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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.
CoFe 박막의 자성과 자기결정이방성에 대한 제일원리계산
김은구(Eun Gu Kim),제갈소영(So Young Jekal),권오룡(Oryong Kwon),홍순철(Soon Cheol Hong) 한국자기학회 2014 韓國磁氣學會誌 Vol.24 No.2
We investigate magnetism and magnetocrystalline anisotropy of CoFe thin films, using VASP code in GGA. In this study Coterminated and Fe-terminated 5-layer CoFe thin films are employed. The Co-terminated CoFe thin film shows two total energy minima at 2-dimensional lattice constants of 2.45 A and 2.76 A. The film of 2.45 A has fcc-like structure and the film of 2.76 A has bcc-like structure similarly to a bulk CoFe alloy. And the fcc-like film is more stable by the energy difference of about 160 meV compared to the bcc-like film. The Fe-terminated CoFe film shows very complicated behaviour of total energy which is suspected to be closely related to its complex magnetic structure. The Co-terminated CoFe film of 2.76 A shows perpendicular magnetocrystalline anisotropy (MCA), while the film of 2.45 does parallel MCA. The Fe-terminated CoFe film also exhibits similar MCA behaviour.
정태성(Tae Sung Jeong),제갈소영(Soyoung Jekal),임성현(S. H. Rhim),홍순철(S. C. Hong) 한국자기학회 2017 韓國磁氣學會誌 Vol.27 No.2
Recent study shows that ordered alloy of L1₂ XPt₃ (M= V, Cr, Mn, Co, and Fe) exhibits various magnetic phases such as ferromagnetic-to-antiferromagnetic transition at the MnPt₃ surface. Moreover, it has been argued that CrPt₃, in particular, possess large magnetocrystalline anisotropy and Kerr rotation with possible violation of Hund’s rule. As such, we extend our work to thickness dependence of the magnetic structure of CrPt₃ thin film using density functional theory. Magnetic ground state of the bulk CrPt₃ turns out to be ferromagnetic (FM), where other magnetic phases such as A-type (A-AF), C-type (C-AF), and G-type antiferromagnetic (GAF) state have higher total energies than FM by 0.517, 0.591, and 0.183 eV, respectively, and magnetic moments of Cr in bulk are respectively 2.807 (FM), 2.805 (A-AF), 2.794 (C-AF) and 2.869 μ<SUB>B</SUB> (G-AF). We extend our study to CrPt₃(001) thin films with CrPtand Pt-termination. The thickness and surface-termination dependences of magnetism are investigated for 3-9 monolayers (ML), where different magnetic phases from bulk emerge: C-AF for CrPt-terminated 3ML and G-AF for Pt-terminated 5ML have energy difference relative to FM by 8 and 54 meV, respectively. Furthermore, thickness- and surface-termination-dependent magnetocrystalline anisotropies of the CrPt₃(001) films are discussed.