G. W. Kim

Changwon National University, Changnyeong, South Gyeongsang, South Korea

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Publications (12)12.14 Total impact

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    ABSTRACT: A detailed investigation of the magnetic and the magnetocaloric properties of La0.7Sr0.3MnO3, La0.7Ca0.3MnO3 and La0.7Ca0.1Sr0.2MnO3 for possible application in magnetic refrigeration is presented. The adiabatic magnetic entropy change was determined for a magnetic field change from 0 to 2.5 T by using magnetization data. A broad table-like entropy change with a relative cooling power of 111 J/kg in an applied magnetic field of 2.5 T with an operating temperature range of 54 K was observed in the La0.7Ca0.1Sr0.2MnO3 sample. The broadening in the magnetic entropy change in La0.7Ca0.1Sr0.2MnO3 is discussed in light of the tricritical mean-field theory. The tricritical exponent (δ) was calculated for the La0.7Ca0.1Sr0.2MnO3 sample and found to be ∼ 4.76 ± 0.03, which is very close to the proposed value δ = 5 for the tricitical exponent. The studied samples can be considered as giant magnetocaloric materials for room-temperature refrigeration.
    No preview · Article · Jun 2013 · Journal- Korean Physical Society
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    ABSTRACT: Transparent p-type conducting a sandwich structure Al/SnO2/Al multilayer thin films deposited on quartz substrate have been prepared by RF sputtering method using SnO2 (99. 99%) and Al (99. 99%) targets. In order to study the effect of thermal diffusing temperature and time on the structural, electrical and optical performances of Al/SnO2/Al films, the deposited films were annealed at different temperatures for different durations, respectively. X-ray diffraction results show that all p-type conducting films possessed polycrystalline SnO2 with tetragonal rutile structure. The microstructure, Hall effect and optical property were checked by FE-SEM, Hall effect measurement and UV-Visible absorption spectra. As the annealing temperature increases, the transparence (%) and energy band (Eg) increase. Especially, Hall effect results indicate that 450°C for 4 hrs were the optimum annealing temperature for the resistivity (Ω·cm) with a relatively high hole concentration of 2.09 × 1019cm-3 and a low resistivity of 5.38 × 10-1 Ω · cm.
    No preview · Article · Nov 2012 · Journal of Ceramic Processing Research
  • G.W. Kim · C.H. Sung · Y.J. Seo · K.Y. Park · S.N. Heo · S.H. Lee · B.H. Koo
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    ABSTRACT: In this work, we studied the influence of the dopant elements concentration on the properties of SnO2 thin films deposited by pulsed laser deposition. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Hall effect measurement and UV-Vis studies were performed to characterize the deposited films. XRD results showed that the films had polycrystalline nature with tetragonal rutile structure. FE-SEM micrographs revealed that the as deposited films composed of dense microstructures with uniform grain size distribution. All the films show n-type conduction and the best transparent conductive oxide (TCO) performance was obtained on 6 wt% Sb2O5 doped SnO2 film prepared at pO2 of 60mtorr and Ts of 500°C. Its resitivity, optical transmittance, figure of merit are 7.8 × 10-4 Ω cm, 85% and 1.2 × 10-2 Ω-1, respectively.
    No preview · Article · Nov 2012 · Journal of Ceramic Processing Research
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    ABSTRACT: We report microwave assisted hydrothermal synthesis and magnetocaloric properties of La0.67Sr0.33MnO3 manganite. The synthesized La0.67Sr0.33MnO3 nanoparticles was characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS) and magnetization measurements. The XRD results indicated that La0.67Sr0.33MnO3 nanoparticles have polycrystalline nature with monoclinic structure. FE-SEM results suggested that La0.67Sr0.33MnO3 nanoparticles are assembled into rod like morphology. Magnetization measurements show that La0.67Sr0.33MnO3 nanoparticles exhibit transition temperature (Tc) above room temperature. The maximum magnetic entropy change (deltaS(M))max was found to be 0.52 J/kg K near Tc approximately 325 K at applied magnetc field of 20 kOe. This compound may considered as potential material for magnetic refrigeration near room temperature.
    No preview · Article · Jul 2012 · Journal of Nanoscience and Nanotechnology
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    ABSTRACT: We report microwave assisted hydrothermal synthesis and magnetocaloric properties of La0.67Sr0.33MnO3 manganite. The synthesized La0.67Sr0.33MnO3 nanoparticles was characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS) and magnetization measurements. The XRD results indicated that La0.67Sr0.33MnO3 nanoparticles have polycrystalline nature with monoclinic structure. FE-SEM results suggested that La0.67Sr0.33MnO3 nanoparticles are assembled into rod like morphology. Magnetization measurements show that La0.67Sr0.33MnO3 nanoparticles exhibit transition temperature (T c) above room temperature. The maximum magnetic entropy change (ΔS M)max was found to be 0.52 J/kg K near T c ∼ 325 K at applied magnetc field of 20 kOe. This compound may considered as potential material for magnetic refrigeration near room temperature.
    No preview · Article · Jul 2012 · Journal of Nanoscience and Nanotechnology
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    ABSTRACT: In this study, the magnetic and the magnetocaloric properties of the La0.66Sr0.34MnO3 (LSMO) compound were investigated. The X-ray diffraction result indicates that the LSMO sample has a single phase of rhombohedral symmetry without any impurity phase. The magnetic study reveals that the specimen La0.66Sr0.34MnO3 exhibits a ferromagnetic-paramagnetic transition at T C ∼ 376 K. Using Arrott plots, the phase transition from ferromagnetic to paramagnetic is found to be of second order. A maximum magnetic entropy change of 1.25 J/kgK has been observed for a low applied magnetic field of 1T. The relative cooling power values exhibit a nearly linear dependence on the applied magnetic field. Moreover, the analysis of the magnetocaloric effect (MCE) using the Landau theory o f phase transitions shows good agreement with the experimental results, confirming the importance of magnetoelastic coupling and electron interactions in the magnetocaloric properties of perovskite manganites. This investigation suggests that La0.66Sr0.34MnO3 can be used as a potential magnetic refrigeration material.
    No preview · Article · May 2012 · Journal- Korean Physical Society
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    ABSTRACT: The magnetocaloric properties of new complex magnetic material La0.65Sr0.35Cu0.1Mn0.9O3, suitable for the Ericsson cycle, have been investigated. For this material, the effect of Cu doping can be attributed to a combination of doping disorder, Cu-Mn super exchange interactions and a site-percolation mechanism, which suppress the metallic conduction and Curie temperature. The Curie temperature decreases to 355 K. The magnetocaloric study exposes a comparable value of the magnetic entropy change for La0.65Sr0.35Cu0.1Mn0.9O3 sample, the value of the maximum entropy change, increases from 1.132 J/kgK to 3.11 J/kgK as magnetic field increases from 1 T to 4 T. A large relative cooling power (RCP) has been observed for La0.65Sr0.35Cu0.1Mn0.9O3. As a result, the studied sample can be considered as potential material for magnetic refrigeration.
    No preview · Article · Apr 2012 · Journal of Electroceramics
  • G.W. Kim · M.S. Anwar · S.N. Heo · K.Y. Park · B.H. Koo
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    ABSTRACT: We report on a detailed study of the electrical resistivity and the magnetoresistance of the (La 0.67Ca 0.33MnO 3) 0.97/(ZnO) 0.03 (LCMO/ZNO) composite prepared at different sintering temperature by conventional solid state reaction method. The results of scanning electron microscopy indicate that ZnO segregates mostly at the grain boundaries of LCMO. The grain size of the LCMO increases with the sintering temperature. The X-ray diffraction (XRD) result shows that no new phase appeared in the composite except LCMO and ZnO phases. The magnetic and electrical transport was measured over a temperature range of 90-320 K. All samples exhibits characteristic insulator-metal and para-ferromagnetic transition temperature, which decreases with increase in sintering temperature. It is very interesting to note that the MR% is enhanced with the sintering temperature, which is encouraging for potential applications.
    No preview · Article · Jan 2012 · Journal of Ceramic Processing Research
  • M.S. Anwar · S. Kumar · F. Ahmed · N. Arshi · G.W. Kim · C.G. Lee · B.H. Koo
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    ABSTRACT: In this paper, magnetic property and magnetocaloric effect (MCE) in perovskite manganites of the type La (0.75-X)Ce XCa 0.25MnO 3 (x = 0.0, 0.2, 0.3 and 0.5) synthesized by using the standard solid state reaction method have been reported. From the magnetic measurements as a function of temperature and applied magnetic field, we have observed that the Curie temperature (T C) of the prepared samples strongly dependent on Ce content and was found to be 255, 213 and 150 K for x = 0.0, 0.2 and 0.3, respectively. A large magnetocaloric effect in vicinity of T C has been observed with a maximum magnetic entropy change (|ΔS M| max) of 3.31 and 6.40 J/kgK at 1.5 and 4 T, respectively, for La 0.55Ce 0.2Ca 0.25MnO 3. In addition, relative cooling power (RCP) of the sample under the magnetic field variation of 1.5 T reaches 59 J/kg. These results suggest that La 0.55Ce 0.2Ca 0.25MnO 3 compound could be a suitable candidate as working substance in magnetic refrigeration at 213 K.
    No preview · Article · Dec 2011
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    ABSTRACT: We report structural, magnetic and electronic structure study of Mn doped TiO2 thin films grown using pulsed laser deposition method. The films were characterized using X-ray diffraction (XRD), dc magnetization, X-ray magnetic circular dichroism (XMCD) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements. XRD results indicate that films exhibit single phase nature with rutile structure and exclude the secondary phase related to Mn metal cluster or any oxide phase of Mn. Magnetization studies reveal that both the films (3% and 5% Mn doped TiO2) exhibit room temperature ferromagnetism and saturation magnetization increases with increase in concentration of Mn doping. The spectral features of XMCD at Mn L3,2 edge show that Mn2+ ions contribute to the ferromagnetism. NEXAFS spectra measured at O K edge show a strong hybridization between Mn, Ti 3d and O 2p orbitals. NEXAFS spectra measured at Mn and Ti L3,2 edge show that Mn exist in +2 valence state, whereas, Ti is in +4 state in Mn doped TiO2 films.Highlights► We have synthesized single phase Mn doped TiO2 thin films using PLD technique. ► XRD results indicate that films exhibit single phase nature with rutile structure and exclude the secondary phase. ► Field cooled magnetization and magnetic hysteresis loop studies infer that Mn doped TiO2 films show room temperature ferromagnetism. ► XMCD measurements at Mn L3,2 edge show that Mn2+ ions contribute to the ferromagnetism. ► NEXAFS spectra measured at Mn and Ti L3,2 edge show that Mn exist in +2 valence state, whereas, Ti is in +4 state in Mn doped TiO2 films.
    Full-text · Article · Oct 2011 · Applied Surface Science
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    Y J Seo · G W Kim · C H Sung · C G Lee · B H Koo
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    ABSTRACT: The magnetic and electrical properties of (La0.7Sr0.3MnO3)1−x/(CuFe2O4)x have been investigated. The (La0.7Sr0.3MnO3)1-x/(CuFe2O4)x composites with x = 0, 0.01, 0.03, 0.06 and 0.09 wt.% were prepared by conventional solid state reaction method, respectively. The structures, morphology, magnetic and electrical properties of the samples have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and physical property measurement system (PPMS). The resistance measured as a function of temperature demonstrates that the pure LSMO samples display metal to semiconductor transition. On the other hand, the others indicated semiconductor/insulator behaviors. The MR was measured in an applied magnetic field of 0.5T. The results clearly show that the magnetoresistance (MR) effect is enhanced at a wide temperature range with CuFe2O4 composition. It may be helpful to improve the low-field magnetoresistance (LFMR) under the influence of the spin-polarized tunneling and the spin-dependent scattering.
    Preview · Article · Jan 2011 · Journal of Physics Conference Series
  • S Kang · M Takeda · Z Hiroi · G W Kim · C G Lee · B H Koo
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    ABSTRACT: The electromagnetic properties and microstructures of a Cu75–Fe5–Ni20 alloy have been investigated on isothermal annealing at 1073 K, using a superconducting quantum interference device magnetometer, quantum design, physical property measurement system and transmission electron microscopy. Nanoscale magnetic particles were formed randomly in the Cu-rich matrix after receiving a short annealing due to the phase decomposition in the alloy. With increasing isothermal annealing time, however, rod-type precipitates aligned along the 1 0 0 directions were observed in the matrix, on isothermal annealing at 1073 K. Although the size of the precipitates became larger (from ~10 nm to >300 nm) after further annealing, no significant change (less than 2%) was detected in the MR value. The largest MR value (MR ~16% at H = 7 T and T = 10 K) was attained, in particular, for the as-quenched specimen. This study revealed that several significant influences were introduced into the magnetic and magnetoresistive properties during the phase decomposition process in the Cu–Fe–Ni alloy.
    No preview · Article · Sep 2010 · Journal of Physics D Applied Physics