M. Zheng

Concordia University–Ann Arbor, Ann Arbor, Michigan, United States

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Publications (34)52.36 Total impact

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    ABSTRACT: This paper compares the images obtained by superparamagnetic and permanent magnetic force microscopy (MFM) tips, that allows us to explain the issues related to the frequency double in some of domain images of the recording media. This paper also investigates the domain structures of high density recording bits (up to 1100 kfci) written on ac and dc-erased CoPtCrO perpendicular magnetic recording (PMR) media with permanent magnet MFM tips under ambient conditions.
    Magnetics Conference, 2006. INTERMAG 2006. IEEE International; 01/2006
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    ABSTRACT: An anisotropy enhanced dual magnetic layer structure is proposed to improve the recording performance of perpendicular media. The initial layer is made of low noise Co-Cr-Pt-O layer and the top layer is made of higher anisotropy Co-Cr-Pt-B layer. A higher nucleation field than the single layer structures is obtained for the dual magnetic layer media. By optimizing the layer structure and properties, better signal-to-noise and lower adjacent track erasure can be achieved for the dual magnetic layer design over single magnetic layer media.
    IEEE Transactions on Magnetics 11/2005; · 1.42 Impact Factor
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    ABSTRACT: A systematic investigation of the structural and recording characteristics of CoPtCrO composite media with various Co alloys as well as continuous TbFeCo capping layers has been performed. For the media with a Co-alloy capping layer (M<sub>s</sub>∼900 emu/cm<sup>3</sup>), the transition jitter remains unaffected and dc noise is greatly reduced. However, for the media with low M<sub>s</sub> capping layer or continuously exchange coupled TbFeCo capping layer, transition jitter is increased. Magnetically continuous but structurally discontinuous Co-alloy capping layers grown onto well-isolated Co grains can reduce dc noise, while maintaining low transition noise.
    IEEE Transactions on Magnetics 11/2005; · 1.42 Impact Factor
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    ABSTRACT: We discuss a method of ac-demagnetizing an entire disk using permanent magnets and compare the demagnetized states of perpendicular media with different levels of intergranular exchange. The method consists of applying a large field perpendicular to the media recording layer anisotropy direction. This bulk ac-erasure technique provides a background noise that is lower than that which can be achieved by ac-erasing with a write head. Highly exchange-coupled media exhibit low dc noise but increased ac-erased background noise. Magnetic force microscopy data show increased magnetic roughness in the ac-erased state with exchange-coupling strength. For the media with least coupling the dc noise is similar to the ac noise.
    IEEE Transactions on Magnetics 11/2005; · 1.42 Impact Factor
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    ABSTRACT: The permeability of the magnetic soft underlayer (SUL) was systematically varied through SUL structure change, and its effect on wide area adjacent track erasure (ATE) and media overwrite (OW) was studied. Results suggest that ATE decreases with SUL permeability, and it involves only a finite thickness of the SUL. OW increases with SUL permeability as well as SUL thickness. In addition, ATE is less affected by magnetic spacing than OW. Antiparallel coupled SUL (APS) improves ATE. A double-APS structure with a permeability gradient is proposed to optimize OW and ATE performance.
    IEEE Transactions on Magnetics 11/2005; · 1.42 Impact Factor
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    ABSTRACT: A systematic investigation of the magnetic and recording characteristics of CoPtCrO media with different Co grain magnetic anisotropy orientation dispersion (AOD) is performed. AOD is characterized by Δθ<sub>50</sub> of the x-ray diffraction Co peak. AOD was systematically varied between 3.3° and 10.5° by varying the growth condition of Ru layer. With increasing AOD, more grain-boundary segregation occurs, resulting in reduced exchange coupling. Micromagnetic simulation and experimentally measured time dependence of coercivity indicate that magnetic switching of well-isolated Co particles is affected by exchange coupling strength as well as Co AOD. Higher coercivity, narrower switching field distribution, and good thermal stability are achieved with lower Co AOD. Reduced Co AOD leads to a substantial decrease of both transition jitter and dc noise.
    Journal of Applied Physics 06/2005; · 2.21 Impact Factor
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    ABSTRACT: The effect of magnetic "preconditioning" on the recording performance of perpendicular media is investigated. Furthermore, the dependence of the magnetic write width (MWW) of shielded-pole heads (SPH) on soft-underlayer (SUL) type and thickness (t<sub>SUL</sub>) and the recording performance of perpendicular media with thin SULs are examined. The MWW dependence on SUL structure is influenced by the pole-to-trailing shield spacing. For a wide-gap (∼80 nm) SPH, thick single-layer, SULs are preferred. For a narrow-gap (∼50 nm) SPH, MWW is less sensitive to the SUL type. For both narrow and wide-gap SPH and for media with an antiferromagnetically-coupled SUL, MWW is reduced for t<sub>SUL</sub><100 nm. Comparable performance is achieved relative to media with t<sub>SUL</sub>∼150 nm.
    IEEE Transactions on Magnetics 03/2005; · 1.42 Impact Factor
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    ABSTRACT: In this work, we discuss a method of ac-erasing an entire disk using permanent magnets and compare the resulting magnetic states of perpendicular media with different levels of intergranular exchange. Co-Cr-Pt-O alloy media were prepared by reactive sputtering.
    01/2005;
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    ABSTRACT: A systematic investigation of the structural, magnetic, and recording properties of CoPtCrO media with different intergranular exchange coupling and anisotropy orientation dispersion is performed. Reduced exchange coupling between grains results in lower media transition jitter and nonlinear transition shift. By narrowing the Co anisotropy orientation dispersion, a substantial reduction of both transition and DC noise is achieved. Media with high squareness, coercivity, nucleation field, low exchange and narrow Co c-axis dispersion have been fabricated. Such media exhibit excellent recording performance and are promising for high-density perpendicular recording.
    Journal of Magnetism and Magnetic Materials 01/2005; · 2.00 Impact Factor
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    ABSTRACT: The effect of the soft underlayer (SUL) permeability on adjacent track erasure (ATE) as well as on the recording performances of the media was investigated and verified. The 3 structures used were: single layer CoZrTa, and the antiferromagnetically coupled SULs (APS) CoZrTa/Ru/CoZrTa and CoZrTa/Ru/Co/CoZrTa. All samples were deposited on NiP-plated AlMg disk substrates. The 240 tracks were written once with 45 mA at 60 kfci and the stressing track was written for 32,000 revolutions with 55 mA at 360 kfci. Similar to previous observations, the APS structure largely suppressed the ATE. It was found that the total amount of flux an SUL is capable of conducting does not correlate well with ATE. However, ATE correlates well with permeability, suggesting that material property alone determines ATE.
    01/2005;
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    ABSTRACT: The effect of oxygen incorporation on the crystallographic, magnetic, and recording performance of perpendicular magnetic recording (PMR) oxide media was investigated. The media were prepared by dc-magnetron sputtering of CoCrPt-SiO<sub>2</sub> targets in an Ar/O<sub>2</sub> gas mixture. X-ray photoelectron spectroscopy (XPS) detected Cr-O peaks in the sputtered film, whereas no strong evidence of SiO<sub>2</sub> is seen. Moderate oxygen incorporation in the film (∼15 at%) promotes Cr-O formation in the grain boundary and results in a dramatic increase of coercivity H<sub>c</sub> and signal-to-noise ratio (SNR). However, as the O<sub>2</sub> content is further increased, oxide incorporates into the core of the grains, resulting in decreased H<sub>c</sub>, magnetization and SNR.
    IEEE Transactions on Magnetics 08/2004; · 1.42 Impact Factor
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    ABSTRACT: A promising soft magnetic under layer (SUL) structure consisting of anti-parallel coupled soft layers (APS) is investigated to improve the performance of high-density perpendicular recording media. CoCrPt-O recording media with average grain diameter ∼6 nm, coercivity of 5 kOe and nucleation field of -2 kOe is fabricated on both conventional SUL and APS. Wide-area adjacent track erasure (ATE) is observed for conventional SUL, which increases at lower interlayer spacing (t<sub>IL</sub>). APS structure enables the suppression of ATE even up to t<sub>IL</sub>=10 nm. Moreover, APS leads to reduced dc erase noise and improved signal to noise ratio particularly for SUL thicknesses <100 nm.
    IEEE Transactions on Magnetics 08/2004; · 1.42 Impact Factor
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    ABSTRACT: In this paper, we address important issues on the performance improvement of perpendicular magnetic recording (PMR) media, such as noise reduction by adjusting the soft magnetic underlayer (SUL) properties and recording layer and intermediate layer modifications for optimum signal-to-noise (SNR) performance. The permeability of the SUL plays a significant role in writability and SNR. Proper pinning of the SUL should be considered for reducing spike noise. A systematic comparison of alloy and oxide granular media shows that optimized oxide media can provide both better SNR and good thermal stability and is promising for high-density recording.
    IEEE Transactions on Magnetics 08/2003; · 1.42 Impact Factor
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    ABSTRACT: The recording performance of perpendicular media with amorphous CoZrNb, nanocrystalline NiFeNb, and exchange-pinned FeMn/NiFeNb soft magnetic underlayers (SULs) has been studied. A method has been proposed to quantify the spike noises from the SUL. We found that media with higher permeability NiFeNb SULs showed 4 db higher signal-to-noise ratio than media with lower permeability CoZrNb SULs. The exchange-pinned NiFeNb SUL suppresses spike noise and offers good recording performance. Furthermore, the recording frequency dependence of dynamic coercivity (H<sub>c</sub>) was strongly affected by the use of different SULs. The increase in dynamic H<sub>c</sub> with linear density is negligibly small for media with NiFeNb SULs, whereas dynamic H<sub>c</sub> sharply increases with increasing density for media with CoZrNb SULs. © 2003 American Institute of Physics.
    Journal of Applied Physics 06/2003; · 2.21 Impact Factor
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    ABSTRACT: A recording density of 130 Gb/in<sup>2</sup> was achieved using thermally stable conventional CoCrPtB longitudinal media. The high in-plane orientation ratio (OR) of the media resulted in an excellent recording performance due to a narrow switching field distribution as well as a high thermal stability caused by narrow energy barrier distribution. The low noise is attributed to the fully isolated fine grains with a narrow size distribution. A good in-plane c axis crystallographic texture was achieved by using an optimum multilayered structure of underlayer and magnetic layers. A detailed study of high OR media is reported by characterizing the magnetic, microstructural, and read/write properties.
    IEEE Transactions on Magnetics 04/2003; · 1.42 Impact Factor
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    ABSTRACT: For perpendicular magnetic recording (PMR) media, SNR improvement is more difficult because of the noise contributions from the soft magnetic underlayer (SUL) and the difficulty of achieving grain segregation in the recording layer for low noise. For the SUL, it is important to reduce the spike noise, but also a suitable SUL is needed to achieve a good head field gradient. A recording layer with small grain size, good grain segregation, and large squareness has to be grown on a very thin intermediate layer on the SUL. In this paper, we address some important issues to improve the performance of PMR media through spike noise reduction by adjusting SUL design, improved recording layer SNR, and intermediate layer (IL) modifications for optimum SNR performance.
    Joint NAPMRC 2003. Digest of Technical Papers [Perpendicular Magnetic Recording Conference 2003]; 02/2003
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    ABSTRACT: For Abstract see ChemInform Abstract in Full Text.
    ChemInform 01/2003; 34(38).
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    ABSTRACT: We have studied thermal stability and recording characteristics of CoCrPtB alloy media with varying c axis anisotropy dispersion, preferred orientation (PO), and in-plane orientation ratio (OR) on both glass and NiP-plated Al substrates. With increasing in-plane c axis orientation and OR, improved recording performances such as lower pulsewidth, lower media transition noise, and lower dc noise are achieved. Thermal decay rate is primarily determined by Co anisotropy dispersion and in-plane OR for the same coercivity media. Media having a large out-of-plane c axis component exhibit a wide switching field distribution and have a high signal decay rate. The PW<sub>50</sub> and dc noise of media with out-of-plane Co grains are very sensitive to changes in write field. On the other hand, highly oriented media having high OR are less sensitive to head writability variations and offer good thermal stability with excellent recording performance.
    IEEE Transactions on Magnetics 10/2002; · 1.42 Impact Factor
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    ABSTRACT: The effects of seedlayer (SL) and preheating on the crystallographic, magnetic, and recording properties of CoCr<sub>18</sub>Pt<sub>16</sub>B<sub>4</sub> granular perpendicular magnetic media have been investigated. For the media with Ru as underlayer, although higher preheating improves both coercivity and squareness, the media is much noisier. With introducing Ta SL, Ru (0002)/Co (0002) crystallography is greatly enhanced. High coercivity, unity squareness, and negative nucleation field are achieved even at low preheating without degrading signal-to-noise ratio. In addition, the media is magnetically stable for a wide range of recording density.
    IEEE Transactions on Magnetics 10/2002; · 1.42 Impact Factor
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    ABSTRACT: Seedlayer and underlayer effects on crystallographic orientation and recording performance were studied for CoCrPtB media sputtered on glass substrates. For this study, the seedlayers are XAl (X=Ni, Co, Ti, and Ru) and the underlayers are CrY (Y=V, Mo, W, and Ti). It was found that not only different seedlayers, but also different combinations of seedlayer and underlayer, led to different magnetic performance. NiAl and CoAl seedlayers orient the Co c axis to (10.0) and TiAl and RuAl seedlayers produce (11.0) Co orientation. For the NiAl and CoAl seedlayer, CrV and CrW underlayers develop less out-of-plane c-axis orientation and higher coercivity and coercive squareness while CrTi and CrMo underlayers work better for TiAl and RuAl seedlayers, respectively. Media with RuAl seedlayers have better parametric performance than media with NiAl and CoAl seedlayers. The detailed relationship between seedlayer and underlayer types and crystal orientation and recording performance is discussed. © 2002 American Institute of Physics.
    Journal of Applied Physics 05/2002; 91(10):7068-7070. · 2.21 Impact Factor