단결정 (單結晶) YFeO₃ 에서 관찰되는 Bubble 자구 ↔ Dumbbell 자구의 형상전이 (形狀轉移) 이력현상에 (履歷現象) 관한 연구

김종오(Chong Oh Kim) 대한공업교육학회 1984 대한공업교육학회지 Vol.9 No.1

A phenomenon of the bubble domain ⇔ dumbbell domain was observed for the single crystal YFeO₃ (thickness: 60㎛) using the magneto-optic Faraday effect when the applied magnetic field was changed. The shape transformation from the bubble to dumbbell and from the dumbbell to bubble was occured at 19.6 0e and 21.6 0e respectively. These magnetic field were independent on the variable range of the applied magnetic field. This domain shape hysteresis was well explained by the Jonge`s calculation of the magnetostatic energy for the bubble domain and dumbbell domain to the applied magnetic field.

논문 : 김리자유화(金利自由化)와 김리변동성(金利變動性)의 특성(特性)에 관한 연구

김종선 ( Chong Sun Kim ),김종오 ( Chong Oh Kim ) 명지대학교 금융지식연구소 2008 금융지식연구 Vol.6 No.2

본 연구는 금리자유화의 진전과 금리변동성의 변화 관계를 포함한 외환위기 이후 장·단기 금리변동성의 몇 가지 特性을 실증적으로 분석하고자 하였다. 우리나라 금리자유화기의 장·단기 금리변동성의 몇 가지 특성을 분석한 결과는 다음과 같다. 첫째로 금리자유화의 진전에 따라 금리변동성이 콜금리의 경우는 탄력적인 축소효과를 보였고, 회사채수익률의 경우는 미미하지만 양(+)의 확대효과를 보였다. 둘째로 외환위기 여파에 따른 금리변동성의 확대효과는 콜금리의 경우가 회사채수익률의 경우보다 크게 나타났다. 셋째로 충격의 半減期 분석을 통해, 콜금리 및 회사채수익률의 경우에 외환위기 이후에는 그 이전에 비해 변동성의 충격이 있을 때 균제상태(steady state)로 돌아오는 기간이 다소 짧아진 것으로 분석되었다. 넷째로 콜금리는 변동성의 비대칭성효과가 외환위기 이후 존재한 것으로 입증되었으나, 회사채의 경우는 외환위기 이후에 금리변동성의 비대칭성효과가 입증되지 않았다. The major object of this study is to analyse some characteristics of the volatility of short & long term interest rates during the interest liberalization period. Therefore, this study focuses on analysing the conditional heteroscedasticity of GARCH models (including the GJR model) using two dummy variables which detect the volatility of short & long term interest rates. After all, some empirical results are summarized as follows; Firstly, the negative relationships was found between the deregulation of interest rate and the enlargement of the volatility of the call rates during the interest liberalization period. Secondly, the positive relationships was found in terms of the enlargement of the volatility of call rates & corporate bond rates during the Korean currency crisis. Thirdly, according to the median lag analysis, the efficiency of call & corporate bond market was detected after the Korean currency crisis. Fourthly, contrary to the corporate bond rates, the confidential evidence was found in detecting the asymmetry effect of volatility of call rates after the Korean currency crisis.

2차원 자기장에 의한 spin-valve 터널링 자기저항 소자의 자유층 반전 거동에 관한 연구

이영우,김철기,김종오,Lee, Young-Woo,Kim, Cheol-Gi,Kim, Chong-Oh 한국재료학회 2003 한국재료학회지 Vol.13 No.6

MTJ devices are fabricated using metal shadow masks and switching characteristics are investigated under 2 dimensional magnetic field. When the hard axis field is less than $\pm$ 16 Oe, switching behavior is similar to that based on the Stoner-Wohlfarth model. As the hard axis field is larger than $\pm$ 16 Oe, deviation from the expectation by Stoner-Wohlfarth model is observed. These phenomena are induced by the generation of multi-domain and inhomogeneous magnetization reversal.

교환 결합력을 갖는 CoFe/MnIr 박막의 마이크로파 투자율 특성

김동영(DongYoung Kim),김종오(Chong-Oh Kim),김철기(CheolGi Kim),M. Tsunoda,M. Takahashi 한국자기학회 2006 韓國磁氣學會誌 Vol.16 No.5

We measured the microwave permeability in the frequency range of 100㎒~9 ㎓ in the exchange biased CoFe/MnIr films. The results were analyzed based on the Landau-Lifshitz-Gilbert theory. The initial permeability and ferromagnetic resonance frequency was tuned by controlling the CoFe thickness and unidirectional anisotropy. The tunable range of ferromagnetic resonance frequency was up to 20 ㎓ in the thin CoFe layer of 1.5 nm. The CoFe/MnIr films showed the high permeability and low loss properties in the microwave frequency range. Thus, this material could be applied to the microwave devices.

자기터널절합에서 자기 및 자기저항의 접합크기 의존성

상카라나라얀,호영강,김철기,김종오,이의복,Sankaranarayanan, V.K.,Hu, Yong-kang,Kim, Cheol-Gi,Kim, Chong-Oh,Lee, Hee-bok 한국재료학회 2003 한국재료학회지 Vol.13 No.6

Magneto-optic Kerr Effect(MOKE), AFM and magnetoresistance measurements have been carried out on as-deposited and annealed Magnetic Tunnel Junctions(MTJs) with junction sizes 180, 250, 320 and 380 $\mu\textrm{m}$ in order to investigate the correlation among interlayer exchange coupling, surface roughness and junction size. Relatively irregular variations of coercivity $H_{c}$ (∼17.5 Oe) and interlayer exchange coupling $H_{E}$ (∼17.5 Oe) are observed over the junction in as-deposited sample prepared by DC magnetron sputtering. After annealing at $200^{\circ}C$, $H_{c}$ decreases to 15 Oe, while $H_{ E}$ increases to 20 Oe with smooth local variation. $H_{E}$ shows very good correlation with surface roughness across the junction in agreement with Neel's orange peel coupling. The increasing slope per $\mu\textrm{m}$ of normalized $H_{c}$ and $H_{E}$ are same near junction edge along free-layer direction irrespective of junction size, giving relatively uniform $H_{c}$ and $H_{ E}$ for wider junction size. Thickness profiles of the junctions measured with $\alpha$-step show increasingly flat top surface for larger junctions, indicating better uniformity for large. junctions in agreement with the normalized$ H_{c}$ and H$/_{E}$ curves. TMR ratios also increase with increasing junction size, indicating improvement for larger uniform junctions.

질화법으로 제작한 강자성 터널링 접합의 국소전도 및 자기저항 특성

윤대식,박범찬,이영우,이영,김종오,Yoon Tae Sick,Tsunoda Masakiyo,Takahashi Migaku,Park Bum Chan,Lee Young-Woo,Li Ying,Kim Chong Oh 한국재료학회 2004 한국재료학회지 Vol.14 No.3

Tunnel junctions with AI-N barriers fabricated by microwave-excited plasma were studied. When the Al thickness, nitridation time, and annealing temperature were 1 nm (0.8 nm), 50 s (35 s), and $280^{\circ}C$ ($300^{\circ}C$), TMR ratio and resistance-area product (RA) were 49% (34%) and $3 ${\times}$ 10^4$ $\Omega$$\mu\m^2$ ($1.5 ${\times}$ 10^4$ $\Omega$$\mu\m^2$), respectively. In order to clarify the annealing temperature dependence of TMR ratio, the local transport properties were measured for Ta 5 nm/Cu 20 nm/Ta 5 nm$29_{76}$ $Fe_{24}$ 2 nm/Cu 5 nm/M $n_{75}$$Ir_{25}$ 10 nm/ $Co_{71}$ $Co_{29}$ 4nm/Al-N junction with Al thickness of 0.8 nm and nitridation time of 35s at various temperatures. The increase of TMR ratio after annealing at $300^{\circ}C$, where the TMR ratio of the corresponding MTJ had the maximum value of 34%, can be well explained by the enhancement of the average barrier height ($\Phi_{ave}$) and the reduction of its fluctuation. After further annealing at $340^{\circ}C$, the leakage current was observed and the TMR ratio decreaseded

고주파용 CoFeAlO계 박막의 자기적 특성

김현빈(Hyun Bin Kim),윤대식(Tae Sick Yoon),N. D. Ha(N. D. Ha),김종오(Chong Oh Kim) 한국자기학회 2005 韓國磁氣學會誌 Vol.15 No.1

The influence of O₂ partial pressure on saturation magnetization, coercivity, anisotropy field and effective permeability(over 1 ㎓) of as-deposited Co-Fe-Al-O thin films, which were fabricated by RF magnetron reactive sputtering method, were investigated. The Co_(69.9)Fe_(20.5)A_(14.4)O_(5.2) thin film fabricated at O₂ partial pressure of 4% exhibits the best magnetic softness with saturation magnetization (4πMs) of 18.1 kG, coercivity (Hc) of 0.82 Oe, anisotropy field (Hk) of 24 Oe, and effective permeability (μeff) about 1,024 above 1 ㎓. The electrical resistivity of Co-Fe-Al-O thin films were increased with increasing O₂ partial pressure, the electrical resistivity of Co_(69.9)Fe_(20.5)A_(14.4)O_(5.2) thin film with the best soft magnetic properties was 560.7 μΩ㎝. Therefore, It is assumed that the good soft magnetic properties of Co_(69.9)Fe_(20.5)A_(14.4)O_(5.2) thin film results from high electrical resistivity and large anisotropy field.

높은 비저항을 갖는 RF 소자용 CoPdAlO 박막의 자기적 특성

김택수(Taek Soo Kim),이영우(Young Woo Lee),김종오(Chong Oh Kim) 한국자기학회 2001 韓國磁氣學會誌 Vol.11 No.3

Presently, an inductor adapted at MMIC (Monolithic Microwave Integrated Circuit) which is used for cellular phone or PHS operates at quasi-microwave range over 800 ㎒. However, a W-CDMA (Wideband Code Division Multiple Access) will use about 2 ㎓ range. Therefore magnetic film device should be compatible up to 2 ㎓. We have deposited Co-Pd-AI-O system film using rf sputtering method which is expected up to 2 ㎓, and investigated the effect of Pd content and magnetic field annealing. When Pd composition is 19%, Hk was 118 Oe, and μ' showed flat frequency characteristics up to 1.5 ㎓. The Q factor (= μ'/μ) was 23.3 at 1 ㎓, 6.7 at 1.5 ㎓ and 1.5 at 2 ㎓, respectively. Resonance frequency was 2.9 ㎓. Therefore Co-Pd-AI-O thin film could be used at over 1 ㎓, and also expected as an inductor material for wide band CDMA type cellular phone.

열처리에 따른 강자성 터널링 접합의 국소전도특성

윤대식,이영,박범찬,김철기,김종오,Yoon, Tae-Sick,Tsunoda, Masakiyo,Takahashi, Migaku,Li, Ying,Park, Bum-Chan,Kim, Cheol-Gi,Kim, Chong-Oh 한국재료학회 2003 한국재료학회지 Vol.13 No.4

Ferromagnetic tunnel junctions, Ta/Cu/Ta/NiFe/Cu/$Mn_{75}$ $Ir_{25}$ $Co_{70}$ $Fe_{30}$/Al-oxide, were fabricated by do magnetron sputtering and plasma oxidation process. The effect of annealing temperature on the local transport properties of the ferromagnetic tunnel junctions was studied using contact-mode Atomic Force Microscopy (AFM). The current images reflected the distribution of the barrier height determined by local I-V analysis. The contrast of the current image became more homogeneous and smooth after annealing at $280^{\circ}C$. And the average barrier height $\phi_{ave}$ increased and its standard deviation $\sigma_{\phi}$ X decreased. For the cases of the annealing temperature more than $300^{\circ}C$, the contrast of the current image became large again. And the average barrier height $\phi_{ave}$ decreased and its standard deviation $\sigma_{\phi}$ increased. Also, the current histogram had a long tail in the high current region and became asymmetric. This result means the generation of the leakage current that is resulted from the local generation of a low barrier height region. In order to obtain the high tunnel magnetoresistance(TMR) ratio, the increase of the average barrier height and the decrease of the barrier height fluctuation must be strictly controlled.led.

Lecithin이 흡착된 나노 자성입자의 특성과 그 자성유체의 생체 친화성

박상임(Sang-Im Park),김종오(Chong-Oh Kim),김종희(Jong-Hee Kim),김성민(Seong-Min Kim),김근호(Keun-Ho Kim) 한국자기학회 2006 韓國磁氣學會誌 Vol.16 No.6

Magnetic nanoparticles were prepared by thermal decomposition and adsorbed with lecithin by applying ultrasonic. The size and saturation magnetization of magnetic nanoparticles were observed with different lecithin concentration, and the maximum tolerated dose (MTD) and toxicity of magnetic fluid was investigated through a biological test. The thickness of lecithin-adsorption layer increased non-linearly with increasing amounts of added lecithin, and the desirable adsorption amount was observed in the lecithin concentration of 20 %(w/v). The dispersibility and magnetic properties of lecithin-adsorbed magnetic nanoparticles were most excellent when the ultrasonic exposure time was 1.5 h. Also, the maximum tolerated concentration with best cell viability was 32 ㎍/㎖ in vitro test, and lecithin-adsorbed magnetic fluids improved the biocompatibility by 1.2 times compared with bare magnetite fluids in vivo.