311 research outputs found

    Spin dynamics under spin Hall effect modulation: Skyrmion oscillator

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    학위논문(박사)--서울대학교 대학원 :자연과학대학 물리·천문학부(물리학전공),2019. 8. 최석봉.A magnet exhibits semi-permanent magnetic field, unless the ordering of magnetic moments does not break by external factors. This so-called non-volatility of magnetization can be harnessed to realize a power-efficient data storage, provided with proper mechanisms to modify the magnetization. These mechanisms were established by the discovery of giant magnetoresistance and spin-polarized current in the 1980s, which enabled the electric detection and control of magnetization, respectively. The concept of spin finally entered the field of electronics, which consecutively led to successful applications in the logic and memory devices. This associated field of study is called the spintronics, also known as the spin electronics. Since the spintronics is involved deeply with the collective ordering of the spins, the constraints bestowed upon the system not only expand the phenomena toward exotic dynamics but also provide design rules to achieve desirable properties. Among various possible constraints, a simple tri-layered system of a ferromagnetic thin film sandwiched between two nonmagnetic layers, exhibit surprisingly complex spin dynamics depending on the choice of the materials and their respective thicknesses. As a result, the current-induced spin dynamics in this tri-layered magnetic system is mainly studied throughout the thesis. Amid the various interesting dynamics of a tri-layered film, the spin-Hall effect (SHE) in the sandwiching heavy metal layers that transfer spin polarized current into the ferromagnetic layer, stands out with its design capabilities. Since the magnitude and sign of the spin polarized current by SHE depends on the material and its thickness, one can manipulate the transferred spin torque by modulating the thickness of the sandwiching layers. This technique is called the spin-Hall-effect modulation and exhibits some interesting features. The thesis is mainly directed on searching what and how exotic spin dynamics happen at the wires with laterally modulated SHE, via micromagnetic simulations and analytic equation analysis. Chapter 2 shows how the current-driven domain wall (DW) pins and depins from various types of spin-Hall-effect-modulation boundaries. The method of unidirectional depinning from given modulation boundaries are investigated. This unidirectional depinning behavior provides a systematic mechanism to precisely move a DW step-by-step toward next modulation boundaries only by alternating the direction of electric current, which will assist the realization of a racetrack memory. Chapter 3 is the highlight of this study where we propose a whole new concept of spin-torque oscillator, based on magnetic skyrmion dynamics subject to lateral modulation of the SHE. In the oscillator, a skyrmion circulates around the modulation boundary between opposite SHE-torque regions, where the SHE pushes the skyrmion in the opposite direction, toward the modulation boundary. A micromagnetic simulation confirms such oscillations. This SHE-modulation-based skyrmion oscillator is expected to overcome the troubling issues of conventional spin-torque oscillators. As part of recent approaches to search for possible applications of spintronic devices, neuromorphic engineering is also briefly discussed in Chapter 4. A neuron device with integrate-and-fire feature is realized via current-driven DW motion in a wire with a magnetic tunnel junction at the end. With the already proposed idea of a DW synapse device, all-DW-based artificial neural network can be realized. Additionally, miscellaneous analytic equations were derived to help magnetic-parameter measurement and to offer design rules for certain properties. The depinning current from a triangle notch, the equations to measure spin-orbit torque at any initial angle and the equations to measure anisotropy field from magneto optical Kerr effect setup are derived from associated analytic models and explained in Chapter 5. Findings analyzed in this thesis provide the latest understanding of the spin-Hall effect modulated systems and some others. The explained spin dynamics in these systems not only exhibit properties that can better the state-of-the-art applications, but also triggers new possibilities to design in completely unconventional ways.강자성 물질은 정렬된 자화의 상태가 외부 요인에 의해 깨지지 않는 한, 반영구적으로 자기장을 가진다. 이것은 자화가 일종의 비휘발성을 가진다는 것을 의미하며, 이는 자화를 변화시킬 수 있는 적절한 방법이 갖추어진다면, 자성물질을 이용하여 저전력의 저장장치를 구현할 수 있다는 것을 의미한다. 실제로 1980 년대부터 거대 자기 저항 효과 (giant magnetoresistance: GMR)와 스핀 토크 현상 (spin torque)이 발견됨으로써, 자화가 각자 전기적으로 측정되거나, 컨트롤 될 수 있게 되었고, 이로 인해, 물질의 스핀 상태가 본격적으로 전기 소자에 응용되기 시작하였다. 주로 논리 및 메모리 소자에 응용되는 관련 분야에 대한 제반의 연구를 스핀트로닉스 (spintronics: spin electronics)라고 부른다. 스핀트로닉스는 언급하였듯이, 스핀들의 집단 현상에 깊게 관계되어 있기 때문에, 시스템에 주어진 여러 제한 조건들에 의해, 완전히 새로운 현상이 발견될 뿐만이 아니라, 그 조건을 이용함으로써, 원하는 특성을 디자인 할 수 있는 가능성까지 갖추고 있다. 여러 제한조건들 중, 간단한 3층 구조의 강자성 박막이 놀랍도록 복잡한 스핀 동역학을 보이는 것이 알려져 있다. 강자성 물질만이 아닌, 이웃하는 층의 물질과 두께, 경계면의 조건 등에 의해 다양한 자성 특징이 관찰되고 있고, 이 논문 또한 이러한 3층 구조에 전류가 주입되었을 때 나타나는 여러 현상을 밝히고 이용하는 데 핵심을 두고 있다. 3층 구조 필름의 여러 흥미로운 동역학 중에서도, 자성층에 이웃한 물질층에서 일어나는 스핀 홀 효과 (spin-Hall effect: SHE)는 디자인의 용이함에 있어 뛰어난 모습을 보인다. 스핀 홀 효과의 크기와 부호는 이웃층의 물질만이 아닌 두께에 따라 바뀌기 때문에, 이웃층의 두께를 바꾸어주는 것만으로, 자성층에 스핀 홀 효과로 인해 전달되는 스핀 토크의 크기 및 부호를 마음대로 바꾸어 줄 수 있다. 이 기술은 스핀 홀 효과 조정 기술 (spin-Hall effect modulation)으로 불리며, 이 논문은 이 스핀 홀 효과 조정 기술을 이용하여 스핀 홀 효과의 크기 및 부호가 다른 영역을 만들어 준 특수한 시스템들에서 어떤 새로운 스핀 동역학이 관찰 될 수 있는지를 미소자기시뮬레이션 (micromagnetic simulation)과 이론적 분석을 통해 연구하였다. 챕터 2는 전류로 구동되는 자구벽 (domain wall: DW)이 어떻게 스핀 홀 효과 조정 경계면에서 피닝 (pinning) 및 디피닝 (depinning) 되는 지 소개한다. 주어진 조정 경계면에서의 한쪽 방향으로 자구벽을 디피닝 시키는 방법이 소개되며, 이를 이용하여, 자구벽을 흐르는 전류의 부호를 바꾸어주는 것만으로 순차적으로 다음 조정 경계면으로 패스해나갈 수 있는 구조적인 방법을 구현함으로써, 학계의 주요 관심사인 레이스트랙 메모리의 실현에 한 걸음 더 다가간다. 챕터 3는 본 논문의 하이라이트로써, 스핀 토크 진동자의 새로운 컨셉을 기울어진 스핀 홀 효과 조정 경계면에서의 자성 스커미온 (skyrmion)의 동역학을 이용하여 구현한 결과를 소개한다. 진동자 구조 내부에서, 스커미온은 서로 스핀 홀 효과의 부호가 반대인 스핀 홀 효과 조정 영역 사이의 경계면을 따라 돌며, 이는 미소자기시뮬레이션을 통해 확인되었다. 새로운 컨셉인 스핀 홀 효과 조정 스커미온 진동자 (spin-Hall-effect-modulation skyrmion oscillator: SHEM-SO)는 현재까지 제시되어 있는 스핀 토크 진동자들의 모든 결함을 극복할 수 있을 것으로 기대된다. 챕터 4에서는 현재 스핀트로닉스가 새로이 역할을 할 바이오 모방 신경 공학 (neuromorphic engineering)에서 얻은 결과를 간략히 소개한다. 전류 구동 자구벽 움직임과 자기 터널 효과를 이용하여, 적분 및 발사 (integrate and fire) 기능을 구현한 뉴론 장비가 이미 제시되어 있는 자구벽 시냅스 디바이스와 합쳐져, 자구벽만으로 구현 가능한 인공 신경망 구조가 소개된다. 추가적으로, 자기 특성을 측정하거나, 새로운 장비의 디자인 룰을 제공하는 데 도움이 될 수 있는 여러 분석식들을 유도한 결과를 소개한다. 삼각 놋치 구조에서의 디피닝 전류의 식과, 임의의 자화 각도에서 스핀 궤도 토크 (spin-orbit torque)를 측정하는 데 필요한 식과, 광자기 컬 효과 (magneto optical Kerr effect: MOKE) 셋업 상에서 수직 비등방성 자기장 (perpendicular magneto anisotropy field)를 측정하는 데 필요한 식 등을 유도한 결과를 챕터 5에서 간략히 다룬다. 본 논문에 기술되어 있는 발견들은 스핀 횰 효과 조정 시스템에 대한 최신의 이해를 제공한다. 이런 시스템들에서 설명되는 스핀 동역학은, 최신 스핀트로닉스 장비들의 기준을 한 단계 업그레이드할 뿐만이 아니라, 틀에 박힌 스핀트로닉스 소자의 디자인 룰을 타파하고, 온전히 새로운 방식의 접근을 가능케 한다는 점에서 의의를 가진다.Contents Abstract 02 List of Figures 08 1. Introduction 11 1.1 Magnetic anisotropy 13 1.2 Spin torque 14 1.2.1 Spin-transfer torque 14 1.2.2 Spin-orbit torque 17 1.2.3 Spin-Hall effect modulation 19 1.3 Magnetic structures 20 1.3.1 Domain wall 20 1.3.2 Dzyaloshinskii-Moriya interaction 22 1.3.3 Skyrmion 23 1.4 Ferrimagnetism 24 1.5 Micromagnetic simulation 26 2. Domain wall pinning/depinning at the spin-Hall-effect-modulation boundary 29 2.1 Introduction 30 2.2 Pinning at the spin-Hall-effect-modulation boundary 31 2.3 Unstable depinning at the spin-Hall-effect-modulation boundary 33 2.4 Unidirectional depinning at three different spin-Hall-effect-modulation boundaries 35 3. Spin-Hall-effect-modulation skyrmion oscillator 44 3.1 Introduction 45 3.2 Skyrmion motion at the tilted spin-Hall-effect-modulation boundary 47 3.3 Properties of the spin-Hall-effect-modulation skyrmion oscillator 51 3.4 Spin-Hall-effect-modulation skyrmion oscillator in the synthetic ferrimagnetic structure 52 3.5 Conclusion 54 3.6 Supplementary analysis 54 3.6.1 Simulation methods 54 3.6.2 Thiele formula for skyrmion motion near modulation boundary 55 3.6.3 Thiele formula for synthetic ferrimagnets 58 3.6.4 Frequency variation with respect to the angle of the modulation boundary 60 4. Domain wall neuron device 62 4.1 Introduction 63 4.2 Synapse device 66 4.3 Neuron device 67 5. Derivation of miscellaneous analytic equations 70 5.1 The analytic formula on depinning current of magnetic domain walls driven by spin-orbit torques from artificial notches 71 5.1.1 Depinning field/current from a notch 72 5.1.2 1st-order approximation for transverse spin-orbit torque 80 5.2 1st-order equation of equilibrium angle under spin-orbit torque from any initial angle 82 5.3 Optical measurement of magnetic anisotropy field in nanostructured-ferromagnetic thin films 86 6. Conclusion 94 References 96 Publication List 107 Abstract in Korean (국문 초록) 108 Acknowledgments (감사의 글) 111Docto

    Strain control of magnetization for magnetoresistive sensors

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    This thesis focuses on the magneto-elastic interactions in thin films and their significance in technological applications with particular focus on sensors. The magnetostriction, constant which determines the strength of these interactions, plays a crucial role in various applications. For instance, strain immunity is essential for magnetic sensors to reduce strain cross-sensitivity, particularly in the case of flexible substrates. On the other hand, to sense strain mate- rials require giant strain effects. The optimization of the magnetic sensing layer, including strain anisotropy, is crucial for magnetic sensors performance, depending on their specific application requirements. The first part of this thesis discusses the characterization and the engineering of the strain-dependent material properties for the development of the free layer of magnetic sensors. The focus is on two material platforms: a Ni/Fe multilayer and Permalloy. The use of He+ ion irradiation as a post-deposition technique is explored to control magnetostriction and enhance magnetic softness. The strain dependence of anisotropy and magnetization compensation is explored in another material platform, Co/Gd synthetic ferrimagnets, that has the unique ability to switch their magnetic state using a laser pulse. In the second part of this thesis, the control of domain walls using strain is extensively studied for their applications in memory devices and magnetic sensors. Domain walls offer non-volatile positioning and energy efficiency in various applications. However, the influence of mechanical strain or stress on these sensing components has been overlooked. In our studies, we highlight the importance of considering mechanical strain in actual devices, exploring the effects of different types of strain on a sensor-type device. Uniform strain and its compensation through material preparation are discussed, along with the conceptualization and realization of a new magnetic sensor based on spatially variant strain. Furthermore, the impact of time-dependent strain on domain wall devices in the presence of surface acoustic waves is investigated. By considering these factors, a comprehensive understanding of the behavior and optimization of free layer of magnetic sensors under different strain conditions is achieved.v, 256 Seiten ; Illustrationen, Diagramm

    Crystalline Field Effects on Magnetic and Thermodynamic properties of a Ferrimagnetic Centered Rectangular Structure

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    The magnetic properties and phase diagrams of the mixed spin Ising model, with spins S=1 and {\sigma}=1/2 on a centered rectangular structure, have been investigated using Monte Carlo simulations based on the Metropolis algorithm. Every spin at one lattice site has four nearest-neighbor spins of the same type and four of the other type. We have assumed ferromagnetic interaction between the same spins type, antiferromagnetic for different spin types. An additional single-site crystal field term on the S=1 site was considered. We have shown that the crystal field enhances the existence of the compensation behavior of the system. In addition, the effects of the crystal field and exchange coupling on the magnetic properties and phase diagrams of the system have been studied. Finally, the magnetic hysteresis cycles of the system for several values of the crystal field have been found.Comment: 19 pages, 12 figures. arXiv admin note: text overlap with arXiv:2012.1092

    Micromagnetic Investigation of the Dynamics of Domain Walls and Substructures in Non-Uniform Magnetic Thin Films

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    Since the discovery of gigantic magnetoresistance and the advent of spintronics, magnetic materials have become ubiquitous in modern technology. As such, a wide range of research areas within this field are being continuously explored for potential means to improve device scaling, decrease energy consumption, or increase speed, as well as to understand the fundamental physics governing spin order at the micro- and nanoscale. One of the avenues under current investigation involves the use of the spin-orbit interaction in systems with broken inversion symmetry through the introduction of interfaces or compositional variations. I will introduce the theory of micromagnetism and demonstrate my recent work utilizing computational micromagnetics to understand the influence of the spin-orbit interaction on domain wall dynamics and domain wall structure. First, the dynamics of domain wall velocities in Bloch and Néel type domain walls are explored without the influence of a Dzyaloshinskii-Moriya Interaction (DMI) with parameters that vary through the film thickness, such as saturation magnetization and exchange stiffness. These results are compared to the domain walls when a DMI is present. We find a shift in domain wall velocities that is similar in magnitude to those with DMI. Next, a mechanism for enhanced velocities without symmetry breaking in-plane fields are further examined to understand how these velocities scale with film thickness. The mechanism for this enhanced velocity is explored, as well as means to obtain further enhancements through controlled in-plane fields. Finally, the role of point-like topological structures known as domain wall skyrmions on kink skyrmion dynamics are detailed, including the emergence of a new, novel resonant mode

    Current-driven domain wall dynamics in ferrimagnets: Micromagnetic approach and collective coordinates model

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    [EN] Theoretical studies dealing with current-driven domain wall dynamics in ferrimagnetic alloys and, by extension, other antiferromagnetically coupled systems as some multilayers, are here presented. The analysis has been made by means of micromagnetic simulations that consider these systems as constituted by two subsystems coupled in terms of an additional exchange interlacing them. Both subsystems differ in their respective gyromagnetic ratios and temperature dependence. Other interactions, as for example anisotropic exchange or spin-orbit torques, can be accounted for differently within each subsystem according to the physical structure. Micromagnetic simulations are also endorsed by means of a collective coordinates model which, in contrast with some previous approaches to these antiferromagnetically coupled systems, based on effective parameters, also considers them as formed by two coupled subsystems with experimentally definite parameters. Both simulations and the collective model reinforce the angular moment compensation argument as accountable for the linear increase with current of domain wall velocities in these alloys at a certain temperature or composition. Importantly, the proposed approach by means of two coupled subsystems permits to infer relevant results in the development of future experimental setups that are unattainable by means of effective models.MAT2017-87072-C4-1-P from the Spanish government SA299P18 from the Junta de Castillay León

    An Investigation of the Structure, Pinning and Magnetoresistance of Domain Walls in Ni81Fe19 Planar Nanowires

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    The research and development of Ni81Fe19 thin films and planar nanowire structures has attracted considerable interest in recent years; in terms of improving the fundamental understanding of the basic physical processes and also for the development of potential applications. Example applications include sensors and the data storage devices. The optimisation of such devices requires detailed knowledge of the thickness dependence and microstructural influences on the magnetic and magnetoresistance properties, along with a thorough understanding of the effect of geometrical confinement on domain wall (DW) structure and pinning behaviour in nanowire structures. The out-of-plane structural properties of thermally evaporated Ni81Fe19 thin films on pre-oxidised silicon substrates have been investigated using x-ray scattering techniques and transmission electron microscopy (TEM). These techniques have been used to provide information on the out-of-plane lattice parameter, the presence and degree of texture and also to quantify the width of the SiO2/Ni81Fe19 interface. Magneto-optical Kerr effect (MOKE) magnetometry, differential phase contrast TEM imaging, micromagnetic simulations and anisotropic magnetoresistance measurements (AMR) have been used to make a detailed study of the thickness dependence of the magnetic behaviour of both thin films and nanowire structures. The resistivity of thin films produced in this study is found to exhibit a higher value and lower mean free path than has previously been reported in the literature, which is attributed to the presence of a microstructure characterised by a small crystallite grain structure. The AMR is strongly thickness dependent for t < 10 nm, and tends toward zero for t < 7 nm. It is suggested that this is due to strain at the SiO2/Ni81Fe19 interface, which changes the magnetostriction and is related to the AMR by spin-orbit effects. The structure and pinning behaviour of DWs has been systematically investigated as a function of nanowire width, thickness and notch geometry. Although the wall structure is sensitive to the nanowire cross-sectional area, the DW depinning behaviour is relatively insensitive to notch geometry and instead is highly sensitive to wall type and chirality. A detailed model has been developed to make predictions for the AMR of individual DWs in nanowires. The model incorporates experimentally derived thickness dependent resistivity parameters and detailed DW spin structures from micromagnetic simulations. The magnitude of DW resistance is sensitive to wire width and the AMR ratio, and is found to be extremely sensitive to the magnitude of the magnetoresistance

    Ferromagnetic Resonance

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