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

ÁÖÁ¦: Spin dynamics from fundamentals to applications : Dynamics of vortices and spin waves ÀÏ½Ã: 2006³â 11¿ù 27ÀÏ(monday) Àå¼Ò: Room #65, Faculty of Eng. Bldg. VI, U of Tokyo, Japan ¹ßÇ¥ÀÚ : Sang-Koog Kim

ÃÊ·Ïº»¹®: 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] ¼º±Õ°ü´ëÇÐ±³ ¹°¸®ÇÐºÎ ÃÊÃ»°¿¬(±¹³»)

ÁÖ Á¦: Wave properties of dipole-exchange spin waves in confined magnetic thin films

ÀÏ ½Ã: 2006³â 10¿ù 25ÀÏ(¼ö¿äÀÏ) ¿ÀÈÄ 430

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ÃÊ ·Ï: 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] ¼¿ï´ëÇÐ±³ Àç·á°øÇÐºÎ ¼¼¹Ì³ª (±¹³»)

ÁÖÁ¦: Vortex Stability and Dynamics in Small Magnetic Particles ÀÏ½Ã: 2006³â 9¿ù 28ÀÏ Àå¼Ò: ÀÌµ¿³ç ¼¼¹Ì³ª½Ç (33µ¿ 208È£)

¹ßÇ¥ÀÚ : K.Y. Guslienko

ÃÊ·Ïº»¹®: 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] ¼¿ï´ë Àç·á°øÇÐºÎ ÃÊÃ»°¿¬(±¹³»)

ÁÖÁ¦: Static and Dynamic Magnetic Microstructures in Ferromagnetic Films ÀÏ½Ã: 2006³â 6¿ù 2ÀÏ(±Ý) 2:00 Àå¼Ò: ¼¿ï´ë ¸ÖÆ¼¹Ìµð¾îµ¿