H. Do

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

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Publications (21)37.15 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 01/2014; 50(3):137-143. · 1.42 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; · 1.42 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; · 2.21 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; · 2.21 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; · 2.21 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. · 2.21 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; · 1.42 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 01/2006; · 2.00 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; · 1.42 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; · 2.21 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 01/2005; · 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. · 2.21 Impact Factor
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    ABSTRACT: The track width performance of giant magnetoresistive (GMR) read heads defined by e-beam lithography was studied in a series of recording tests. Results on longitudinal media showed slow reductions of magnetic versus physical read widths below 100 nm due to spacing loss effects and substantial microtrack widths. These findings led to explorations of different measurement techniques to address the microtrack width issue. Results on perpendicular media showed distinctly narrower magnetic read widths consistent with modeling results assuming negligible microtrack widths. This read width narrowing effect could be another potential advantage for perpendicular media in high density recording. Finally, various parametric effects on track width performance were also explored and compared between perpendicular and longitudinal media.
    IEEE Transactions on Magnetics 02/2004; · 1.42 Impact Factor
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    ABSTRACT: The role of intergranular exchange coupling was experimentally investigated in a coupled granular/continuous (CGC) perpendicular medium structure. By changing the number of Co/Pt bilayers in the continuous layer, the degree of exchange coupling can be systematically controlled. The switching field distribution (SFD) was roughly estimated from Kerr loops. The SFD became narrower for media with a thicker continuous layer. Using a head with a single-pole writer and an 80 nm gap giant magnetoresistance reader, reverse dc erase noise was measured as a function of the reverse dc erase current. The media showed a clear positive noise peak with an increase in reverse erase current. Narrowing of the SFD was reflected by narrowing of the noise peak width. However, the peak value of the medium’s noise also increased with an increase in the thickness of the continuous layer. Although exchange coupling increases the average magnetic cluster size in the demagnetization condition, CGC media with moderate exchange coupling showed no degradation of the signal to noise ratio compared to the base granular medium. © 2003 American Institute of Physics.
    Journal of Applied Physics 05/2003; 93(10):7855-7857. · 2.21 Impact Factor
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    ABSTRACT: In this paper, we show in this study that it is possible to vary PW50 by up to 20% to values below 90 nm and have a thermally stable media that produce an SNR gain of ∼3dB at 700 KFCI.
    01/2003;
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    ABSTRACT: In this study, the magnetic layer was nominally the same, only the thickness of SUL, and EBL were varied. The EBL should be thin to minimize the write gap but thick enough to break the direct exchange coupling between the SUL and the recording layer. We explored the thickness effects of EBL and SUL on recording using multilayer structures.
    Magnetics Conference, 2003. INTERMAG 2003. IEEE International; 01/2003
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    ABSTRACT: The thermal stability of coupled granular/continuous (CGC) perpendicular media is supported by fundamental modeling and experimental results, including spin-stand testing. By incorporating the interlayer exchange coupling into the model, the simulation result suggests that the CGC structure is capable of achieving the energy barrier of K<sub>u</sub>V/k<sub>B</sub>T required for 1 Tbit/in<sup>2</sup> recording density. To demonstrate the CGC approach, we investigate a new class of CGC perpendicular media consisting of a Pt-rich CoPtCr layer with poor Co-Cr phase segregation and a thin Pt layer. The addition of these layers improves the nucleation field of the CoCr<sub>18</sub>Pt<sub>12</sub> medium from +420 to -600 Oe and the thermal decay of the output is reduced from 2.23% to 0.10% per decade. Unity squareness was obtained by using a thin Pt capping layer and resulted in a small decay rate of 0.21% per decade. The new CGC media showed no degradation of SNR compared to the base granular medium. Similar to CGC media utilizing a multilayer capping structure, the CGC medium with a Pt-rich CoPtCr or Pt capping structure improved the thermal stability without compromising SNR. The simplicity of these new CGC structures also greatly simplifies the deposition process.
    IEEE Transactions on Magnetics 10/2002; · 1.42 Impact Factor
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    ABSTRACT: Fabrication, magnetic properties, and read/write characteristics of coupled granular/continuous (CGC) perpendicular media are described. The media consist of continuous Co/Pt multilayers, which have a strong in-plane exchange coupling, and a granular CoCrPt layer on top of a soft magnetic underlayer. These continuous and granular layers are magnetically coupled, thus the switching field distribution becomes effectively narrower. Recording measurements using a single-pole writer and a giant magnetoresistive reader show substantial improvement in the head output and resolution of the CGC medium, compared to the plain granular medium. Transition jitter and medium noise are reduced in the CGC media. As a result, the CGC media have 3–5 dB higher signal-to-noise ratio (SNR) and better thermal stability than the plain granular medium. The concept of CGC perpendicular media is a viable way to achieve both the thermal stability and medium SNR requirements for high-density recording. © 2002 American Institute of Physics.
    Journal of Applied Physics 05/2002; 91(10):8055-8057. · 2.21 Impact Factor
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    ABSTRACT: Perpendicular medium with an average grain size of 8.2 nm have been produced by using a-CoCrPt/Ti/NiAl tri-layer structure. The signal-to-noise ratio for the tri-layer medium is 2-7 dB higher than CoCrPt/Ti bi-layer medium at low deposition temperatures. The medium noise is dominated by dc noise and exhibits little transition noise for densities less than 500 kFCI. The ultra-small grain size will improve the performance of perpendicular media for future high-density recording applications
    IEEE Transactions on Magnetics 08/2001; · 1.42 Impact Factor