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Lee, Sangyeop,Lee, Hakjoon,Yoo, Taehee,Lee, Sanghoon,Liu, X.,Furdyna, J. K. American Institute of Physics 2013 JOURNAL OF APPLIED PHYSICS - Vol.113 No.17
<P>Magnetic anisotropy of ferromagnetic semiconductor GaMnAs film with a low Mn concentration grown on a (001) GaAs substrate was investigated by Hall effect measurements. The presence of domains with in-plane and out-of-plane easy axes was identified in the film by analyzing hysteresis loops observed via the Hall resistance measured in various geometries. Quantitative analysis of the planar Hall resistance showed that the fraction of the sample with magnetic domains having a dominant out-of-plane easy axis was about 6 times larger than the fraction corresponding to domains with easy axis in the sample plane. (C) 2013 American Institute of Physics.</P>
Design of HTS Magnets for a 2.5 MJ SMES
Sangyeop Kwak,Seyeon Lee,Sangyeop Lee,Woo-Seok Kim,Ji-Kwang Lee,Chan Park,Joonhan Bae,Jung-Bin Song,Haigun Lee,Kyeongdal Choi,Kichul Seong,Hyunkyo Jung,Song-yop Hahn IEEE 2009 IEEE transactions on applied superconductivity Vol.19 No.3
<P>A 600 kJ HTS SMES system has been developed for power system stabilization as a national project in Korea. Successful operating tests of the 600 kJ were recently completed. In this paper, a 2.5 MJ class SMES with HTS magnets of single solenoid, multiple solenoid and modular toroid type were optimized using a recently developed multi-modal optimization technique named multi-grouped particle swarm optimization (MGPSO). The objective of the optimization was to minimize the total length of HTS superconductor wires satisfying some equality and inequality constraints. The stored energy and constraints were calculated using 3-D magnetic field analysis techniques and an automatic tetrahedral mesh generator. Optimized results were verified by 3D finite element method (FEM).</P>
Manipulation of magnetization in GaMnAs films by spin-orbit-induced magnetic fields
Lee, Sangyeop,Yoo, Taehee,Bac, Seul-Ki,Choi, Seonghoon,Lee, Hakjoon,Lee, Sanghoon,Liu, X.,Furdyna, J.K.,Dobrowolska, M. Elsevier 2017 Current Applied Physics Vol.17 No.5
<P>We have investigated the effect of spin-orbit-induced (SOI) magnetic fields on magnetization switching in GaMnAs films. The sign of such SOI fields depends on the direction of the current flowing in the film, thus providing a handle for electrically manipulating magnetization in ferromagnetic GaMnAs films. Specifically, when an applied magnetic field is swept along the current direction, magnetization reversal occurs via rotations in opposite sense (i.e., clockwise (CW) or counterclockwise (CCW)) depending on the sign of the current, thus leading to opposite signs of the planar Hall resistance (PHR) measured on the film. The effect of SOI fields also manifests itself through hysteretic behavior of PHR for two opposite currents as a fixed magnetic field is rotated in the film plane. The width of the resulting hysteresis between two current directions then allows us to estimate the magnitude of the SOI field at current density of 1.0 x 10(5) A/cm(2) as similar to 1.2 Oe in our GaMnAs film. Such switching of magnetization between two magnetic easy axes induced by switching the sign of an applied current provides a means of electronically controlling the value of film resistance (in this case of PHR), a process that can be exploited in spintronic devices. (C) 2017 Elsevier B.V. All rights reserved.</P>
불화탄소 전극의 열분해 연료유 기반 탄소 코팅이 리튬일차전지의 고율속 성능에 미치는 영향
이상엽 ( Sangyeop Lee ),하나은 ( Naeun Ha ),명성재 ( Seongjae Myeong ),임채훈 ( Chaehun Lim ),이세현 ( Sei-hyun Lee ),이영석 ( Young-seak Lee ) 한국공업화학회 2024 공업화학 Vol.35 No.4
불화탄소 기반 리튬일차전지(Li/CFX)의 활물질로 이용되는 불화탄소는 낮은 전도성에 기인한 열악한 율속 특성으로 방전 성능이 제한적이다. 따라서, 본 연구에서는 이를 극복하기 위하여 불화탄소에 열분해 연료유를 이용하여 탄소코팅을 진행하였고, 전기화학적 성능을 고찰하였다. 탄소 코팅에 의하여 불화탄소 표면에 무정형 탄소층이 형성되었으며, 열처리 온도에 따른 불화탄소의 표면 물리화학적 특성을 면밀히 고찰하였다. 상용 불화탄소를 450 ℃에서 열처리한 ARC@C450 샘플은 sp2 탄소 결합의 함량이 62%로 가장 크게 증가하였으며, 반이온성 C-F 결합이 가장 많이 형성되었다. 또한, ARC@C450 샘플을 환원극 활물질로 이용한 일차전지는 가장 높은 5 C 율속(392 mAh/g)에서 안정적인 방전 특성을 보였으며, Rct 값은 미처리 시료에 비하여 53% 감소하였다. 따라서, 본 연구에서는 불화탄소의 낮은 전도성을 극복하기 위한 방법으로 열분해 연료유 기반 탄소 코팅을 제안하며, 탄소 코팅된 불화탄소는 우수한 율속성능을 나타냄으로 고출력 일차전지로의 응용 가능성을 제시한다. The performance of carbon fluoride-based lithium primary batteries (Li/CFX) is limited due to poor rate capability resulting from the low conductivity of carbon fluoride, which is used as the active material. Therefore, in this study, we applied a carbon coating using pyrolysis fuel oil on carbon fluoride to overcome this limitation and considered its electrochemical performance. An amorphous carbon layer was formed on the surface of the carbon fluoride through carbon coating, and the surface physicochemical properties of the carbon fluoride were meticulously considered based on the heat treatment temperature. The advanced research chemical 1000 heat treated at 450 ℃ (ARC@C450) sample, which was commercial carbon fluoride heat-treated at 450 ℃, showed the largest increase in the concentration of sp2 carbon bonds (62%) and the highest formation of semi-ionic C-F bonds. Also, the primary battery using the ARC@C450 sample as a cathode active material exhibited stable discharge capability at the highest rate of 5 C (392 mAh/g), and the Rct value was reduced by 53% compared to the untreated sample. Therefore, we proposed pyrolysis fuel oil-based carbon coating as a method to overcome the low conductivity of carbon fluoride, and the carbon-coated carbon fluoride showed excellent rate performance, suggesting its potential application in high-power primary batteries.
Antiferromagnetic Interlayer Exchange Coupling in Ferromagnetic GaMnAs/GaAs:Be Multilayers
Lee, Hakjoon,Lee, Sangyeop,Choi, Seonghoon,Yoo, Taehee,Lee, Sanghoon,Liu, Xinyu,Furdyna, Jacek K. IEEE 2015 IEEE transactions on magnetics Vol.51 No.11
<P>Interlayer exchange coupling (IEC) between the GaMnAs layers in GaMnAs/GaAs:Be multilayer systems has been investigated using magnetotransport experiments. The observation of a stable antiparallel magnetization alignment state from the systems indicates the presence of antiferromagnetic (AFM) IEC between the GaMnAs layers. The transitions between parallel and antiparallel alignments of GaMnAs magnetic layers in the system were carefully investigated by measuring resistance change with increasing temperature under various bias magnetic fields. From the dependence of the transition temperature on bias fields, we have estimated the magnitude of AFM IEC and its temperature behavior of our GaMnAs/GaAs:Be multilayers.</P>
Lee, See Hi,Hwang, Joonki,Kim, Kihyun,Jeon, Jinhyeok,Lee, Sangyeop,Ko, Juhui,Lee, Jichul,Kang, Minhee,Chung, Doo Ryeon,Choo, Jaebum American Chemical Society 2019 ANALYTICAL CHEMISTRY - Vol.91 No.19
<P>A surface-enhanced Raman scattering-based lateral flow assay (SERS-LFA) technique has been developed for the rapid and accurate diagnosis of scrub typhus. Lateral flow kits for the detection of <I>O. tsutsugamushi</I> IgG (scrub typhus biomarker) were fabricated, and the calibration curve for various standard clinical sera concentrations were obtained by Raman measurements. The clinical sera titer values were determined by fitting the Raman data to the calibration curve. To assess the clinical feasibility of the proposed method, SERS-LFA assays were performed on 40 clinical samples. The results showed good agreement with those of the standard indirect immunofluorescence assay (IFA) method. SERS-LFA has many advantages over IFA including the less sample volume, simpler assay steps, shorter assay time, more systematic quantitative analysis, and longer assay lifetime. As SERS strips can be easily integrated with a miniaturized Raman spectrophotometer, field serodiagnosis is also more feasible.</P> [FIG OMISSION]</BR>