Interfacing ultracold atoms with nanomagnetic domain walls

This thesis presents the first realisation of a new type of hybrid quantum device based on spintronic technology. We demonstrate an interaction between the magnetic fringing fields produced by domain walls within planar permalloy nanowires and a cloud of ultracold Rubidium 87 atoms. This interaction is manifested through the realisation of a magnetic atom mirror produced by a two-dimensional domain wall array. The interaction is tuned through the reconfiguration of the micromagnetic structure. Analytic modelling of the fringing fields is developed and shows good agreement with calculations based on micromagnetically simulated structures. The accurate and rapid calculation of the fringing fields permits simulation of the resulting atom dynamics, which agrees well with data. In turn, we use the atom dynamics as a probe of the micromagnetic reconfiguration processes that take place and observe a collective behaviour which is both reliably reproducible and in agreement with alternative, conventional magnetometry. We also observe evidence of stochastic behaviour, characteristic of superparamagnetic systems. We consider the development of a more advanced spintronics-based atom chip which will allow for the creation of extremely tight mobile atom traps. We consider the problems associated with ensuring that the trapping potential is adiabatic, sufficiently deep, and technically feasible. In particular we examine techniques to circumvent losses due to Majorana spin-flip transitions. As a result of this study we propose a novel scheme for creating time-averaged potentials via the piezoelectric actuation of magnetic field sources. We show that this technique presents significant fundamental and technical advantages over conventional time-averaging schemes

Magnetization dynamics in racetrack memory

Various devices have been proposed which use magnetic domain walls (DWs) in nanosized magnetic structures to perform logic operations or store information. In particular in ‘Racetrack memory’ bits of information represented by DWs are shifted in a magnetic wire to be stored. For these memory and logic devices to be successful, great control of DW motion is of vital importance. In cooperation with IBM’s Almaden research laboratory a pump-probe Kerr magnetooptical scanning microscope has been developed. In order to control DW injection, motion and reset, magnetic fields have to be applied locally on the nanowire. For this a special Damascene CMOS chip has been fabricated at the 200 mm wafer facility at IBM Microelectronics Research Laboratory (MRL). Probing of the local magnetization is done with a focused pulsed laser spot of 400 nm diameter where the polarization rotation caused by the Kerr effect is measured after reflection. In order to achieve optimal focusing a perpendicular incident laser beam is focused with a high numerical aperture objective. Synchronized ‘pumping’ in this scheme is achieved by successively: 1 injecting a DW; 2 propagate the DW down the nanowire with either current through or an applied field pulse over the nanowire; 3 and finally resetting the whole nanowire to its original magnetization by applying a large field together with the injection of an opposite magnetic domain. With this setup field and current induced DW motion is studied in permalloy nanowires ranging in width from 200 to 700 nm and thickness of 20 nm. For control of DWs in Racetrack memory it is important to understand the different mechanism for driving a DW already in motion (dynamic) and driving a DW that is currently at rest (static). The propagation field, the minimum field below which no DW motion takes place, is measured for both dynamic DWs and static DWs. It is found that Static DWs require a much higher field than DWs already in motion. A model is build where this effect is related to the wire roughness, successfully describing the existence of a propagation field, the difference between both propagation fields and a specific effect related to the method of injection. For Racetrack memory to be successful the critical current needs to be small (the current needed to move a DW solely by current) and the DW velocity high. Much of the influence of intrinsic magnetic properties of materials on DW dynamics is unknown. One important property affecting DW velocity and possibly also the critical current is Gilbert damping. Gilbert damping in permalloy can be tuned by doping the nanowires with osmium. This is used to prepare a sample series with increasing Gilbert damping. Measurement of the field induced DW velocity revealed a profile well known that includes the Walker breakdown (a maximum field where further increasing field strength does not further increase the DW velocity). From this profile the dependence of the Walker breakdown, DW mobility and maximum DW velocity on Gilbert damping has been determined. With the same sample series also current induced field assisted DW motion has been measured. Current induced DW motion is known to be driven by two effects: adiabatic and ballistic- spin momentum transfer (SMT) which relative contribution is parameterized by beta in the Landau Lifshitz Gilbert equation (LLG). Measurement of DW velocity depending on current density revealed the relative contribution of the two SMT schemes. Also the influence of Gilbert damping on the relative contribution of both schemes has been explored. A pronounced dependence of the measured spin torque efficiency on osmium concentration was found. This result may be interpreted as a sign that the intensively debated ratio ¿ / ¿ is far from constant over the range of ¿ studied

Micro-nano biosystems: silicon nanowire sensor and micromechanical wireless power receiver

Silicon Nanowire-based biosensors owe their sensitivity to the large surface area to volume ratio of the nanowires. However, presently they have only been shown to detect specific bio-markers in low-salt buffer environments. The first part of this thesis presents a pertinent next step in the evolution of these sensors by presenting the specific detection of a target analyte (NT-ProBNP) in a physiologically relevant solution such as serum. By fabrication of the nanowires down to widths of 60 nm, choosing appropriate design parameters, optimization of the silicon surface functionalization recipe and using a reduced gate oxide thickness of 5 nm; these sensors are shown to detect the NT-ProBNP bio-marker down to 2ng/ml in serum. The observed high background noise in the measured response of the sensor is discussed and removed experimentally by the addition of an extra microfabrication step to employ a differential measurement scheme. It is also shown how the modulation of the local charge density via external static electric fields (applied by on-chip patterned electrodes) pushes the sensitivity threshold by more than an order of magnitude. These demonstrations bring the silicon nanowire-based biosensor platform one step closer to being realized for point-of-care (POC) applications. In the second half of the thesis, it is demonstrated how silicon micromechanical piezoelectric resonators could be tasked to provide wireless power to such POC bio-systems. At present most sensing and actuation platforms, especially in the implantable format, are powered either via onboard battery packs which are large and need periodic replacement or are powered wirelessly through magnetic induction, which requires a proximately located external charging coil. Using energy harnessed from electric fields at distances over a meter; comprehensive distance, orientation, and power dependence for these first-generation devices is presented. The distance response is non-monotonic and anomalous due to multi-path interferences, reflections and low directivity of the power receiver. This issue is studied and evaluated using COMSOL Multiphysics simulations. It is shown that the efficiency of these devices initially evaluated at 3% may be enhanced up to 15% by accessing higher frequency modes

Domain walls in spin-valve nanotracks: characterisation and applications

Magnetic systems based on the manipulation of domain walls (DWs) in nanometre-scaled tracks have been shown to store data at high density, perform complex logic operations, and even mechanically manipulate magnetic beads. The magnetic nano-track has also been an indispensable model system to study fundamental magnetic and magneto-electronic phenomena, such as field induced DW propagation, spin-transfer torque, and other micromagnetic properties. Its value to fundamental research and the breath of potentially useful applications have made this class of systems the focus of wide research in the area of nanomagnetism and spintronics. This thesis focuses on DW manipulation and DW-based devices in spin-valve nanotracks. The spin-valve is a metallic multi-layered spintronic structure, wherein the electrical resistance varies greatly with the magnetisation of its layers. In comparison to monolayer tracks, the spin-valve track enables more sensitive and versatile measurements, as well as demonstrating electronic output of DW-based devices, an achievement of crucial interest to technological applications. However, these multi-layered tracks introduce new, potentially disruptive magnetic interactions, as well as fabrication challenges. In this thesis, the DW propagation in spin-valve nanotracks of different compositions was studied, and a system with DW propagation properties comparable to the state-of-the-art in monolayer tracks was demonstrated, down to an unprecedented lateral size of 33nm. Several DW logic devices of variable complexity were demonstrated and studied, namely a turn-counting DW spiral, a DW gate, multiple DW logic NOT gates, and a DW-DW interactor. It was found that, where the comparison was possible, the overall magnetic behaviour of these devices was analogous to that of monolayer structures, and the device performance, as defined by the range of field wherein they function desirably, was found to be comparable, albeit inferior, to that of their monolayer counterparts. The interaction between DWs in adjacent tracks was studied and new phenomena were observed and characterised, such as DW depinning induced by a static or travelling adjacent DW. The contribution of different physical mechanisms to electrical current induced depinning were quantified, and it was found that the Oersted field, typically negligible in monolayer tracks, was responsible for large variations in depinning field in SV tracks, and that the strength of spin-transfer effect was similar in magnitude to that reported in monolayer tracks. Finally, current induced ferromagnetic resonance was measured, and the domain uniform resonant mode was observed, in very good agreement to Kittel theory and simulations

Dispersive 1D Majorana modes with emergent supersymmetry in 1D proximitized superconductors via spatially-modulated potentials and magnetic fields

In condensed matter systems, zero-dimensional or one-dimensional Majorana modes can be realized respectively as the end and edge states of one-dimensional and two-dimensional topological superconductors. In this $\textit{top-down}$ approach, $(d-1)$-dimensional Majorana modes are obtained as the boundary states of a topologically nontrivial $d$-dimensional bulk. In a $\textit{bottom-up}$ approach instead, $d$-dimensional Majorana modes in a $d$-dimensional system can be realized as the continuous limit of a periodic lattice of coupled $(d-1)$-dimensional Majorana modes. We illustrate this idea by considering one-dimensional proximitized superconductors with spatially-modulated potential or magnetic fields. The ensuing inhomogenous topological state exhibits one-dimensional counterpropagating Majorana modes with finite dispersion, and with a Majorana gap which can be controlled by external fields. In the massless case, the Majorana modes have opposite Majorana polarizations and pseudospins, are conformally invariant, and realize centrally extended quantum mechanical supersymmetry. The supersymmetry exhibits spontaneous partial breaking. Consequently, the massless Majorana fermion can be identified as a Goldstino, i.e., the Nambu-Goldstone fermion associated with the spontaneously broken supersymmetry.Comment: 21 pages, 10 figures, minor revision, references adde

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

The Role of Interdigitated Electrodes in Printed and Flexible Electronics

Flexible electronics, also referred to as printable electronics, represent an interesting technology for implementing electronic circuits via depositing electronic devices onto flexible substrates, boosting their possible applications. Among all flexible electronics, interdigitated electrodes (IDEs) are currently being used for different sensor applications since they offer significant benefits beyond their functionality as capacitors, like the generation of high output voltage, fewer fabrication steps, convenience of application of sensitive coatings, material imaging capability and a potential of spectroscopy measurements via electrical excitation frequency variation. This review examines the role of IDEs in printed and flexible electronics since they are progressively being incorporated into a myriad of applications, envisaging that the growth pattern will continue in the next generations of flexible circuits to come.Project P21_00105 funded by the Consejería de Universidad, Investigación e Innovación of Regional Government of AndalusiaSpanish Ministry of Sciences and Innovation through the Ramón y Cajal fellows RYC2019-027457-I and RYC2021-032522-

Development and applications of microwave impedance microscopy for imaging emergent properties in quantum materials

Near-field scanning microwave microscopy (NSMM) detects local physical properties of materials through electromagnetic interaction between the tip and the sample at a length scale much smaller than the freespace wavelength of the microwave radiation. However, previous implementations of NSMM have suffered from poor resolutions, low sensitivity, and unreliable tip-sample contact conditions. In this dissertation, I will first briefly review the prior research of NSMM (Chapter 1) and then naturally move on to the main theme — the basic principles and technical details of the recently developed microwave impedance microscope (MIM) (Chapter 2). I will present the development of MIM instrumentation including quantitative measurement with tuning-fork-based probes, broadband impedance microcopy, and implementation in cryogenic environment (Chapter 3), which are utilized in research described in the following chapters. The application of MIM will be demonstrated by a number of scientific studies in two general categories, emergent phenomena at ferroelectric domain walls and electrical inhomogeneity in nanodevices. Chapter 4 describes the discovery of low-energy structural dynamics of ferroelectric domain walls in hexagonal rare-earth manganites (h-RMnO₃) by broadband impedance microscopy. Chapter 5 includes direct visualization of sketched conductive nanostructures at the LaAlO₃/SrTiO₃ heterostructure and nanoscale conductance evolution in ion-gel-gated oxide transistors, demonstrating the capability of MIM to image buried structures. I will conclude the dissertation with a short summary and outlook for the future.Physic