M. Zheng

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

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Publications (13)27.81 Total impact

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    ABSTRACT: For Abstract see ChemInform Abstract in Full Text.
    ChemInform 01/2003; 34(38).
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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. · 2.21 Impact Factor
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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. · 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; · 1.42 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. · 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 01/2001; 44:1347-1351. · 2.82 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). · 3.66 Impact Factor
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    ABSTRACT: Magnetization reversal in transition-metal nanowires is investigated. Model calculations explain why magnetization reversal is localized, as opposed to the sometimes assumed delocalized coherent-rotation and curling modes. The localization is a quite general phenomenon caused by morphological inhomogenities and occurring in both polycrystalline and single-crystalline wires. In the polycrystalline limit, the competition between interatomic exchange and anisotropy gives rise to a variety of random-anisotropy effects, whereas nearly single-crystalline wires exhibit a weak localization of the nucleation mode. Model predictions are used to explain the coercive and magnetic-viscosity behavior of Co (and Ni) nanowires electrodeposited in self-assembled alumina pores.
    Physical Review B 08/2000; 62(6):3900-3904. · 3.66 Impact Factor
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    ABSTRACT: Ferromagnetic Co nanowires have been electrodeposited into self-assembled porous anodic alumina arrays. Due to their cylindrical shape, the nanowires exhibit perpendicular anisotropy. The coercivity, remanence ratio, and activation volumes of Co nanowires depend strongly on the length, diameter, and spacing of the nanowires. Both coercivity and thermal activation volume increase with increasing wire length, while for constant center-to-center spacing, coercivity decreases and thermal activation volume increases with increasing wire diameter. The behavior of the nanowires is explained qualitatively in terms of localized magnetization reversal. © 2000 American Institute of Physics.
    Journal of Applied Physics 06/2000; · 2.21 Impact Factor
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    ABSTRACT: Electrodeposition of metals into self-assembled porous anodic alumina is considered as one of the most convenient ways to produce the nanostructures, especially nanowire arrays[1]. Previously, much work has been done on fabrication of Fe, Co or Ni nanowire arrays in such templates. However, since alternating current is imposed in fabricating nanowires in alumite templates, it is difficult to use the conventional dc reduction method, e.g. controlling the reduction potential, to produce multilayers or alloys of two metals with large different reduction potential. A different deposition method must be employed. In this work, we report the magnetic properties of self-assembled multilayered Fe/Pt nanowire arrays. The thicknesses of individual layers are varied from 10-30 nm with diameter of each wire between 10-100 nm. Each wire is polycrystalline as shown by high resolution TEM. Coercivity measured along wire axis ranges from 1500 to 2500 Oe, depending on the thickness of Fe and Pt layer. Magnetic properties have been investigated as a function of different Fe and Pt thicknesses as well as annealing conditions. Hysteresis loops after annealing indicate the presence of two magnetic phases. This behavior depends on the thickness of Fe and Pt layer thickness and their ratio. Reference: [1] D. AlMawlawi, et al. J. Appl. Phys. 70, 4421(1991); H. Masuda and K. Fukuda, Science, 268, 1466 (1995).
    03/2000;
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    ABSTRACT: A quasi-periodic array of Fe quantum dots of cylindrical shape has been synthesized by electrodeposition of Fe in porous alumina. By controlling the fabrication parameters, we have controlled the length, diameter, and spacing of the dots. The magnetic properties are shown to depend on these parameters. It has been found that at room temperature, there exists a critical diameter of the dots for which the coercivity is a maximum. The largest value of coercivity obtained at room temperature is 2640 Oe which rises to 2900 Oe on annealing. At a low temperature of 5K, an increase in coercivity is observed for most of the samples. The largest value is 3800 Oe which rises to a value of 4100 Oe in the corresponding annealed counterpart. At 5K, no maximum is seen in the coercivity as a function of diameter. Instead, the coercivity is found to decrease with increasing diameter. This dependence of the coercivity on diameter is discussed in terms of localized reversal effects.
    Journal of Electronic Materials 01/2000; 29(5):510-515. · 1.64 Impact Factor
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    ABSTRACT: Magnetization processes in Ni nanowire arrays are investigated. The wires are produced by electrodepositing Ni into porous anodic alumina and exhibit coercivities of the order of 0.05 T (500 Oe) along the wire axis. Transmission-electron microscopy of freed wires shows that the wires are polycrystalline and resemble a chain of nanocrystallites. To model the hysteresis loops, the wires are treated as one-dimensional random-anisotropy magnets where the magnetocrystalline bulk anisotropy is a weak perturbation to the leading anisotropy contribution. The calculation yields an analytical equation for the magnetization as a function of the applied magnetic field. For small and moderate reversed fields, the agreement between theory and experiment is very good, but the applicability of the model breaks down close to the coercive field. This failure is explained by the neglect of higher-order perturbation terms describing, for example, magnetic localization effects.
    Journal of Physics Condensed Matter 01/2000; 12(30). · 2.22 Impact Factor
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    ABSTRACT: Magnetization processes in Ni nanowire arrays are investigated. The wires are produced by electrodepositing Ni into porous anodic alumina and exhibit coercivities of the order of 0.05 T (500 Oe) along the wire axis. Transmission-electron microscopy of freed wires shows that the wires are polycrystalline and resemble a chain of nanocrystallites. To model the hysteresis loops, the wires are treated as one-dimensional random-anisotropy magnets where the magnetocrystalline bulk anisotropy is a weak perturbation to the leading anisotropy contribution. The calculation yields an analytical equation for the magnetization as a function of the applied magnetic field. For small and moderate reversed fields, the agreement between theory and experiment is very good, but the applicability of the model breaks down close to the coercive field. This failure is explained by the neglect of higher-order perturbation terms describing, for example, magnetic localization effects.
    Journal of Physics Condensed Matter 01/2000; 12(30). · 2.22 Impact Factor