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홍순철(Soon Cheol Hong) 한국자기학회 2016 韓國磁氣學會誌 Vol.26 No.1
Pd has the highest magnetic susceptibility among single element metals and often shows ferromagnetism under some special environments. In this paper, we report magnetism of 5- and 9-monolayers (ML) calculated by using full-potential linearized augmented plane wave method. Exchange-correlation interaction is taken into account in local density approximation (LDA) and generalized gradient approximation (GGA) and calculational results in LDA and GGA are compared with each other. It is found that calculations by LDA are more reliable compared to those by GGA because LDA prediction of paramagnetism of bulk Pd is consistent with experiments, whereas GGA predicts wrongly ferromagnetim of bulk Pd. Calculational results in LDA on a 5-ML Pd(111) thin film shows a ferromagnetic ground state unlike a paramagnetic ground state of bulk Pd. The center Pd layer of the 5-ML Pd(111) thin film has the largest magnetic moment (0.273 μB) among the layers and |m| = 1 orbital states play a dominant role in stabilizing the ferromagnetism of the 5-ML Pd(111) thin film. A 9-ML Pd(111) thin film in a ferromagnetic state has almost the same total energy as in a paramagnetic state. Since the magnetization of the 9-ML Pd(111) thin film is stable, the ferromagnetic state may be meta-stable.
홍순철(Soon Cheol Hong),엇후(Dorj Odkhuu) 한국자기학회 2019 韓國磁氣學會誌 Vol.29 No.4
Since Nd₂Fe<SUB>14</SUB>B permanent magnet was developed in 1982, a new permanent magnet with higher performance than Nd2Fe14B has not been reported yet. On the other hand, new permanent magnet materials, which do not include rare-earth elements but are competible to ferrites in price, also are needed to be searched urgently. In this paper, we will review required material properties for good permanent magnets and discuss ways to search new permanent magnets, using first principles calculations. The goal to search the new permanent magnets should be set to find materials to have higher performance than Nd₂Fe<SUB>14</SUB>B or to be consisted of 3d transition metal as magnetic elements with compatible performance to Nd₂Fe<SUB>14</SUB>B to replace high performance rare-earth magnets or cheap ferrites.
사이클 및 CFD 과도상태 해석을 이용한 주택환경 냉방성능 예측에 대한 연구
홍순철(Soon-Cheol Hong),김태헌(Tae-Hun Kim),김중호(Jung-Ho Kim),이창선(Chang-Seon Lee),윤백(Baek Youn),박현연(Hyun-Yeon Park),안중현(Jung-Hyeon Ahn) 대한설비공학회 2013 대한설비공학회 학술발표대회논문집 Vol.2013 No.6
Predicting performance of air conditioning in thermal environment is very important to improve energy efficiency. Even the energy may include the effort, process, cost and much more time. In this simulation, the boundary condition equivalent to the actual housing environment with latent heat needs to be set accurately to solve the real heat transfer problem using 3D CFD transient analysis coupled with 1D cycle. As the result, air conditioning performance predicted by co-simulation is ±5% accuracy level comparing with the actual test value. It will effect on the development process and project planing.
Ni/Fe(001)의 자성과 자기이방성에 대한 제일원리계산
권오룡(Oryong Kwon),홍순철(Soon Cheol Hong) 한국자기학회 2015 韓國磁氣學會誌 Vol.25 No.4
Recent theoretical calculations predicted that a system composed exclusively of 3d transition metals without 4d/5d transition metals or rare earth metals can have strong perpendicular magnetocrystalline anisotropy (MCA) if Fe and Ni layers are arranged appropriately. They considered only Fe-terminated surfaces, noting that Fe/MgO(001) and CoFeB/MgO(001) show strong perpendicular MCA. In this paper, we investigate magnetism and MCA of Ni/Fe(001) surface where Ni layer is positioned at the surface, by using complementarily the first principles calculational methods of Vienna Ab-initio Simulation Package (VASP) and Fullpotential Linearized Augmented Plane Wave (FLAPW) method. Comparing results of magnetism and MCA obtained by VASP with the results by FLAPW method, we find the VASP results do not show big difference from results by FLAPW method. Magnetic moments of Fe and Ni are enhanced due to strong hybridization between Fe and Ni bands. MCA of Ni/Fe(001) is parallel to the surface, which implies the surface termination plays a crucial role in determining MCA of a system.
임성현(S. H. Rhim),홍순철(Soon Cheol Hong) 한국자기학회 2017 韓國磁氣學會誌 Vol.27 No.5
This review aims a brief and short-handed introduction to nonexperts, where most contents are based on talk given in Summer School 2017 in Pohang. Detailed derivations and formalism are avoided unless it is necessary to present. Historical evolution of density functional theory (DFT) is provided to grasp the main motivation of first-principles calculations in a more broad perspective. By doing so, a list of acronyms is shown, which in many cases become an obstacle to newcomers. Furthermore, a classification of DFT packages according to vendors, methods, and others is provided for better understanding. Next, the practical employment of ab initio calculations in magnetism is introduced, which includes spintronics, for more specifically, spin and anomalous Hall effect (SHE and AHE), magnetocrystalline anisotropy (MCA), magneto-optical Kerr effect (MOKE). Also, ingredients for DFT calculations - human, package, and infrastructure (cluster or supercomputing center) are briefly outlined. To do so, we expect long-term improvement and investment in our field, which in turn help other expertises.
페로브스카이트 구조를 가지는 CoFeX₃(X = O, F, S, Cl) 합금의 자성과 전자구조에 대한 제일원리계산
제갈소영(Soyoung Jekal),홍순철(Soon Cheol Hong) 한국자기학회 2016 韓國磁氣學會誌 Vol.26 No.6
For an industrial spin-transfer torque (STT) MRAM, low switching current and high thermal stability are required, simultaneously. For this point of view, it is essential to find magnetic materials which satisfy high spin polarization and strong perpendicular magnetocrystalline anisotropy (MCA). In this paper, we investigate electronic structures and MCA energies of perovskite CoFeX₃ (X = O, F, S, Cl). For X = F and Cl, spin polarization at the Fermi level are 97 % and 96 %, respectively, which are close to a half metal. Furthermore, Co-terminated 5-monolayer (ML) CoFeX₃ (X = O, F, S, Cl) films show perpendicular MCA. In particular, the MCA energy of the Co-terminated CoFeCl₃ is about 1.0 meV/cell which is three times larger than that of a 5-ML CoFe film. Therefore, we expect to realize a magnetic material with high spin polarization and strong perpendicular MCA energy by utilizing group 6 and 7 elements in the periodic table, and to contribute to commercializing of the STT-MRAM.
제일원리계산에 의한 단층 MoS₂ 의 1H/1T 경계 자성
제갈소영(Soyoung Jekal),홍순철(Soon Cheol Hong) 한국자기학회 2016 韓國磁氣學會誌 Vol.26 No.3
Monolayer MoS₂ is energetically most stable when it has a 1H phase, but 1H to 1T phase transition (1H→1T) is easily realized by various ways. Even though magnetic moment is not observed during 1H→1T, 0.049 μ<SUB>B</SUB>/MoS₂ is obtained in local 1T phase; 75% 2H and 25% 1T phases are mixed in (2 × 2) supercell. Most magnetic moment is originated from the 1T phase Mo atom in the supercell, while the magnetic moments of other atoms are negligible. As a result, magnetic/non-magnetic boundary is created in the monolayered MoS₂. Our result suggests that MoS₂ can be applied for spintronics such as a spin transistor.