-granular exchange coupling with grain size. With the reduction of the grain size below 8.0 nm, higher inter-granular exchange coupled media showed a large increase in magnetic cluster size, while lower exchange media did not show as dramatic a cluster size change. This cluster size “knee” position was consistent with reduction in signal-to-noise ratio. To utilize small grain media for high density recording, it is desirable to push this “knee” position toward smaller grain size.
    IEEE Transactions on Magnetics 11/2012; 48(11):3185-3187. · 1.21 Impact Factor
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    ABSTRACT: Polarized small-angle neutron scattering has been used to measure the magnetic structure of a CoCrPt – SiO <sub> x </sub> thin-film data storage layer, contained within a writable perpendicular recording media, at granular (<10 nm ) length scales. The magnetic contribution to the scattering is measured as the magnetization is reversed by an external field, providing unique spatial information on the switching process. A simple model of noninteracting nanomagnetic grains provides a good description of the data and an analysis of the grain-size dependent reversal provides strong evidence for an increase in magnetic anisotropy with grain diameter.
    Applied Physics Letters 10/2010; · 3.52 Impact Factor
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    ABSTRACT: This paper demonstrates that electrochemical impedance spectroscopy combined with atomic force microscopy analysis can successfully characterize the coverage ability of an overcoat on perpendicular magnetic recording media. Rougher media, brought about by lower surface energy oxide segregants, can adversely impact the overcoat integrity. The role of the capping layer, and its ability to somewhat planarize the overall structure, is also discussed.
    IEEE Transactions on Magnetics 02/2010; · 1.21 Impact Factor
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    ABSTRACT: Polarized small-angle neutron scattering has been used to measure the local magnetic structure of writable thin-film perpendicular media with a granular CoCrPt-SiO <sub>x</sub> recording layer. By exploiting the cross terms between the nuclear and magnetic scattering, we are able to probe simultaneously both the grain structure and the subgranular magnetic structure of the recording layer, which has a thickness of only 15 nm and which is embedded within a full perpendicular media structure including soft underlayer. Two models are used to analyze the data, one analytical and the other a numerical approach based on transmission electron microscopy measurements of the grains. Both models show that the recording layer consists of ferromagnetically ordered core regions that are smaller in extent than the corresponding grains and allow a direct, quantitative comparison of these two length scales.
    Journal of Applied Physics 10/2009; · 2.19 Impact Factor
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    ABSTRACT: We report the magnetic properties of perpendicular media with a layered structure in which a high anisotropy, segregated, granular CoCrPt-oxide base layer is capped by a lower anisotropy CoCrPt-based film. Anisotropy field (H<sub>k</sub>) data show that for the thickness of oxide media studied here, the measured value of H<sub>k</sub> remains constant as cap thickness increases. This provides strong evidence that the anisotropy of the composite grain is controlled by the hard oxide layer and is not a simple average of the anisotropy of the oxide and cap layers. The reversal mechanism is explored by determining the angle dependent switching as a function of cap thickness. In the absence of a cap layer, the media show a Stoner–Wohlfarth-like reversal which becomes more Kondorsky-like [1/ cos (θ)] when a critical cap thickness is reach, which we interpret as indicating greater lateral exchange coupling.
    Journal of Applied Physics 05/2008; · 2.19 Impact Factor
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    ABSTRACT: A perpendicular recording system that allows areal densities beyond 200 Gbit/in2 has been designed and tested to investigate the major challenges in perpendicular magnetic recording. The integrated write/read head has a trailing shield to improve the write head field gradient and a conventional CIP–GMR reader. The medium is a low-noise CoPtCr-based oxide medium with a CoTaZr soft underlayer. On track byte error rates at ∼ 50 Mb/s are better than 10−4 at ∼1000 kbpi. Using a 15% off-track criterion at 10−2 byte error rate, track densities between 200–240 ktpi are realized, yielding areal densities of 210–233 Gbit/in2. High-resolution magnetic force microscopy (hrMFM) has been employed to investigate the write characteristics of these heads with improved cross-track resolution. Using a quantitative analysis method, many parameters, such as transition curvature and transition width, are estimated from the hrMFM image. Significant transition curvature is found, which increases the width of the read head response to a transition, T50, by 2–3 nm.These results give insights into the recording physics of perpendicular recording and in particular point out improvements required for achieving even higher areal densities.
    Journal of Magnetism and Magnetic Materials 08/2006; · 2.00 Impact Factor
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    ABSTRACT: We have evaluated the magnetic properties and recording potential of longitudinal exchange spring recording media. The media structure consists of a low anisotropy Co Cr X spring layer ferromagnetically coupled to a high anisotropy CoPtCrB hard layer via a CoRu interlayer. The interlayer exchange coupling is tuned by varying the CoRu layer thickness. The short-time coercive field (H<sub>c0</sub>) as well as the energy barrier (K<sub>u</sub>V) of the structure are furthermore tuned by varying the thickness of the spring layer. We show that by changing the Co Cr X layer thickness from 1 to 4 nm , H<sub>c0</sub> can be decreased by a factor of 2 while the energy barrier can be increased by up to 50% hence improving the stability of the overall structure. More importantly, it is possible to reduce the switching field distribution width leading to an improved overall signal to noise ratio in this Co Cr X thickness range.
    Journal of Applied Physics 05/2006; · 2.19 Impact Factor
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    ABSTRACT: Two media, A and B, are discussed to illustrate the media requirements and properties for the 230 Gb/in2 demonstration. A and B had similar layer structures except that the recording layers had different oxide compositions and that the exchange break layer thicknesses were 21 and 13 nm, respectively. They had similar coercivity and intrinsic switching field but showed large difference in writability. Consequently, even though they achieved similar performance for heads down to a track width of 150 nm, medium A could not be saturated adequately for heads with a track width of 120 nm. On the other hand, medium B possessed the desirable properties for high density recording and formed the base for the media that were used in the 230 Gb/in2 demonstration.
    Journal of Applied Physics 04/2006; 99(8):08E712-08E712-3. · 2.19 Impact Factor
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    ABSTRACT: To characterize the magnetodynamic properties of a perpendicular recording system, consisting of a pole head, a recording layer, an exchangebreak layer, and a soft underlayer (SUL), we have built a test vehicle by depositing either the SUL, or the full medium stack on the air-bearing surface of the recording head. Using ultra-high-speed scanning Kerr microscopy, the intrinsic switching characteristics of the SUL and the impact of the recording layer on the former has been measured. One important feature found is the formation of vortices during reversal which are much larger than the pole area as well as evidence for spin wave excitation in the SUL. Both fast write current steps, and complex write current waveforms have been applied and significant differences of the magnetic responses are observed. Though the Kerr signal probes the temporal evolution of the SUL surface magnetization, it is possible to determine the onset of the recording layer switching by taking its magnetostatic coupling to the SUL into account. The magnetization state of the recording layer also impacts the switching speed of the SUL. The noise characteristics of the Kerr signal gives information about nonreproducible magnetization processes in the SUL.
    IEEE Transactions on Magnetics 03/2006; · 1.21 Impact Factor
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    ABSTRACT: To explore recording head challenges for perpendicular recording at 200 Gb/in<sup>2</sup> and beyond, the design, fabrication and performance of narrow track dual-element heads were studied using an ABS trailing shield writer design and a conventional CIP-GMR reader design. Parametric recording tests of these heads on low noise CoCrPt/SUL media show that, with the trailing shield design, good writability and low disk transition jitter around 2.5 nm were achieved at narrow write trackwidths down to 120 nm. In addition, peak-to-peak signal amplitudes around 1 mV and T<sub>50</sub> widths around 28 nm were also achieved at read trackwidths around 60 nm. The areal density potential of these heads was studied using a PRML channel at ∼50 MB/s data rate. Results show linear densities around 1000 Kbpi at ontrack byte error rates of 10<sup>-4</sup>, and track densities around 200-240 ktpi using a criterion of 15% offtrack to trackpitch ratio. In all, areal densities of 210-230 Gb/in<sup>2</sup> were achieved with these head and disk components.
    IEEE Transactions on Magnetics 03/2006; · 1.21 Impact Factor
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    ABSTRACT: We have investigated the magnetic reversal and recording properties of layered ferromagnetically exchange coupled (or exchange spring) structures made of a hard layer and a soft layer. The hard layer is a longitudinal antiferromagnetically-coupled (AFC) media which is exchange coupled to a soft CoCrX layer via a CoRu interlayer. We find that optimizing the CoCrX and CoRu layer thicknesses is critical to achieve optimum improved writability while maintaining the media signal to noise ratio (SNR). Furthermore although we successfully decrease substantially the media coercive field (Hc) of these exchange spring structures, the energy barrier K<sub>u</sub>V appears unchanged. Finally, we show that these structures have the potential of improved SNR.
    IEEE Transactions on Magnetics 11/2005; · 1.21 Impact Factor
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    ABSTRACT: Minimum soft underlayer thickness for adequate flux conduction was established through experiment and calculation. Methods for suppressing stripe domains were discussed. Characteristics of spike noise in laminated soft underlayers were described, and the impact of spike noise on error rate was assessed using a software channel. The correlation between spike noise and errors was weak.
    IEEE Transactions on Magnetics 11/2005; · 1.21 Impact Factor
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    ABSTRACT: We determine the lateral correlation length L of the magnetization reversal process in thin magnetic films. For this purpose, we have devised an experimental system, which allows tuning of the effective disorder level by means of the local bias field distribution acting upon the magnetic film. This disorder is hereby counteracting the intergranular exchange coupling within the film, which tends to synchronize the reversal process. We observe a sharpening of the switching field distribution upon narrowing the exchange bias field distribution. Quantitative comparisons between these two quantities allow the determination of L without the need for adjustable parameters.
    Journal of Applied Physics 06/2005; · 2.19 Impact Factor
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    ABSTRACT: Recording performance as a function of write current is studied for single-pole and trailing-shield heads. Optimal write current is not obvious from saturation curves. Media noise, jitter, and error rate give better guidance. For a single-pole head, there is an optimal write current, beyond which, the field strength may continue to increase, but field gradient degrades leading to higher noise. For trailing shield achieved by reverse-flying longitudinal heads, the field magnitude and gradient from the main pole continue to improve with increasing write current. However, the return pole erasure also increases. There is a critical current, above which, partial erasure from the return pole leads to excessive noise. Finite-element modeling of these two types of write poles has been performed to confirm and understand the experimental findings.
    Journal of Applied Physics 06/2005; · 2.19 Impact Factor
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    ABSTRACT: Lamination of multiple isolated magnetic layers has been shown to be an effective method to significantly increase signal-to-noise ratio in longitudinal media. These laminated media, however, are accompanied by low overwrite and wide magnetic pulse width, mainly as a result of poor writing of the bit transitions in the magnetic layer further away from the head and an offset in the transition position in the multiple magnetic layers resulting from head field spacing loss. We have demonstrated that the transition writing and transition alignment in the multiple magnetic layers of the laminated antiferromagnetically coupled (AFC) media can be optimized by adjusting the magnetic anisotropy of the relevant magnetic layers to compensate for the reduction of the head field magnitude with spacing. Such optimization results in significant improvements in media recording performance, leading to successful application of this medium technology. In this paper, we will highlight some of these improvements and discuss our approaches to further improve the recording performance by reducing the thicknesses of the magnetic layers and the lamination spacer layer in the laminated AFC film stack and by introducing additional elements in the magnetic layer.
    IEEE Transactions on Magnetics 03/2005; · 1.21 Impact Factor
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    ABSTRACT: A study has been conducted on the characteristics of the reversal dynamics for a perpendicular recording system consisting of a pole head, a recording layer, and a soft under layer (SUL) using time-resolved Kerr microscopy. It was found that the local step response as well as the 2-D magnetization distribution of the SUL in the pole area during reversal depend on the initial magnetization state of the recording layer. This phenomenon can cause pattern-dependent nonlinear timing shifts of the write process. The precessional frequency of the SUL response also differs for the two initial states. In the 2-D time-slices of the magnetization reversal we find evidence for the generation of spin wave modes which can limit the achievable data rate.
    Journal of Magnetism and Magnetic Materials 02/2005; · 2.00 Impact Factor
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    ABSTRACT: High-resolution magnetic force microscopy (hrMFM) has been used to investigate the write characteristics of forward and reverse flying heads on perpendicular recording media with soft underlayers. hrMFM has the advantage that it provides much better resolution than the read element of a head, particularly in the cross-track direction. Using a quantitative analysis method, many parameters, such as transition position jitter, transition width and signal-to-noise ratio are estimated from the hrMFM image. Furthermore, these parameters, which can also be measured using a spinstand, are compared to the micromagnetic properties of the transitions, such as track curvature, transition width and the local noise associated with transitions. The results give insights into the recording physics and, in particular, in the write process of shielded and un-shielded write heads in perpendicular recording.
    Journal of Magnetism and Magnetic Materials 02/2005; · 2.00 Impact Factor
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    ABSTRACT: We have utilized antiferromagnetically coupled bilayer structures to magnetically tune hysteresis loop properties. A key element of this approach is the nonoverlapping switching field distribution of the two magnetic layers, which allows for the measurement of magnetic reversal of the soft Co film at low fields while keeping the magnetization state of the hardmagnetic CoPtCrB film unperturbed. The effective disorder level in the Co film can be tuned easily by applying an appropriate field sequence to set the desired magnetization state of the CoPtCrB layer prior to hysteresis loop measurements of the Co film. This tuning procedure is fully reversible, temperature independent, and highly effective. © 2004 American Institute of Physics.
    Journal of Applied Physics 05/2004; 95(11):6660-6662. · 2.19 Impact Factor