Chang K. Kim

Hanyang University, Ansan, Gyeonggi, South Korea

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Publications (11)25.35 Total impact

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    ABSTRACT: Femtosecond laser-interference induced amorphization (FLIA) was used to form a spatially periodic magnetic structure by selectively amorphizing the surface of bulk Co2MnSi. Regularly spaced alternating lines with a periodicity of 2 μm were produced by FLIA. Magnetic force microscopy of the samples clearly revealed one-dimensional periodic magnetic domains resulting from the modulated surface structure which was confirmed by transmission electron microscopy. The periodicity of the modulated structure can be further decreased or the process can be modified to write two-dimensional pattern so that the resulting magnetic structure can be potentially utilized for a permanent magnetic identification.
    Journal of Applied Physics 01/2008; 103(7):07E701-07E701-3. · 2.21 Impact Factor
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    ABSTRACT: Using X-ray magnetic circular dichroism (XMCD), effective magnetic moments for Co and Mn in Co78−xMnxB10Si12 amorphous alloys were estimated in order to explain the different magnetic states found in the amorphous alloy depending on the Mn content. As the Mn content was increased, the Co–Mn amorphous alloy changed from a ferromagnet to a paramagnet and then to a mictomagnetic state (x = 30 at%) at room temperature. When x > 25 at%, a reentrant spin glass behavior was also observed below 50 K. XMCD analysis showed that the magnetic moments for both Co and Mn were severely reduced by the structural disorder. It was also found that the Mn–Mn pair exchange interaction was ferromagnetic, regardless of the composition (up to 30 at%); hence, antiferromagnetic JCoMn exchange interaction is believed to be responsible for the reentrant spin glass behavior as well as the mictomagnetic state observed for the alloys with high Mn content.
    Journal of Alloys and Compounds. 07/2007; 439(s 1–2):171–175.
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    ABSTRACT: A monolayer of Ni Fe alloy nanoparticles was fabricated on a polyimide (PI) film via depositing a 1 nm thick Fe thin film on top of a preexisting nano-sized Ni particle template, followed by thermal annealing to induce coalescence of the Fe film onto the preexisting Ni seed particles. The resulting alloy nanoparticles were 6.5 nm in size and exhibited an increase in saturation magnetization due to the incorporation of Fe compared to the initial Ni seed particles. The nanoparticle template process presented here can be easily extended to other alloy systems to potentially alter the magnetic properties of the nanoparticles as needed.
    Journal of Magnetism and Magnetic Materials 01/2007; 310:2402-2404. · 2.00 Impact Factor
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    ABSTRACT: A monolayer of Co-Pt alloy nanoparticles in the nanometer-size regime was fabricated using a nanotemplate approach. 1.7-nm-thick Co46Pt54 film was deposited onto a preexisting array of Ni seed particles embedded in a polyimide film. During subsequent annealing, the deposited Co46Pt54 film coalesced onto the seed particles to produce a monolayer of Co-Pt alloy particles. Deposition and annealing were repeated to increase both average particle size and volume fraction of the alloy particles. It was also shown that the annealing temperature was critical in controlling the particle size distribution and the final composition of the nanoparticles. This method of forming a single layer of vertically aligned nanoparticles can be easily extended to a large area as well as to produce a different combination of alloy particles on a polymer film.
    Journal of Colloid and Interface Science 12/2006; 303(1):131-6. · 3.55 Impact Factor
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    ABSTRACT: Femtosecond laser-interference crystallization (FLIC) was used to form a spatially periodic magnetic structure by selectively crystallizing a paramagnetic amorphous Co2MnSi thin film. Regularly spaced alternating lines of polycrystalline and microcrystalline regions with a periodicity of 2 μm were produced by FLIC. The crystalline region composed of ∼ 100-nm-sized grains contained a nonequilibrium ferromagnetic phase intermixed with β-Mn. The areas between the crystallized lines also received sufficient energy, crystallizing into a microcrystalline state with its grain size ranging from 1 to 5 nm. The magnetic force microscopy of the samples clearly revealed the one-dimensional periodic magnetic domains resulting from the modulated microstructure.
    Journal of Applied Physics 04/2006; 99(8):08G311-08G311-3. · 2.21 Impact Factor
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    ABSTRACT: A mono-layer of nano-sized metal particles was prepared on the surface of a polyimide film by simply depositing a thin film of Ni80Fe20 on top of the polyamic acid that was spin coated onto a Si wafer. During thermal imidization of the polyamic acid film, Fe was selectively etched by reacting with the carbonyl group of the polyamic acid to leave behind uniformly distributed Ni-rich metallic particles. The average diameter of the particles was 4 nm and the particles were confined into a single layer on top of the polymer film. Moreover, it was also shown that the morphology of the nanoparticles can be substantially altered by curing the precursor film in a hydrogen atmosphere, without significantly damaging the polymer film. Thus produced nanoparticles lay exposed on top of the electrically insulating and chemically stable polymer film so that it is possible that the nanoparticles can be directly used for fabricating a nonvolatile flash memory device or as a template for building functional nano-structures.
    Journal of Colloid and Interface Science 04/2006; 295(1):108-14. · 3.55 Impact Factor
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    ABSTRACT: A mono-layer of metallic Ni nanoparticles with different average particle sizes was prepared on top of a polyimide film by depositing a Ni100−xFex alloy thin film on the polyimide precursor. During imidization of the precursor, Fe was selectively etched from the alloy film, leaving behind Ni-enriched nanoparticles. Average particle size of the thus-produced nanoparticles ranged from 5 to 7 nm depending on the Fe content in the initial metal film and increasing Fe content also resulted in formation of spinel NiFe2O4 particles intermixed with the Ni particles. Because these particles are located at the very top of the polyimide film, it was demonstrated that the nanoparticles can be used as a template for building functional nanostructures.
    Colloids and Surfaces A: Physicochemical and Engineering Aspects. 01/2006;
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    ABSTRACT: Metallicnanoparticles embedded in a polyimide (PI) matrix were fabricated through selectively oxidizing a layer of Ni80Fe20 metal film sandwiched between two PI precursor layers. Ni nanoparticles, formed in a monolayer between two PI layers, had an average particle size of 5 nm. X-ray photoelectron spectroscopy confirmed that Fe in the film was preferentially consumed, resulting in formation of Ni nanoparticles. Similar experiments to produce uniform-sized Co nanoparticles failed to produce uniform-sized Co nanoparticles when a Co80Fe20 film was inserted because, unlike Ni, Co was partially oxidized during imidization. Transmission infrared spectroscopy suggested that thermal degradation of the PI matrix was catalyzed by the Co80Fe20 film, leading to the Co oxidation together with Fe.
    The Journal of Physical Chemistry B 11/2004; 108(47):18179-18184. · 3.61 Impact Factor
  • Sung K Lim, Chong S Yoon, Chang K Kim
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    ABSTRACT: Ni nanoparticles embedded in a polyimide (PI) matrix were fabricated by selectively oxidizing a layer of Ni(80)Fe(20) metal film sandwiched between two PI precursor layers. Ni nanoparticles, formed in a monolayer between two PI layers, had an average particle size of approximately 5 nm. X-Ray photoelectron spectroscopy confirmed that Fe in the film was preferentially consumed, resulting in the formation of Ni nanoparticles.
    Chemical Communications 05/2004; · 6.38 Impact Factor
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    ABSTRACT: Ultra-low thermal expansion ceramics based upon CaZr4(PO4)6–Li2O system were synthesized through solid state sintering. Lithium oxide was introduced to the CaZr4(PO4)6 lattice to promote liquid phase sintering to form CaZr4(PO4)6 with strongly negative thermal expansion. A small addition of Li2O has improved the densification rate, but also caused an excessive grain growth, which resulted in extensive microcracking. The microcracking lowered the bulk thermal expansion to −6×10−6°C−1. Excessive amount of Li2O content also led to formation of the secondary phase, Li2Zr(PO4)2, and also caused the swelling of the CaZr4(PO4)6 phase to lower the sintered density. An addition of over 5% of lithium oxide, the glass phase surrounding the CaZr4(PO4)6 grain, together with the secondary phase increased the thermal expansion. At 2.5% addition, there was an indication that the viscous glassy phase has partially relieved the stresses associated with the thermal expansion anisotropy during cooling from the processing temperature. It was demonstrated that the microstructure of the CaZr4(PO4)6 could be altered by adding varying amount of lithium oxide to tailor the thermophysical properties of the material.
    Materials Science and Engineering B 01/2001; 79(1):6-10. · 1.85 Impact Factor
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    ABSTRACT: Ultra-low thermal expansion ceramics based upon CaZr4(PO4)6–Li2O system were synthesized through solid state sintering. Lithium oxide was introduced to the CaZr4(PO4)6 lattice to promote liquid phase sintering to form CaZr4(PO4)6 with strongly negative thermal expansion. A small addition of Li2O has improved the densification rate, but also caused an excessive grain growth, which resulted in extensive microcracking. The microcracking lowered the bulk thermal expansion to −6×10−6°C−1. Excessive amount of Li2O content also led to formation of the secondary phase, Li2Zr(PO4)2, and also caused the swelling of the CaZr4(PO4)6 phase to lower the sintered density. An addition of over 5% of lithium oxide, the glass phase surrounding the CaZr4(PO4)6 grain, together with the secondary phase increased the thermal expansion. At 2.5% addition, there was an indication that the viscous glassy phase has partially relieved the stresses associated with the thermal expansion anisotropy during cooling from the processing temperature. It was demonstrated that the microstructure of the CaZr4(PO4)6 could be altered by adding varying amount of lithium oxide to tailor the thermophysical properties of the material.
    Materials Science and Engineering B-advanced Functional Solid-state Materials. 01/2001; 79(1):6-10.