自旋波驱动畴壁运动动力学的微磁学研究

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 재료공학부, 2021. 2. Chan Park.자벽 이동은 오랫동안 차세대 논리 및 메모리 장치를 개발하는 데에 가능한 해결책으로 여겨져 왔다. 자벽 이동을 구동하기 위하여, 최근 스핀파가 새로운 원동력으로 제안되고 있다. 그러나, 자벽이동의 기구와 원리 관련 이해가 부족하며, 스핀파를 이용하여 자벽이동을 정밀하게 제어하는 것은 많은 해결되지 못한 문제를 가지고 있다. 이 논문에서는 자성 나노스트립 （magnetic nanostrip） 에서 스핀파로 인한 자벽 이동의 동역학을 미시 자기 시뮬레이션 (micromagnetic simulation) 을 이용하여 아래와 같이 세 가지 문제를 중심으로 조사하였다. 첫째, 스핀파가 구동된 자벽 이동의 물리적 메커니즘; 둘째, 스핀파로 인한 자벽 이동의 관성 변위; 셋째, 임의의 스핀파 (arbitrary spin waves) 와 여러 종류의 자벽이 포함된 시스템에서의 자벽 이동 거동; 첫 번째 문제와 관련하여, 스핀파의 흡수를 계산하였고 스핀파 펄스를 사용했다는 점에서 기존 연구와 차별화된다. 계산된 스핀파 흡수는 자벽 이동 속도와 동일한 경향을 가지며, 자벽 이동은 spin-transfer torque (STT) 를 제공하기 위하여 스핀파 흡수를 필요로 한다는 것이 확인되었다. 두 번째 문제와 관련하여, 유발된 스핀파 펄스가 자벽 이동을 구동할 수 있는 것과 자벽 이동의 가속과 감속 현상이 관찰되었다. 스핀파 펄스가 가해지면, 자벽이 가속과 감속을 한다는 것을, 1차원 모델을 이용하여, 설명하였다. 특히, 감속 과정은 자벽의 이완 (domain wall relaxation) 의 결과로 발생하는 것을 확인하였다 세 번째 문제와 관련하여, 서로 다른 파형의 스핀파와 다양한 형태의스택형 자벽 구조가 사용되었다. 그리고 임의의 스핀파에 의한 자벽이동을 푸리에 분석을 이용하여 정량화하였으며, 다양한 형태의 자벽이 포함된 자벽이동은 resonant 픽의 움직임이 변형된다는 것이 확인되었다. 이외에, 스택형 자벽 구조의 움직임은 속도 스펙트럼 (velocity spectrum) 에 변화를 나타내는 것을 확인하였다. 이 연구는 스핀파와 자벽 이동의 상호작용에 대한 이해를 높이고 다양한 구조의 자벽이 포함된 시스템에서의 자벽이동을 제어하는 것에 실질적으로 활용될 수 있으며, 자벽 이동을 이용하는 장치의 개발에 큰 도움을 줄 수 있을 것이다.Magnetic domain wall motion has long been considered a feasible solution to developing next-generation logic and memory devices. Recently spin wave has been proposed as a new driving force for the domain wall motion. Due to the unclear physics, however, it is currently still immature to achieve reliable control of domain wall motion using spin wave. In this thesis, the dynamics of spin wave-induced domain wall motion in a magnetic nanostrip is investigated using micromagnetic simulation. Particularly, three important problems are studied: (1) mechanism of spin wave-induced domain wall motion, (2) spin wave-induced domain wall inertial displacements, and (3) domain wall motion in cases with arbitrary spin waves and multiple domain walls. As regards the first problem, spin wave absorption by domain wall is for the first time calculated and is compared with the forward domain wall velocity. The excellent agreement between the two quantities suggests that forward domain wall motion necessarily consumes spin wave absorption for the required magnonic spin-transfer torque. Concerning the second problem, a spin wave pulse is generated to drive domain wall motion. Negligible acceleration and inevitable deceleration are observed. Such inertial displacements can be understood based on a 1-D model developed and used in this study. Particularly, the deceleration process is found to be a result of domain wall relaxation which includes the release of domain wall internal energy and reduction of the out-of-plane tilting of domain wall. Concerning the third problem, spin waves of different waveforms are generated and stacked domain wall structures are formed. It is found that spin wave harmonic is the basic element when interacting with domain wall and an arbitrary spin wave-induced domain wall motion can be quantified based on the Fourier analysis. The motion of the stacked domain walls is shown to exhibit modifications in the velocity spectrum, which can be ascribed to a changed property of spin wave reflection. This thesis aims to shed further light on the interaction between spin waves and domain walls and pave the way for future development of domain wall motion-based applications.Abstract i Acknowledgement ii Lsit of Figures iii List of Tables xv Chapter 1. Introduction 1 1.1 Motivation 1 1.1.1 Novel data storage based on domain wall motion 1 1.1.2 Other applications based on domain wall motion 7 1.2 Background 10 1.2.1 Domain wall 10 1.2.2 Domain wall motion 16 1.2.3 Spin wave-induced domain wall motion 22 1.3 Research objectives 28 1.4 Scope of this thesis 29 Reference 31 Chapter 2. Theoretical fundamentals 36 2.1 Basics of magnetism 36 2.1.1 Magnetic field 38 2.1.2 Magnetic moment 41 2.1.3 Magnetic interactions 52 2.1.4 Magnetic order 65 2.2 Theory of micromagnetism 77 2.2.1 Assumptions in the continuum theory of micromagnetism 79 2.2.2 Thermodynamics in micromagnetism 80 2.2.3 Landau free energy and effective field 81 2.2.4 Static micromagnetism 95 2.2.5 Dynamic micromagnetism 100 2.2.6 Micromagnetic simulation 135 Reference 138 Chapter 3. Mechanism of spin wave-induced domain wall motion 145 3.1 Introduction 145 3.2 Micromagnetic simulation 147 3.3 Results and discussion 147 3.4 Conclusion 153 Reference 155 Chapter 4. Spin wave-induced domain wall inertial displacements 157 4.1 Introduction 157 4.2 Micromagnetic simulation 159 4.3 Results and discussion 160 4.4 Conclusion 172 Reference 173 Chapter 5. Domain wall motion in cases with arbitrary spin wave and multiple domain walls 177 5.1 Introduction 177 5.2 Micromagnetic simulation 179 5.3 Results and discussion 180 5.4 Conclusion 195 Reference 197 Chapter 6. Conclusion and future works 200 6.1 Conclusion 200 6.2 Future works 201 List of Publications 203 Abstract in Korean 204Docto

Interplay between spin wave and magnetic vortex

In this paper, the interplay between spin wave and magnetic vortex is studied. We find three types of magnon scatterings: skew scattering, symmetric side deflection and back reflection, which associate with respectively magnetic topology, energy density distribution and linear momentum transfer torque within vortex. The vortex core exhibits two translational modes: the intrinsic circular mode and a coercive elliptical mode, which can be excited based on permanent and periodic magnon spin-transfer torque effects of spin wave. Lastly, we propose a vortex-based spin wave valve in which via inhomogeneity modulation we access arbitrary control of the phase shift.Comment: 33 pages, 23 figures, 1 tabl

Seizing the window of opportunity to mitigate the impact of climate change on the health of Chinese residents

The health threats posed by climate change in China are increasing rapidly. Each province faces different health risks. Without a timely and adequate response, climate change will impact lives and livelihoods at an accelerated rate and even prevent the achievement of the Healthy and Beautiful China initiatives. The 2021 China Report of the Lancet Countdown on Health and Climate Change is the first annual update of China’s Report of the Lancet Countdown. It comprehensively assesses the impact of climate change on the health of Chinese households and the measures China has taken. Invited by the Lancet committee, Tsinghua University led the writing of the report and cooperated with 25 relevant institutions in and outside of China. The report includes 25 indicators within five major areas (climate change impacts, exposures, and vulnerability; adaptation, planning, and resilience for health; mitigation actions and health co-benefits; economics and finance; and public and political engagement) and a policy brief. This 2021 China policy brief contains the most urgent and relevant indicators focusing on provincial data: The increasing health risks of climate change in China; mixed progress in responding to climate change. In 2020, the heatwave exposures per person in China increased by 4.51 d compared with the 1986–2005 average, resulting in an estimated 92% increase in heatwave-related deaths. The resulting economic cost of the estimated 14500 heatwave-related deaths in 2020 is US$176 million. Increased temperatures also caused a potential 31.5 billion h in lost work time in 2020, which is equivalent to 1.3% of the work hours of the total national workforce, with resulting economic losses estimated at 1.4% of China’s annual gross domestic product. For adaptation efforts, there has been steady progress in local adaptation planning and assessment in 2020, urban green space growth in 2020, and health emergency management in 2019. 12 of 30 provinces reported that they have completed, or were developing, provincial health adaptation plans. Urban green space, which is an important heat adaptation measure, has increased in 18 of 31 provinces in the past decade, and the capacity of China’s health emergency management increased in almost all provinces from 2018 to 2019. As a result of China’s persistent efforts to clean its energy structure and control air pollution, the premature deaths due to exposure to ambient particulate matter of 2.5 μm or less (PM2.5) and the resulting costs continue to decline. However, 98% of China’s cities still have annual average PM2.5 concentrations that are more than the WHO guideline standard of 10 μg/m3. It provides policymakers and the public with up-to-date information on China’s response to climate change and improvements in health outcomes and makes the following policy recommendations. (1) Promote systematic thinking in the related departments and strengthen multi-departmental cooperation. Sectors related to climate and development in China should incorporate health perspectives into their policymaking and actions, demonstrating WHO’s and President Xi Jinping’s so-called health-in-all-policies principle. (2) Include clear goals and timelines for climate-related health impact assessments and health adaptation plans at both the national and the regional levels in the National Climate Change Adaptation Strategy for 2035. (3) Strengthen China’s climate mitigation actions and ensure that health is included in China’s pathway to carbon neutrality. By promoting investments in zero-carbon technologies and reducing fossil fuel subsidies, the current rebounding trend in carbon emissions will be reversed and lead to a healthy, low-carbon future. (4) Increase awareness of the linkages between climate change and health at all levels. Health professionals, the academic community, and traditional and new media should raise the awareness of the public and policymakers on the important linkages between climate change and health.</p

Geometric Metric Learning for Multi-Output Learning

Due to its wide applications, multi-output learning that predicts multiple output values for a single input at the same time is becoming more and more attractive. As one of the most popular frameworks for dealing with multi-output learning, the performance of the k-nearest neighbor (kNN) algorithm mainly depends on the metric used to compute the distance between different instances. In this paper, we propose a novel cost-weighted geometric mean metric learning method for multi-output learning. Specifically, this method learns a geometric mean metric which can make the distance between the input embedding and its correct output be smaller than the distance between the input embedding and the outputs of its nearest neighbors. The learned geometric mean metric can discover output dependencies and move the instances with different outputs far away in the embedding space. In addition, our objective function has a closed solution, and thus the calculation speed is very fast. Compared with state-of-the-art methods, it is easier to explain and also has a faster calculation speed. Experiments conducted on two multi-output learning tasks (i.e., multi-label classification and multi-objective regression) have confirmed that our method provides better results than state-of-the-art methods

Current driven properties and the associated magnetic domain walls manipulation in U-shaped magnetic nanowires

Based on the extended Landau–Lifshitz–Gilbert method, the properties of current driven domain wall movement in U-shaped magnetic nanowires and the effect of spin wave assistance on their properties have been investigated. The results show that changes of the curvature radius of magnetic nanowire can cause the additional pinning action and the pinning action will weaken the speed of current driven domain wall movement. For U-shaped magnetic nanowires, the changes of curvature radius can be represented by the radius R at the bend. The results show that the decline of its speed non-monotonically increases with the decrease of the bending radius of magnetic nanowires. On the other hand, the assistance of applying spin waves not only enhances the movement of magnetic domain walls but also weakens the pinning action. Further research has shown that applying the appropriate spin waves at the bend changing point can completely eliminate the influence induced by bend changing, in order to ensure uniform and stable movement of current driven magnetic domain walls in U-shaped magnetic nanowires, and achieve the current driven three-dimensional racetrack memory technology

Effect of Type I Diabetes on the Proteome of Mouse Oocytes

Background: Type I diabetes is a global public health concern that affects young people of reproductive age and can damage oocytes, reducing their maturation rate and blocking embryonic development. Understanding the effects of type I diabetes on oocytes is important to facilitate the maintenance of reproductive capacity in female diabetic patients. Methods: To analyze the effects of type I diabetes on mammalian oocytes, protein profile changes in mice with streptozotocin-induced type I diabetes were investigated using proteomic tools; non-diabetic mouse oocytes were used as controls. Immunofluorescence analysis for the spindle and mitochondria of oocytes. Results: We found that type I diabetes severely disturbed the metabolic processes of mouse oocytes. We also observed significant changes in levels of histone H1, H2A/B, and H3 variants in diabetic oocytes (fold change: > 0.4 or Conclusion: Our results indicate that type I diabetes disrupts metabolic processes, spindle formation, mitochondria distribution and modulates epigenetic code in oocytes. Such effects could have a major impact on the reproductive dynamics of female patients with type I diabetes