M. Zheng

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

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Publications (25)36.98 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: 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; DOI:10.1109/TMAG.2005.855283 · 1.21 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; DOI:10.1063/1.1847911 · 2.19 Impact Factor
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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 02/2005; DOI:10.1016/j.jmmm.2004.10.026 · 2.00 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.
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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.
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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; DOI:10.1109/TMAG.2004.832167 · 1.21 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; DOI:10.1109/TMAG.2004.832165 · 1.21 Impact Factor
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    ABSTRACT: For Abstract see ChemInform Abstract in Full Text.
    ChemInform 09/2003; 34(38). DOI:10.1002/chin.200338211
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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; DOI:10.1109/TMAG.2003.813777 · 1.21 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; DOI:10.1109/TMAG.2003.808984 · 1.21 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: 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; DOI:10.1109/TMAG.2002.802790 · 1.21 Impact Factor
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    ABSTRACT: The nanomagnetic behavior of Co–C nanodot arrays was investigated by magnetic force microscopy (MFM) and an alternative gradient force magnetometer. The direction of the easy axis can be observed directly with MFM by comparing the saturated magnetization state and the remanent magnetization state. Interaction of the domain wall with local defects was observed by field dependent MFM measurements. Some types of defects that can pin domain wall movement were identified. © 2002 American Institute of Physics.
    Journal of Applied Physics 05/2002; 91(10):7311-7313. DOI:10.1063/1.1452259 · 2.19 Impact Factor
  • G. Choe, M. Zheng, K. E. Johnson
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    ABSTRACT: Summary form only given. It has been reported that media with high in-plane orientation ratio (OR) may enhance thermal stability and strong in-plane orientation of the Co c-axis in isotropic glass media is needed to improve recording performance. In this work, a systematic study of thermal stability and recording characteristics was performed on glass media with varying in-plane crystallographic orientation [random 2D-to-3D] and preferred orientation [(10.0) vs (11.0)]. In addition, the effect of in-plane OR on thermal stability was investigated for the media deposited onto both mechanically textured and non-textured polish only AlNiP substrates.
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    ABSTRACT: Periodic magnetic nanodot arrays have been produced on an area as large as 1 cm×1 cm by direct nanolithography using interferometric laser radiation. The dots are formed by the local annealing of sputtered amorphous Co–C films in regions where the laser intensity is highest. At room temperature the dots exhibit ferromagnetic order and are embedded in a paramagnetic matrix. The onset of room-temperature ferromagnetism is caused by nanoscale chemical and morphological changes during dot formation and reflects the phase separation of magnetic Co-rich clusters. The present single-step nanolithography is potentially an efficient method for fabrication of patterned magnetic arrays. © 2001 American Institute of Physics.
    Applied Physics Letters 10/2001; 79(16):2606-2608. DOI:10.1063/1.1409948 · 3.52 Impact Factor
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    ABSTRACT: Magnetic properties of Co-C nanodot arrays produced by direct interferometric laser annealing are investigated by magnetic force microscopy (MFM) and magnetization measurements. The dots are formed by locally annealing sputtered amorphous Co-C films in regions where the laser intensity is highest. As-sputtered Co-C films do not exhibit ferromagnetic order at room temperature, but MFM shows that the dots become magnetic upon annealing, possibly due to the agglomeration or phase separation of Co-rich clusters. The dots are embedded in either a paramagnetic or weakly magnetic matrix. The magnetic properties of the generated pattern can be changed by varying the laser power. The present results show that direct interferometric lithography may become a useful tool for fabricating future patterned magnetic nanostructures
    IEEE Transactions on Magnetics 08/2001; DOI:10.1109/20.951056 · 1.21 Impact Factor
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    ABSTRACT: Recent work on magnetic properties of transition-metal nanowire arrays produced by electro-deposition is reviewed. The wires, which are electro-deposited into self-assembled porous anodic alumina, form nearly hexagonal arrays characterized by wire diameters down to less than 10 nm, wire lengths up to about 1 µm, and variable centre-to-centre spacings of the order of 50 nm. The fabrication and structural characterization of the arrays is summarized, magnetic data are presented and theoretical explanations of the behaviour of the wires are given. Emphasis is on extrinsic phenomena such as coercivity, magnetization reversal and interactions of the magnetic nanowires. In particular, we analyse how wire imperfections give rise to magnetic localization and dominate the hysteresis behaviour of the wires. Potential applications are outlined in the last section.
    Journal of Physics Condensed Matter 06/2001; 13(25):R433. DOI:10.1088/0953-8984/13/25/201 · 2.22 Impact Factor
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    ABSTRACT: This paper presents recent results on magnetic interactions in CoPt:C nanocrystalline films and Co:C magnetic nanodot arrays. The local magnetic properties were studied by magnetic force microscopy. We discuss the limit of the using ΔM for characterization of magnetic interactions in these CoPt:C nanocrystalline films. The magnetic force microscopy images of patterned Co:C nanodot arrays show the presence of “macro”-domains (domains formed by several separate dots). This behavior is attributed to the exchange coupling between the dots.
    Scripta Materialia 05/2001; 44:1347-1351. DOI:10.1016/S1359-6462(01)00698-4 · 2.97 Impact Factor
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    ABSTRACT: Magnetic properties of Ni nanowires electrodeposited into self-assembled porous alumina arrays have been investigated. By anodizing aluminum in sulfuric acid and immersing the as-anodized template into phosphoric acid for different lengths of time, we are able to vary the diameters of the subsequently deposited nanowires between 8 and 25 nm. The coercivity measured along wire axis first increases with the wire diameter, reaches a maximum of 950 Oe near a diameter of 18 nm, and then decreases with further increase of wire diameter. The dependence of the magnetization of Ni nanowires is found to follow Bloch's law at low temperature but with the Bloch exponent decreasing from the bulk value and the Bloch constant increasing from the bulk value by an order of magnitude.
    Physical Review B 10/2000; 62(18). DOI:10.1103/PhysRevB.62.12282 · 3.66 Impact Factor