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유천열(Chun-Yeol You) 한국자기학회 2005 韓國磁氣學會誌 Vol.15 No.4
Formation of a narrow magnetic domain wall is demonstrated by micromagnetics simulations. It is found that the domain wall width can be shrunk in a local exchange coupled system. The local exchange coupled system means that only a part of a ferromagnetic layer has an exchange coupling with another ferromagnetic layer. The system can be considered as two parts in the lateral dimensions: one is an exchange coupled region and another is a free region. Since the two regions have quite different local switching fields, the domain wall will be formed at the interface between the two regions at moderate field ranges.
마그네트론 스퍼터링법으로 증착한 Au 박막의 전기전도특성에 미치는 열처리 온도와 Ta 삽입층의 영향
최혁철,유천열,Choi, Hyeok-Cheol,You, Chun-Yeol 한국진공학회 2007 Applied Science and Convergence Technology Vol.16 No.6
We fabricated thin films of Au and Ta/Au with thicknesses of 30 nm and 5 nm/30nm, respectively on Si(100) or Si(111) substrates using a dc magnetron sputtering system. Grain sizes, roughness and conductivity for Au thin films are measured as a function of the annealing temperatures. We observed that the grain size of samples enlarged and the surface became rougher with increasing annealing temperature. The grain size and roughness were improved in the structure of Si/Ta/Au than Si/Au. Furthermore, the Si(100) substrate was more effective for decreasing the resistance for Ta/Au system than Si(111) substrate. We confirm that by inserting a Ta buffer layer in Si(100)/Au, surface roughness was reduced and by adjusting the annealing temperature the grain size were enlarged. Consequently, the Au thin-film has improved conductivity. 열처리 온도에 따른 Au 결정립 크기의 변화와 표면 거칠기 및 전기전도도를 연구하기 위해 dc 마그네트론 스퍼터링법을 사용하여 Si(111) 또는 Si(100) 기판위에 Au (30nm) 와 Ta (5 nm)/Au (30 nm) 를 증착하였다. 열처리 온도가 증가함에 따라 시료의결정립 크기가 증가하였고, 박막 표면 거칠기 또한 증가함을 확인하였다. Si/Au보다Si/Ta/Au구조에서 결정립 크기가 증가하였고 표면거칠기는 감소되었으며 Si(111)기판보다 Si(100) 기판위의 Ta/Au구조에서 전기 저항이 감소되었다. Si(100)/Au구조에 5 nm 두께의 Ta의 buffer layer를 삽입하여 표면 거칠기 정도를 낮춤과 동시에 열처리 온도를 적절히 조절하여 결정립 크기를 증가시킴으로서 전도성이우수한 양질의 Au 박막을 얻을 수 있었다.
미세자기 동역학을 이용한 강자성 나노선의 자기 잡음 연구
윤정범(Jungbum Yoon),유천열(Chun-Yeol You),조영훈(Younghun Jo),박승영(Seung-Young Park),정명화(Myung-Hwa Jung) 한국자기학회 2010 韓國磁氣學會誌 Vol.20 No.1
We investigate the spin dynamics of the magnetic domain wall using the magnetic noise in the magnetic nanowire structure by employing micromagnetic simulations. Magnetic noise due to the thermal fluctuations in ferromagnetic materials is related to magnetic susceptibility and resonance frequency, which are important physical quantities in the study of the spin dynamics. In this study, we present the magnetic noise of the single domain without magnetic domain wall, and with the magnetic domain wall between two magnetic domains in ferromagnetic nanowires. It is confirmed that the Kittel equation with simple ellipsoid model with demagnetizing factor well describe the resonance frequency due to magnetic noise of the single domain. Besides, we find that there is a distinguishable additional resonance frequency, when a magnetic domain wall exists. It is verified that the additional resonance frequency is originated from the magnetic domain wall, and it is lower than one of the single domain. It implies that the spins inside the domain wall have a different effective field.
교환 바이어스 인위적 준강자성 기준층을 포함한 자기 터널 접합의 강자성 공명
윤정범(Jungbum Yoon),유천열(Chun-Yeol You),정명화(Myung-Hwa Jung) 한국자기학회 2011 韓國磁氣學會誌 Vol.21 No.4
Spin dynamics of magnetic tunnel junctions with free and fixed reference layers is investigated by ferromagnetic resonance micromagnetic simulations. First, in magnetic tunnel junctions with an exchange biased synthetic ferrimagnetic reference layer, a magnetization direction of each layer and the tunneling magnetoresistance are calculated for a DC magnetic field. To investigate the spin exciting modes in magnetic tunnel junctions, we simulate the ferromagnetic resonance frequency spectra with small RF magnetic fields. Exciting modes of the tunneling magnetoresistance calculated by an included angle between free and reference layers is interpreted from those of each layer. Spin exciting modes are different according to a signs of the DC magnetic field. In a negative magnetic field, FMR frequency spectra of free and reference layers are well elucidated by the modified Kittel’s equation. However, in a positive magnetic field, there is no simple analytic solution related to FMR frequency spectra due to the coupled modes. Since ferromagnetic layers in magnetic tunnel junctions are interactive each other, careful considerations of the reference and fixed layer as well as the free layer are required for understanding on the spin dynamics of magnetic tunnel junctions with an exchange biased synthetic ferrimagnetic reference layer.
Brillouin Light Scattering을 이용한 GaAs/Fe/Au 구조의 자기이방성
하승석(Seung-Seok Ha),유천열(Chun-Yeol You),이석목(Sukmock Lee),Kenta Ohta,Takayuk Nozaki,Yoshishige Suzuki,W. Van Roy 한국자기학회 2008 韓國磁氣學會誌 Vol.18 No.4
It has been well-known that the Fe/GaAs heterostructure has a small lattice mismatch of 1.4 % between Fe and GaAs, and the Fe layer is grown epitaxially on the the GaAs substrate. There are rich physics are observed in the GaAs/Fe interface, and the spininjection is actively studied due to its potential applications for spintronics devices. We fabricated Fe wedge layer in the thickness range 0~3.4 ㎚ on the GaAs (100) surface with 5-㎚ thick Au capping layer. The magnetic anisotropy of the Fe/GaAs system was investigated by employing Brillouin light scattering (BLS) measurements in this study. The spin wave excitation of Fe layer was studied as the function of intensity and the in-plane angle of external magnetic field, and thickness of Fe layer. Also these various dependences were analyzed with analytic expression of spin wave surface mode in order to determine the magnetic anisotropies. It has been found that the GaAs/Fe/Au system has additional uniaxial magnetic anisotropy, while the bulk Fe has biaxial anisotropy. The uniaxial anisotropy shows increasing dependency respected to decreasing thickness of Fe layer while biaxial anisotropy is reduced with Fe film thickness. This result allows the analysis that the uniaxial anisotropy is originated from interface between GaAs surface and Fe layer.
하승석(Seung-Seok Ha),유천열(Chun-Yeol You) 한국자기학회 2006 韓國磁氣學會誌 Vol.16 No.2
We theoretically demonstrate that the interlayer exchange coupling (IEC) energy can be manipulated by means of an external bias voltage in a F₁/NM/F₂/S (F₁: ferromagnetic, NM: nonmagnetic metallic, F₂: ferromagnetic, S: semiconductor layers) four-layer system. It is well known that the IEC energy between two ferromagnetic layers separated by nanometer thick nonmagnetic layer depends on the spin-dependence of reflectivity to the F₁/NM/F₂/S four-layer system, where the reflectivities at the interface in NM/F₂ interface also depends on F₂/S interface due to the multiple reflection of an electron-like optics. Finally, the IEC energy depends on the spindependent electron reflectivity not only at the interfaces of F₁/NM/F₂, but also at the interface of F₂/S. Naturally the Schottky barrier is formed at the interface between metallic ferromagnetic layer and semiconductor, the Schottky barrier height and thickness can be tailored by an external bias voltage, which causes the change of the spin-dependent reflectivity at F₂/S interface. We show that the IEC energy between two ferromagnetic layers can be controlled by an external bias voltage due ti the electron-optics nature using a simple free-electron-like one-dimensional model.
임호택(Ho-Tack Lim),유천열(Chun-Yeol You) 한국자기학회 2005 韓國磁氣學會誌 Vol.15 No.6
Effect of a ferromagnetic layer thickness on a narrow domain wall width is investigated. It is found that the narrow domain wall is formed in ferromagnetic/nonmagnetic/ferromagnetic multilayer structure with a local interlayer exchange coupling, and that the width of the narrow domain wall is affected by the ferromagnetic layer thickness. We performed micromagnetics simulations for the Fe₁/Cr/Fe₂ system with the local interlayer exchange coupling, with fixed thickness (20-㎚) of Fe₂ layer and various Fe₁ layer thickness (1, 2, 4, and 6 ㎚). Consequently, we confirmed that the thinner the Fe₁ layer thickness, the thinner the width of the domain wall is formed, because of the surface energy nature of the interlayer exchange coupling.
Super-ROM/RENS 디스크 구조의 재생신호 해석을 위한 유한차분시간구역 (fdtd) 방법을 이용한 시뮬레이터 개발
안덕원 ( Duck Won Ahn ),유천열 ( Chun Yeol You ) 정보저장시스템학회 2005 추계학술대회논문집 Vol.2005 No.-
We developed a numerical simulator in order to study the Super- RENS/ROM (Super REsolution Near-Field Structure, Read Only Memory) using 3-dimensional FDTD (finite difference time domain) method. The simulation can be performed by three steps. In the first step, we utilized the vector-diffraction theory to calculate the characteristics of incident laser beam from the object-lens to the surface of the disk. At the second step, we fed the calculated result as an input for the main FDTD simulations on the optical layers in the disk structure. After performed the FDTD simulations, we took near-to-far field transformation for the reflected signal, from the surface of the disk to the detector. Finally, we can get reflected signal at the photo-diode. Using this developed simulator, we were able to study about the reading signal from various disk structures as a function of a laser beam position. We calculated reading signals for various pit sizes for Super-ROM structure, and it is found that the simple optical diffraction theory can not explain the reading mechanism of Super-ROM, and more complicated temperature dependent physics must be involved.