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[2006] 21st COE Seminar Series #124 (University of Tokyo, Japan)

주제: Spin dynamics from fundamentals to applications : Dynamics of vortices and spin waves
일시: 2006년 11월 27일(monday)
장소: Room #65, Faculty of Eng. Bldg. VI, U of Tokyo, Japan
발표자 : Sang-Koog Kim

초록본문:
 Spin dynamics in bulk and geometrically confined micrometer-size (or less) magnetic structures has been one of hot topics inthe current research areas of magnetism and magnetic materials. Spin motions excited in magnetic nanostructures have been well described by the Landau-Lifschitz-Gilbert equation. By numerical micromagneticcalculations using a variety of confined geometry models based on the equation of motions of individual spins interacting with another, we have studied on dynamic motions of magnetic vortices and their interactions with their counterpart structures, i.e., antivortices, the reversal of the polarization of vortex cores, and spin-wave emission followed by the annihilation of a vortex-antivortexpair, as well as the wave properties of spin waves and the design of spin-wave logic devices. In the first part of my talk, I will give general introductions of basic concepts of spin dynamics, and will focus on vortex dynamics in the second part. The vortex structure, which is characterized by in-plane spiral orientations of local magnetizations and a core region ofa few tens of nm with an orientation perpendicular to the plane, is easily found in nature in different systems ranging from ~ 10 nm to extremely large universe scales, such that quantized magnetic flux lines in superconductors, through water whirlpools and atmospheric tornadoes to spiral galaxies. In magnetic media, the vortices have also beenfound in different environmental microstructures such as in a segment of cross -tie walls and near or at domain boundaries between differently orientated domains, and in geometrically confined submicron-size thin-film elements as a nontrivial unique structure. In our successive previous works we found many interesting magnetic features and novel phenomena associated with the magnetic vortices. As examples, I will report on experimentally observed arrays of the vortices and antivorticesand their interacting signature at a frozen state of demagnetized Fe thin films, and numerical studies on attractive interactions between vortices and antivorticesand spin-wave radiations by the annihilation of their pairs, and a new dynamic mechanism of the vortex-core reversals (core-polarization switching) followed by spin-wave emission. In the last part, I will present the radiation and wave behaviors of spin waves and their conceptual applications to spin-wave logic devices. Spin waves (or magnons) have attracted much attention because of their fundamental interests as well as technological applications to magnetic logic and sensor devices. Accordingly, studies on the eigenmodesof spin -wave excitations have been intensively performed both theoretically and experimentally for quasi-two-dimensional micrometer-size ferromagnetic elements with various shapes. However, those spin waves have not been studied in terms of light wave characteristics. I will show micromagneticsimulation results on the characteristic wave properties of spinwaves traveling in various model systems made of patterned magnetic films such as nanowires. Radiation mechanism of the spin waves and their wave properties such as propagation, transmission, reflection, refraction, interference will be presented with some examples of their conceptual applications to logic devices. These results offer a preview of the forthcoming era of magnetic logic devices using spin waves traveling at ultrahigh speeds.
 
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[2006] 성균관대학교 물리학부 초청강연(국내)

주 제: Wave properties of dipole-exchange spin waves in confined magnetic thin films

일 시: 2006년 10월 25일(수요일) 오후 430

장 소: 성균관대 물리학과세미나실(31355호실)

초 록: Dipole-exchange spin waves in confined ferromagnetic thin-film structures have attracted much attention because of their fundamental interests as well as technological applications to magnetic logic and sensor devices [1]. Accordingly, studies on the eigenmodes of spin-wave excitations have been intensively performed both theoretically and experimentally for quasi-two-dimensional micrometer-size films with various shapes. However, those spin waves have not been studied in terms of its wave behaviors, such as radiation, propagation, transmission, reflection, and interference. In this talk, I will present micromagnetic simulation results on the characteristic wave properties of spin waves traveling in various model systems made of geometrically confined magnetic films [2]. It is found that strong spin waves can be radiated from the core of a single magnetic vortex by virtue of large torques localized at the core region, and that the radiated spin waves can be injected into and propagate well through variously shaped nanowire-type magnetic waveguides. Furthermore, their transmission through heterostructures, and its related filtering of spin waves with lower frequencies are investigated. Finally, spin-wave interference is demonstrated with a model system, which is analogous to Young’s double slit experiment with light. These results offer a preview of the forthcoming era of magnetic logic devices using spin waves traveling at ultrahigh speeds [3, 4].
 
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[2006] 서울대학교 재료공학부 세미나 (국내)

주제: Vortex Stability and Dynamics in Small Magnetic Particles
일시: 2006년 9월 28일
장소: 이동녕 세미나실 (33동 208호)

발표자 : K.Y. Guslienko

초록본문:
Understanding the fundamentals of the magnetization dynamics in systems with reduced dimensionality is central to the future advancement of the field of nanomagnetism and spintronics. Thus, it becomes important to calculate and observe the eigenfrequencies and eigenmodes of magnetization oscillations of sub-micron magnetic particles. The magnetization distribution in small ferromagnetic particles (dots) depends on their size and shape. For mesoscopic sizes and non-elliptical particle shapes, non-uniform magnetization distributions with zero remanence ("vortex" state) are often observed.

These states can be stable within a wide range of dot sizes from a few tens of nm up to a few tens of microns. In this talk I will present a review of calculations and measurements of the low-frequency (sub-GHz range) vortex dynamic excitations in soft magnetic dots. Particular cases of the circular, elliptic dots and tri-layer ferromagnet/spacer/ferromagnet dots will be considered. The vortex eigenfrequencies are quantized due to geometrical confinement and depend on the dot sizes and geometry. The observed vortex core position low-frequency oscillations are described as the gyrotropic modes of the magnetic vortex motions around the equilibrium positions induced by a gyroforce and dynamic magnetostatic restoring forces. The role of the vortex topological charges in the magnetization dynamics will be also
discussed.
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[2006] 서울대 재료공학부 초청강연(국내)
주제: Static and Dynamic Magnetic Microstructures in Ferromagnetic Films
일시: 2006년 6월 2일(금) 2:00
장소: 서울대 멀티미디어동
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[2006] KAIST 신소재공학과 초청강연(국내)
주제: Static and Dynamic Magnetic Microstructures in Ferromagnetic Films
일시: 2006년 5월 30일(화) 4:00
장소: KAIST  신소재공학과
초록본문:
        우리 주위에서 흔히 볼 수 있는 소용돌이 구조는 태양 크기의 1조 배에 해당하는 은하계부터 싱크대 배수구의 물 소용돌이까지 그 크기가 다양하다.  그 중에 하나가 한반도의 여름이 끝나갈 무렵에 적도 부근에서 발생하여 우리나라 근방을 거쳐 지나가는 태풍이다.   거시 세계의 은하계나 태풍의 소용돌이 형상과 유사한 소용돌이 구조가 자성박막내에도 존재하며 이를 자기소용돌이(magnetic vortex)라 한다. 자기소용돌이는 수십 나노미터 두께의 자성박막에서 스핀방향의 배열 상태가 마치 태풍이나 토네이도와 같은 소용돌이의 형상을 가지고 있어 붙여진 이름이다. 자기소용돌이의 크기는 수 마이크로에서 작게는 수 십 나노미터이며 그 중심에는 태풍과 같이 핵을 가지고 있다. 현재는 이 자기소용돌이의 구체적인 미세구조가 실험적으로 관찰되고 있다. 이러한 자기소용돌이는 학문적 관심뿐만 아니라 초고밀도 메모리 소자와 스핀트로닉스 소자의 응용 가능성 때문에 수년 전부터 주목을 받기 시작하였다. 지금까지는 마이크로미터 크기의 패턴된 자성 박막에서  형성된 자기소용돌이의 정적 구조에 관해 많은 연구가 이루어져 왔다. 그러나 반소용돌이(antivortex)와 소용돌이 구조간의 동적 상호작용에 대한 연구는 미흡한 상태이다.
    본 연구실에서는 세계 최초로 방사광 연 X-선을 이용하여 Fe 자성박막내의 국소영역 스핀방향을 관찰하고, 서로 다른 방향을 가지는 자구(magnetic domain)들의 경계면에 수많은 자기 소용돌이, 반소용돌이가 존재함을 실험적으로 관찰하였다. 또한, 이 자기 소용돌이와 반소용돌이 간의 상호작용이 자성박막의 자화반전에 동역학적으로 매우 중요한 역할을 하고 있음을 미소자기학 정밀 전산모사를 통하여 입증하였다. 본 연구는 자화반전에 있어서 상호작용하는 자기 소용돌이-반소용돌이(vortex-antivortex) 쌍의 핵심 역할을 규명함으로써 연속 박막에서의 자기 동역학을 이해하는데 있어서 중요한 방향을 제시하고 있으며, 실제 연속 박막에서 발견되는 스핀 구조들의  형성과정을 자세하게 기술하고 있어 자기구조 관찰 및 미소자기 동력학 분야에 지대한 공헌을 하였다.  이를 인정받아 본 연구 결과는 전 세계 응용물리학 분야의 우수한 연구결과만을 엄선해 게재하는 미국 응용물리학회지 “Applied Physics Letters” 저널에 2005년 1월 31일자 표지 논문으로 소개되었다.  
        본 강좌에서는 연 X-선을 이용하여 관찰한 다양한 형태의 자기소용돌이와 수 십 피코초 (1 ps) 시간 간격에서 일어나는 소용돌이-반소용돌이 쌍(vortex-antivortex pair)의 생성, 진행, 소멸하는 과정에 대한 미소자기 동역학 전산모사결과를 발표할 것이다.
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[2006] 국민대 나노전자물리학과 학부생 초청강연(국내)
주제: Spin World in Our Life
일시: 2006년 5월18일(목) 4:00
장소: 국민대학교 과학관
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