The 2020 magnetism roadmap

Following the success and relevance of the 2014 and 2017 Magnetism Roadmap articles, this 2020 Magnetism Roadmap edition takes yet another timely look at newly relevant and highly active areas in magnetism research. The overall layout of this article is unchanged, given that it has proved the most appropriate way to convey the most relevant aspects of today's magnetism research in a wide variety of sub-fields to a broad readership. A different group of experts has again been selected for this article, representing both the breadth of new research areas, and the desire to incorporate different voices and viewpoints. The latter is especially relevant for thistype of article, in which one's field of expertise has to be accommodated on two printed pages only, so that personal selection preferences are naturally rather more visible than in other types of articles. Most importantly, the very relevant advances in the field of magnetism research in recent years make the publication of yet another Magnetism Roadmap a very sensible and timely endeavour, allowing its authors and readers to take another broad-based, but concise look at the most significant developments in magnetism, their precise status, their challenges, and their anticipated future developments. While many of the contributions in this 2020 Magnetism Roadmap edition have significant associations with different aspects of magnetism, the general layout can nonetheless be classified in terms of three main themes: (i) phenomena, (ii) materials and characterization, and (iii) applications and devices. While these categories are unsurprisingly rather similar to the 2017 Roadmap, the order is different, in that the 2020 Roadmap considers phenomena first, even if their occurrences are naturally very difficult to separate from the materials exhibiting such phenomena. Nonetheless, the specifically selected topics seemed to be best displayed in the order presented here, in particular, because many of the phenomena or geometries discussed in (i) can be found or designed into a large variety of materials, so that the progression of the article embarks from more general concepts to more specific classes of materials in the selected order. Given that applications and devices are based on both phenomena and materials, it seemed most appropriate to close the article with the application and devices section (iii) once again. The 2020 Magnetism Roadmap article contains 14 sections, all of which were written by individual authors and experts, specifically addressing a subject in terms of its status, advances, challenges and perspectives in just two pages. Evidently, this two-page format limits the depth to which each subject can be described. Nonetheless, the most relevant and key aspects of each field are touched upon, which enables the Roadmap as whole to give its readership an initial overview of and outlook into a wide variety of topics and fields in a fairly condensed format. Correspondingly, the Roadmap pursues the goal of giving each reader a brief reference frame of relevant and current topics in modern applied magnetism research, even if not all sub-fields can be represented here. The first block of this 2020 Magnetism Roadmap, which is focussed on (i) phenomena, contains five contributions, which address the areas of interfacial Dzyaloshinskii-Moriya interactions, and two-dimensional and curvilinear magnetism, as well as spin-orbit torque phenomena and all optical magnetization reversal. All of these contributions describe cutting edge aspects of rather fundamental physical processes and properties, associated with new and improved magnetic materials' properties, together with potential developments in terms of future devices and technology. As such, they form part of a widening magnetism 'phenomena reservoir' for utilization in applied magnetism and related device technology. The final block (iii) of this article focuses on such applications and device-related fields in four contributions relating to currently active areas of research, which are of course utilizing magnetic phenomena to enable specific functions. These contributions highlight the role of magnetism or spintronics in the field of neuromorphic and reservoir computing, terahertz technology, and domain wall-based logic. One aspect common to all of these application-related contributions is that they are not yet being utilized in commercially available technology; it is currently still an open question, whether or not such technological applications will be magnetism-based at all in the future, or if other types of materials and phenomena will yet outperform magnetism. This last point is actually a very good indication of the vibrancy of applied magnetism research today, given that it demonstrates that magnetism research is able to venture into novel application fields, based upon its portfolio of phenomena, effects and materials. This materials portfolio in particular defines the central block (ii) of this article, with its five contributions interconnecting phenomena with devices, for which materials and the characterization of their properties is the decisive discriminator between purely academically interesting aspects and the true viability of real-life devices, because only available materials and their associated fabrication and characterization methods permit reliable technological implementation. These five contributions specifically address magnetic films and multiferroic heterostructures for the purpose of spin electronic utilization, multi-scale materials modelling, and magnetic materials design based upon machine-learning, as well as materials characterization via polarized neutron measurements. As such, these contributions illustrate the balanced relevance of research into experimental and modelling magnetic materials, as well the importance of sophisticated characterization methods that allow for an ever-more refined understanding of materials. As a combined and integrated article, this 2020 Magnetism Roadmap is intended to be a reference point for current, novel and emerging research directions in modern magnetism, just as its 2014 and 2017 predecessors have been in previous years

Spin dynamics under spin Hall effect modulation: Skyrmion oscillator

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

Ferrimagnetic rare-earth-transition-metal heterostructures: implications for future data storage, sensors, and unconventional computing

In this work, different ferrimagnetic rare-earth-transition-metal heterostructures are investigated. The findings provide implications for future data storage, sensor, and unconventional computing devices. In the first part, ferri- and ferromagnetic films are exchange-coupled and studied as potential composite media for magnetic recording technologies. For this, the underlying individual layers are examined, too. Within this study, the influence of Pd and Pt insertion layers in ferromagnetic Co/Ni multilayers is investigated. In these systems, the maximum effective magnetic anisotropy is more than doubled by the introduced insertion layers, while the initial saturation magnetization and Curie temperature are reduced. Further, amorphous Tb-FeCo alloys and multilayers are studied as the second building block of the desired composite medium. In particular, the structural and magnetic properties are analyzed upon post-annealing. At temperatures above 400 K, irreversible effects on the structural properties are found, which also influence the magnetic properties. It is shown that these changes in properties cannot be prevented by tuning the composition or by a multilayer structure of the film. However, key insights on the structural and magnetic properties upon annealing are provided for future high-temperature devices. Afterward, the exchange-coupled ferrimagnetic/ferromagnetic bilayer is studied. Measurements on the dependency on temperature, the ferrimagnetic composition, and the thickness of the ferromagnet are carried out. Two distinct magnetic reversal mechanisms are revealed. The reversal characteristics depend critically on the thickness of the ferromagnetic layer. The underlying microscopic origin is revealed by high-resolution magnetic force microscopy. Above a certain thickness of the ferromagnet, the switching process is driven by in-plane domain wall propagation. In contrast, thinner ferromagnetic layers exhibit a nucleation-dominated reversal due to grain-to-grain variations in magnetic anisotropy. Although the realization of an exchange-coupled composite medium for magnetic recording can not be achieved, insights for the future realization of sub micron high energy density permanent magnets and spintronic devices are gained. In the second part of this work, topologically protected spin structures, including skyrmions and antiskyrmions, are investigated in Fe/Gd-based multilayers. Particularly in coexisting phases, different topologically protected magnetic quasi-particles may show fascinating physics and potential for spintronic devices. While skyrmions are observed in a wide range of materials, until now, antiskyrmions have been exclusive to materials with D2d or S4 symmetry. In this work, first and second-order antiskyrmions are stabilized for the first time by magnetic dipole-dipole interaction. Using Lorentz transmission electron microscopy imaging, coexisting first- and second-order antiskyrmions, Bloch skyrmions, and type-2 bubbles are observed, and the range of material properties and magnetic fields where the different spin objects form and dissipate is determined. The discovered phase pocket of metastable antiskyrmions for low saturation magnetization and uniaxial magnetic anisotropy values is confirmed by micromagnetic simulations and represents a recipe, which has to be satisfied for the stabilization of antiskyrmions by dipole-dipole interaction in other material systems. Furthermore, the nucleation process of the spin objects and the influence of an in-plane magnetic field are studied. Additionally, post-deposition techniques are employed to locally change the anisotropy of the samples and influence the nucleation and stability range of the spin objects. The gained knowledge significantly simplifies future investigations of antiskyrmions. Moreover, the coexisting phases of different topologically protected spin objects and their controlled nucleation provide great potential for further studies on magnetic quasi-particle interactions, spin dynamics, as well as for possible future applications in spintronics, namely the racetrack memory, skyrmionic interconnections, skyrmion-based unconventional computing, and sensor devices

Quantum materials for energy-efficient neuromorphic computing

Neuromorphic computing approaches become increasingly important as we address future needs for efficiently processing massive amounts of data. The unique attributes of quantum materials can help address these needs by enabling new energy-efficient device concepts that implement neuromorphic ideas at the hardware level. In particular, strong correlations give rise to highly non-linear responses, such as conductive phase transitions that can be harnessed for short and long-term plasticity. Similarly, magnetization dynamics are strongly non-linear and can be utilized for data classification. This paper discusses select examples of these approaches, and provides a perspective for the current opportunities and challenges for assembling quantum-material-based devices for neuromorphic functionalities into larger emergent complex network systems