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[2013] 스핀파 동역학-소자 연구단 외부인사 초청강연

주제: 미정

연사: Yang-Ki Hong (The University of Alabama)
일시: 2013년 4월 29일 (월) 오후 10시
장소: 미정
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[2010] 제 25회 스핀파 동역학-소자 연구단 외부인사 초청강연

주제: RF‐Induced Vortex Dynamics in Individual  Micromagnetic Structures
연사: 임상현 박사 (National Institute of Standards and Technology)
일시: 2010년 6월 24일 (금) 오후 4시
장소: 서울대학교 33동 307호


내용: We investigated the magnetization dynamics of single, patterned, thin‐film Permalloy (Ni80Fe20) elements embedded in a coplanar waveguide (CPW). Such structures are considered as potential candidates for “spin bus” and “spin interconnect” applications. Depending on the dimensions of the patterned elements one of the eigen‐mode excitations is a vortex. In this talk I will present the results of electrical properties of magnetic vortices in such structures as a function of device size and shape. We used anisotropic magnetoresistance (AMR) effect as the detection mechanism in current‐modulated differential resistance (dV/dI) measurements. This approach is well established, simple, and sensitive to the formation and dislocation of vortices in a single element. Each element is represented through a Sshaped two‐port CPW segment with different size of gaps in the middle of center conductor. The devices were fabricated using conventional photolithography techniques on a polished quartz substrate. Driving the system with a radio frequency (RF) signal tuned to the resonance frequency of the vortex motion resulted in fluctuations in the vortex average positions between distinct states representing the change in the path of vortex pair motion during their interaction.
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[2009] 제 24회 스핀파 동역학-소자 연구단 외부인사 초청강연

주제: Observing Magnetic Vortex and Antivortex Dynamics in Real-Time with Time-resolved X-Ray Microscopy and Micromagnetic Simulations
연사: Markus Bolte (Universität Hamburg, Germany)
일시: 2009년 6월 5일 (금) 오후 4시
장소: 서울대학교 33동 307호

 

내용: Magnetic vortices are interesting because of their potential application in non-volatile memory devi¬ces. Vortex cores can switch their polarization when the excitation exceeds a cer¬tain threshold amplitude, however, the exact mechanism of the switching as well as of the deve¬lop¬ment towards the switching is still being actively investigated. Recently we have shown by time-resolved X-ray microscopy that vortices also gyrate when excited by in-plane alter¬nating spin-polarized cur¬rents, but it was experimentally shown that a part of the exci¬ta¬tion derives from the currents’ attending Oersted field. This has grave consequences for the interpretation of current-driven vortex-core switching in experiments. The findings could be matched to micromagnetic simulations as well as an analytical model that describes the gyra¬ting vortex by a harmonic oscillator. This model could also be extended to predict the vortex’s dynamics well into the nonlinear regime and for coupled vortices. Using rotating spin-polarized currents to lock into the vortex’s particular eigenmode, we also observed vortex-core switching at sufficiently high current densities. The dynamics of the antivortex, on the other hand, has been studied much less, even though both occur naturally in thin films, e.g., in crosstie domain walls. This is in part due to the difficulty to isolate individual antivortices. We isolated individual antivortices in specially shaped microstruc¬tures and investigated their dynamics by time-resolved X-ray microscopy. Using rotating currents, it was possible to, for the first time, observe antivortex-core switching experimentally.
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[2008] 제 23회 스핀파 동역학-소자 연구단 외부인사 초청강연

주제: Magnetic Tunnel Junctions for STT-RAM
연사: Kyung-Ho Shin     (KIST (Korea Institute of Science and Technology), Seoul, KOREA)
일시: 2008년 9월 9일 (화) 오후 4시 30분
장소: 서울대학교 33동 307호


내용: Magnetic RAM is expected to be the most promising candidate among the next generation memory thanks to the realization of ‘spin-transfer-torque’ phenomena and the success in the MgO barrier fabrication. A critical current density for switching a magnetic bit in STT(Spin-Transfer-Torque)-RAM is one of the most serious challenges and should be reduced to the order of 1 MA/cm2 prior to a commercialization. A magnetic tunnel junction (MTJ) structure with layers having an out-of-plane anisotropy or with a synthetic free layer has turned out to provide potential solutions for the reduction of the switching current density. We have fabricated MgO-based MTJs with various synthetic free layers and those with layers having out-of-plane anisotropy. The paper will report how their tunnel magenetoresistance (TMR) and critical switching current density (Jc) are influenced by the structure and materials of multilayers. A combination of DC magnetron sputtering(a Singulus TIMARIS II sputtering cluster system or AJA International), ion milling, photo- and electron beam lithography is applied to prepare the magnetic tunnel junctions with a nanoscaled size, for example, about 90 nm x 150 nm. Details of our fabrication process are described elsewhere. The magnetic properties of the multilayers are characterized by M (H) curves measured by VSM or AGM. Resistance changes upon applied field CPP-R (H), and CIMS signals, in the form of resistance changes versus injected direct current R (I), are detected by using a 4-point probe measurement system. In the MTJs consisting of CoFeB/ MgO/ CoFeB/ nonmagnetic spacer / ferromagnet (CoFeB/ MgO/ CoFeB/ NM/ FM2), we show that the TMR is dependent on the kind of materials used for NM and FM2, while the resistance area product in the low-resistance state remains constant. We illuminate how the TMR is affected by the structural and magnetic properties of the CoFeB/ NM/ FM2 free layer. In the MTJs with a layer having out-of-plane anisotropy, we demonstrate that a proper device structure and suitable material selection present TMR higher than 100%. We discuss the consequences of the use of synthetic free layers and the out-of-plane anisotropy on the reduction of the Jc. 
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[2008] 제 22회 스핀파 동역학-소자 연구단 외부인사 초청강연

주제: Novel phenomena in spintronic tunneling devices
연사: See-Hun Yang
일시: 2008년 9월 8일 (월) 오전 10시 30분
장소: 서울대학교 33동 307호

내용: Tremendous efforts and studies have been made to take advantage of the other degree of freedom of carriers with the solid state device in addition to charge: spin. In particular, magnetic tunneling junctions using spin-dependent tunneling have been recognized to be the most promising building block in non-volatile memory device. However, only little has been yet understood about underlying physics on this quantum phenomenon. In this seminar, I will discuss novel phenomena originated from spin dependent tunneling in a variety of spintronic devices. Also some new type of materials and devices that may be potentially useful for application will be briefly addressed.
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[2008] 제 21회 스핀파 동역학-소자 연구단 외부인사 초청강연

주제: Modelling of laser-induced magnetization dynamics
연사: Oksana Chubykalo-Fesenko (Instituto de Ciencia de Materiales de
        Madrid, CSIC, Cantoblanco, Madrid, Spain)
일시: 2008년 7월 24일 (목) 오전 10시
장소: 서울대학교 33동 307호

내용: Recent advances in ultra-fast pulsed laser experiments in magnetic media have opened new possibilities of controlling magnetisation dynamics in the femto and pico-second regime. The physical processes underlying the response of the magnetization on this ultra-short timescale are complicated and  far from being understood, but clearly involve the excitation and consequent non-equilibrium interaction of electron, phonon and spin sub-systems. An essential part of these experiments is a heat pulse, produced by the laser, acting on the magnetic material.
In the present talk we show that magnetisation dynamics at high temperatures can reasonably be understood taking into account that during ultra-fast laser-induced processes, the electron temperature rapidly increases up to and often above the Curie temperature Tc. Standard micromagnetic models, based on the Landau-Lifshitz-Gilbert (LLG)  equation cannot account for effects taking place at high temperature. This is basically because of the assumption of a constant magnetisation length and the fact the that high-frequency spin waves are not included into simulations.
In the present work, using the Langevin dynamics simulations with an atomistic spin model, we demonstrate the occurrence of several important effects of the macrospin dynamics at elevated temperatures. It is found that the linear demagnetization can occur on a time scale of less than one picosecond, in agreement with published experimental data. However, the recovery of the magnetisation can take place on very different time scales, depending on the magnetic state after heating. It is on the time scale of picoseconds if the system retains some “memory”' of the initial state, but can take up to two orders of magnitude longer if the magnetic state is completely disordered after heating. The existence of a slow recovery of the magnetisation after full demagnetisation is experimentally confirmed. The relaxation times are compared with experimental measurements.
However, the ultrafast (femto-second) dynamics occurs in extreme conditions of strong and rapidly varying fields and high temperatures.  This situation may require a new description of magnetization dynamics, even on a phenomenological level of the atomistic Landau-Lifshitz-Gilbert equation, taking into account that the correlation time for electron system could be of the order of the inverse characteristic spin frequency. For this case we introduce the thermodynamically correct phenomenological approach for spin dynamics based on the Landau-Lifshitz-Miyasaki-Seki equation.  The influence of the electron-electron correlation time on  longitudinal and transverse magnetisation relaxation is  investigated.
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[2008] 제 20회 스핀파 동역학-소자 연구단 외부인사 초청강연

주제: Towards multiscale modelling of magnetic materials:: magnetisation 
         dynamics of FePt at elevated temperatures
연사: Oksana Chubykalo-Fesenko (Instituto de Ciencia de Materiales de
         Madrid, CSIC, Cantoblanco, Madrid, Spain)
일시: 2008년 7월 23일 (수) 오전 10시
장소: 서울대학교 33동 307호

내용: Well-established methods for the evaluation of magnetic material properties include spin density functional theory, spin models, and micromagnetics. Each of these methods covers only a certain range of times, lengths, and temperatures.  In the present paper we introduce a multi-scale modelling approach, bridging some of the gaps between the three approaches above. The goal is to describe thermodynamic equilibrium and non-equilibrium properties of magnetic materials for temperatures up to and even above the Curie temperature and on length scales from the single atom reaching to micrometres, starting from first principles.
Since it was shown experimentally that the magnetic state of a material can be changed on a picosecond timescale using pulsed lasers there has been an increasing experimental and theoretical interest in this phenomenon. Especially from a technological point of view an understanding of fast laser processes is made pressing by the fact that ultra-high density information storage may require Heat Assisted Magnetic Recording (HAMR), which involves heating up to or beyond the Curie temperature. For a theoretical desrciption of magnetisation dynamics following a heat pulse in the picosecond regime there is a need to go beyond the formalism of conventional micromagnetics since complete demagnetization is possible following a laser pulse and it is known that micromagnetics is not capable of dealing with phase transitions.
In the first step we model, as an example, bulk FePt in the ordered L10 phase using an effective, classical spin Hamiltonian.  This atomistic model was constructed on the basis of first-principles methods. The next step is to simulate this spin model using the stochastic Landau-Lifshitz-Gilbert (LLG) equation. Due to the atomic resolution, this approach covers only length-scales up to some nanometres. However, the thermodynamic equilibrium properties for the magnetisation, the parallel and perpendicular susceptibilities and the exchange stiffnessare then used to develop a micromagnetic approach, based on the Landau-Lifshitz-Bloch (LLB) equation. With the latter approach, we model the laser-induced magnetisation dynamics at all emperatures and show the agreement with both atomistic approach and experiment.
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[2008] 제 19회 스핀파 동역학-소자 연구단 외부인사 초청강연

주제: Probing magnetic microdots and their arrays by ferromagnetic resonance
연사: Dr. Gleb Kakazei (Department of Physics, Oviedo University, Oviedo, Spain)
일시: 2008년 1월 18일 (금) 오후 2시
장소: 서울대학교 33동 307호

내용: Continuous wave ferromagnetic resonance (cw – FMR) at 10 GHz was used to characterize static and dynamic properties of rectangular and square arrays of circular nickel and permalloy microdots. In the case of a rectangular lattice, as interdot distances in one direction decrease, the in-plane uniaxial anisotropy field increases, in good agreement with a simple theory of magnetostatically interacting uniformly magnetized dots. In the case of a square lattice a four-fold anisotropy of the in-plane FMR field Hr is found when the interdot distance a gets comparable to the dot diameter D. This anisotropy, not expected in the case of uniformly magnetized dots, is explained by a non-uniform magnetization m(r) in a dot in response to dipolar forces in the patterned magnetic structure. It is well described by an iterative solution of a continuous variation procedure. In the case of perpendicular magnetization multiple sharp resonance peaks are observed below the main FMR peak in all the samples, and the relative positions of these peaks are independent of the interdot separations. Quantitative description of the observed multiresonance FMR spectra is given using the dipole-exchange spin wave dispersion equation for a perpendicularly magnetized film where in-plane wave vector is quantized due to the finite dot radius, and the inhomogenetiy of the intradot static demagnetization field in the nonellipsoidal dot is taken into account. It is also found that with an increase of θ (angle between magnetic field and sample normal), higher-order peaks approach the main one, cross it (each one at its own θ) and continue their shift to higher (in comparison with the main peak) fields. At θ > 15o the spectra have a reversed order – the main peak has the lowest resonance field value, and the intensities of higher-order peaks decrease with increasing peak number. Such structure of spin wave spectra is preserved until θ = 90o.
In addition, the same arrays of circular permalloy microdots were characterized by Vector Network Analyzer (VNA) technique. Vibrating sample magnetometry measurements demonstrate a typical magnetization curve for the vortex ground state. High frequency vortex resonance (two modes) with positive dispersion law was observed below the nucleation field. The broad uniform procession resonance peak occurs above nucleation field and reaches the similar to cw – FMR values of linewidth above annihilation peak. Its Hr in the whole frequency range (4 – 10 GHz) was perfectly described by Kittel formula that takes into account the demagnetizing factor of individual magnetic dot. An additional field independent low frequency peak was observed in a high density sample.
It was also demonstrated that ferromagnetic resonance force microscopy (FMRFM) can be used to determine both local and global properties of patterned submicron ferromagnetic samples. Local spectroscopy together with the possibility to vary the tip-sample spacing enables the separation of those two contributions to a FMRFM spectrum.

 
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[2008] 제 18회 스핀파 동역학-소자 연구단 외부인사 초청강연

주제: Application of ferromagnetic resonance techniques to the studies of magnetic thin films and nanostructures
연사: Dr. Gleb Kakazei (Department of Physics, Oviedo University, Oviedo, Spain)
일시: 2008년 1월 17일 (목) 오후 2시
장소: 서울대학교 33동 307호

내용:  Ferromagnetic resonance (FMR) has proven to be a powerful technique in the investigation of the magnetic parameters of continuous thin films and multilayers, to determine exchange interactions and different types of magnetic anisotropy fields. In the case of narrow resonance linewidth, FMR experiments have the capability of obtaining the resonance positions with an extremely high degree of accuracy (± few Oersted). It is very important that the measurements of resonance field angular dependence, both polar and azimuthal, and temperature dependence can be easily done. FMR was also successfully used to study standing spin waves in continuous thin magnetic films, both single layered and multilayers. And finally, FMR is a judicious technique to investigate the dynamics of internal spin interactions in ferromagnets that determine the relaxation processes.
The most conventional and frequently used FMR setups is so called continuous wave spectrometer (cw – FMR), where the frequency is fixed and the sample is placed inside cavity. Usually commercial electron spin resonance spectrometers that operate at frequencies 10 - 36 GHz are used for these purposes. This method provides the highest possible sensitivity. One of advantage of the high frequency FMR (10 GHz and above) is that the resonance field in most cases is larger than the saturation field of the sample, thus the influence of domain structures is avoided.
Another type of FMR setups is broadband spectrometer, where frequency can be changed in the broad range (0 - 40 GHz). This can be reached by placing the sample directly over stripline or coplanar waveguide. In this case it is possible to investigate the frequency dependence of resonance position and linewidth as well as to study samples in unsaturated state. The drawback of this technique is a lower homogeneity of radiofrequency (rf) field and lower sensitivity. Recently the sensitivity of this method was improved by using Vector Network Analyzer as a source/detector of rf radiation.
To study individual micron-size elements by FMR a new experimental technique, ferromagnetic resonance force microscopy (FMRFM) was recently developed. FMRFM is a variation of magnetic resonance force microscopy (MRFM) that was first proposed by J.A. Sidles in 1991 as a noninductive method to detect nuclear magnetic resonance in microscopic samples. Since then the technique has evolved into a powerful microscopy technique for electron spin resonance, nuclear magnetic resonance and ferromagnetic resonance. In 2004 D. Rugar et al. reported the detection of a single unpaired electron spin in silicon dioxide, achieving a spatial resolution of 25 nm.
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[2007] 제 17회 스핀파 동역학-소자 연구단 외부인사 초청강연

주제: Shape Effect on Magnetization and Magnetization Ringing in “Pac-man” Element
연사: Prof. Yang-Ki Hong (University of Alabama, USA)
일시: 2007년 6월 5일 (수) 오후 5시
장소: 서울대학교 33동 307호

내용: The size effect is a critical issue in the design of magnetic tunneling junction (MTJ) memory cells of an MRAM device. The magnetization reversal of submicron sized, elongated “Pac-man” (EPM) shaped Ni80Fe20 element was studied using micromagnetic simulation. Micromagnetic simulation was performed on the EPM-I 180 shape (1.4 times elongated, 180°slot angle) to understand the effects of lateral dimension while maintaining an aspect ratio of 2.8.  It was found that the reduction in lateral dimension decreases the possibility of domain wall formation, consequently resulting in sharper switching and faster reversal. However, the smaller the dimension, the more magnetization ringing. There is no significant difference between lateral and film thickness effects on the coercivity.  Suppression of magnetization ringing and spin polarized current switching will be introduced.
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[2007] International Workshop on Spin Dynamics in Restricted Geometry

International Workshop on Spin Dynamics in Restricted Geometry

Schedule : May 25th (Fri) 2007  09:00 AM ~17:20 PM
                May 26th (Sat) 2007 09:00 AM ~ 15:00 PM

Location : Bldg. 37-201-1, Seoul National University, Seoul, Korea

Invited Speakers
Prof. Burkard Hillebrands (University of Kaiserslautern, Germany)
Prof. Andrei Slavin (Oakland University, USA)
Dr. Peter Fischer (Lawrence Berkeley National Laboratory, USA)
Dr. Hermann Stoll (Max Planck Institute for Metals Research, Germany)
Dr. O. Chubykalo-Fesenko (Spanish National Research Council, Spain)
Prof.  Hideo Ohno (Tohoku University, Japan)
Prof.  Theo Rasing (Radboud University  Nijmegen, Netherlands)
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[2007] 제 10회 스핀파 동역학-소자 연구단 외부인사 초청강연

주제: Pulsed Laser Deposition과 High Resolution X-ray Diffraction을 사용한 메모리용 산화물 박막의 연구소개
연사: 정창욱 교수님 (한국외국어대학교)
일시: 2007년 5월 2일 (수) 오후 4시
장소: 서울대학교 공대 Cyber 강의실 (37동 201-1호)

내용: ABO3산화물 중 Perovskite구조를 가지는 것들은 다양한 고체물리학적 특성을 보이는 데 이중에서 강유전체와 강자성체를 보이는 것들이 흥미롭다. 이들을 thin film으로 만들어 비휘발성 메모리를 구현하려는 연구가 각광받고 있다. Pulsed Laser Deposition과 High Resolution X-ray Diffraction을 각각 사용하여 박막을 제작하고 구조를 분석할 수 있다. 부피가 매우 작은 박막시료의 자성특성은 초전도양자간섭장치(Superconducting quantum inferference device)를 사용하여 분석한다. 강자성체금속인 SrRuO3와 강자성체 절연체인 YTiO3에 대한 연구결과를 간략하게 소개한다. 그리고 최근 PLD를 이용한 산화물 박막 연구에 사용되는 다양한 기술 중 한 두가지를 소개하고자 한다.
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[2007] 제 9회 스핀파 동역학-소자 연구단 외부인사 초청강연

주제: Enhanced Thermodynamic Stability of Epitaxial Oxide Thin Films
연사: 송종현 교수 (충남대학교)
일시: 2007년 4월 23일 (월) 오후 4시
장소: 서울대학교 공대 Cyber 강의실 (37동 201-1호)

내용: Oxide thin films have been attracting much attention due to their exotic physical properties ranging from insulator to superconductor and possibility of device applications1. Here we reexamine the growth phase diagram of La0.7Sr0.3MnO3 thin films grown by pulsed laser deposition, which are heavily studied for their magnetoresistive properties even at room temperature. Unlike previous reports, we obtained stoichiometric films as grown, despite extremely reducing growth conditions, with bulk-like Curie temperatures exceeding 360oC. Masking this stability is a particularly strong dependence of film stoichiometry on the laser profile at the target and also on kinetics of ablated species on the substrate surface. These results indicate that bulk thermodynamic limitations can be overcome in perovskite films using epitaxial stabilization.
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[2007] 제 8회 스핀파 동역학-소자 연구단 외부인사 초청강연

주제: Recent studies on spin-torque-related (transport) experiments
연사: 정명화 박사 (한국기초과학지원연구원)
일시: 2007년 4월 13일 (금) 오후 4시
장소: 서울대학교 공대 Cyber 강의실 (37동 201-1호)

내용: 한국기초과학지원연구원(KBSI: Korea Basic Science Institute) 양자물성팀이 현재 소유하고 있는 스핀토크 기술 관련 연구장비를 소개한다. Microwave device 중에서 magnetic nano-oscillator와 spin-torque diode에 관해서 설명하고, race track device 중에서 straight nanowire, notch를 가지고 있는 straight nanowire, curved nanowire, semicircular nanowire 등에서 보이는 domain wall motion에 대해서 설명하고자 한다. 더불어 최근에 양자물성팀에서 연구한 스핀토크 기술 관련 실험 결과에 대해서 논의하고자 한다.
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[2007] 제 7회 스핀파 동역학-소자 연구단 외부인사 초청강연

주제: Self-consistent feedback between inhomogeneous magnetization and spin-transfer torque
연사: 이경진 교수 (고려대학교)
일시: 2007년 4월 13일 (금) 오후 3시
장소: 서울대학교 공대 Cyber 강의실 (37동 201-1호)

내용 : As predicted by Slonczewski and Berger, magnetization switching and microwave excitation in a nanomagnet by a spin-polarized current has been experimentally demonstrated. It is ascribed to spin-angular-momentum transfer of incoming electrons to local magnet, i.e. spin-transfer torque. The spin-transfer torque is attracting a considerable interest because of its applicability to various spintronic devices. A micromagnetic study is essential to understand the new physics because the magnetic dynamics cannot be described within the framework of single domain state.
The Slonczewski’s spin torque term was originally suggested within the context of homogeneous magnetic domain. Micromagnetic and experimental studies have revealed that the magnetizations excited by the spin torque could be inhomogeneous. Therefore we have to find a way of correcting the Slonczewski’s term in describing the magnetization dynamics. We show the self-consistent model to numerically solve the equations of motion of local magnetization and spin accumulation. The self-consistent model enables us to consider the feedback between inhomogeneous magnetization and the spin torque. We found the feedback is crucial in the magnetization dynamics induced by the spin torque. We will show the numerical evidence of the importance of the feedback for the current-induced magnetic excitation in a single Co layer and a spin valve structure.
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