The 2017 Magnetism Roadmap

Building upon the success and relevance of the 2014 Magnetism Roadmap, this 2017 Magnetism Roadmap edition follows a similar general layout, even if its focus is naturally shifted, and a different group of experts and, thus, viewpoints are being collected and presented. More importantly, key developments have changed the research landscape in very relevant ways, so that a novel view onto some of the most crucial developments is warranted, and thus, this 2017 Magnetism Roadmap article is a timely endeavour. The change in landscape is hereby not exclusively scientific, but also reflects the magnetism related industrial application portfolio. Specifically, Hard Disk Drive technology, which still dominates digital storage and will continue to do so for many years, if not decades, has now limited its footprint in the scientific and research community, whereas significantly growing interest in magnetism and magnetic materials in relation to energy applications is noticeable, and other technological fields are emerging as well. Also, more and more work is occurring in which complex topologies of magnetically ordered states are being explored, hereby aiming at a technological utilization of the very theoretical concepts that were recognised by the 2016 Nobel Prize in Physics. Given this somewhat shifted scenario, it seemed appropriate to select topics for this Roadmap article that represent the three core pillars of magnetism, namely magnetic materials, magnetic phenomena and associated characterization techniques, as well as applications of magnetism. While many of the contributions in this Roadmap have clearly overlapping relevance in all three fields, their relative focus is mostly associated to one of the three pillars. In this way, the interconnecting roles of having suitable magnetic materials, understanding (and being able to characterize) the underlying physics of their behaviour and utilizing them for applications and devices is well illustrated, thus giving an accurate snapshot of the world of magnetism in 2017. The article consists of 14 sections, each written by an expert in the field and addressing a specific subject on two pages. Evidently, the depth at which each contribution can describe the subject matter is limited and a full review of their statuses, advances, challenges and perspectives cannot be fully accomplished. Also, magnetism, as a vibrant research field, is too diverse, so that a number of areas will not be adequately represented here, leaving space for further Roadmap editions in the future. However, this 2017 Magnetism Roadmap article can provide a frame that will enable the reader to judge where each subject and magnetism research field stands overall today and which directions it might take in the foreseeable future. The first material focused pillar of the 2017 Magnetism Roadmap contains five articles, which address the questions of atomic scale confinement, 2D, curved and topological magnetic materials, as well as materials exhibiting unconventional magnetic phase transitions. The second pillar also has five contributions, which are devoted to advances in magnetic characterization, magneto-optics and magneto-plasmonics, ultrafast magnetization dynamics and magnonic transport. The final and application focused pillar has four contributions, which present non-volatile memory technology, antiferromagnetic spintronics, as well as magnet technology for energy and bio-related applications. As a whole, the 2017 Magnetism Roadmap article, just as with its 2014 predecessor, is intended to act as a reference point and guideline for emerging research directions in modern magnetism

Optimization of spin-orbit magnetic-state readout in metallic nanodevices

183 p.This thesis presents the first steps of the optimization of the magnetic-state readout component for the envisioned magneto-electric spin-orbit (MESO) logic device. We established that (i) reducing the device dimension of ferromagnetic materials/ strong spin-orbit coupling non-magnetic materials nanostructured devices leads to an enhancement of the output signals; (ii) spurious effects in the device due to the local configuration can be avoided by proper design of the ferromagnetic and spin-orbit coupling material electrodes; (iii) interface properties and interfacial spin-charge interconversion have to be carefully considered when studying spin transport in metallic devices and such interface might be applicable for the MESO-logic devices. Even tough, we did not achieve the required values for the realization of cascaded gates with MESO devices, we did find a guideline for further improvement of the output signals. Besides the independent scaling laws for voltage and charge output signals, the use of other materials systems with large spin-charge interconversion efficiency and high resistivities seems to be promising for enhanced output signal. Further experiments are required to demonstrate the use of our device as a curren

Resistance switching devices based on amorphous insulator-metal thin films

Nanometallic devices based on amorphous insulator-metal thin films are developed to provide a novel non-volatile resistance-switching random-access memory (RRAM). In these devices, data recording is controlled by a bipolar voltage, which tunes electron localization length, thus resistivity, through electron trapping/detrapping. The low-resistance state is a metallic state while the high-resistance state is an insulating state, as established by conductivity studies from 2K to 300K. The material is exemplified by a Si3N4 thin film with randomly dispersed Pt or Cr. It has been extended to other materials, spanning a large library of oxide and nitride insulator films, dispersed with transition and main-group metal atoms. Nanometallic RRAMs have superior properties that set them apart from other RRAMs. The critical switching voltage is independent of the film thickness/device area/temperature/switching speed. Trapped electrons are relaxed by electron-phonon interaction, adding stability which enables long-term memory retention. As electron-phonon interaction is mechanically altered, trapped electron can be destabilized, and sub-picosecond switching has been demonstrated using an electromagnetically generated stress pulse. AC impedance spectroscopy confirms the resistance state is spatially uniform, providing a capacitance that linearly scales with area and inversely scales with thickness. The spatial uniformity is also manifested in outstanding uniformity of switching properties. Device degradation, due to moisture, electrode oxidation and dielectrophoresis, is minimal when dense thin films are used or when a hermetic seal is provided. The potential for low power operation, multi-bit storage and complementary stacking have been demonstrated in various RRAM configurations.Comment: 523 pages, 215 figures, 10 chapter

Electrical characterization of metal-oxide-polymer devices for non-volatile memory applications

The objective of this thesis is to study the properties of resistive switching effect based on bistable resistive memory which is fabricated in the form of Al2O3/polymer diodes and to contribute to the elucidation of resistive switching mechanisms. Resistive memories were characterized using a variety of electrical techniques, including current-voltage measurements, small-signal impedance, and electrical noise based techniques. All the measurements were carried out over a large temperature range. Fast voltage ramps were used to elucidate the dynamic response of the memory to rapid varying electric fields. The temperature dependence of the current provided insight into the role of trapped charges in resistive switching. The analysis of fast current fluctuations using electric noise techniques contributed to the elucidation of the kinetics involved in filament formation/rupture, the filament size and correspondent current capabilities. The results reported in this thesis provide insight into a number of issues namely: (i) The fundamental limitations on the speed of operation of a bi-layer resistive memory are the time and voltage dependences of the switch-on mechanism. (ii) The results explain the wide spread in switching times reported in the literature and the apparently anomalous behaviour of the high conductance state namely the disappearance of the negative differential resistance region at high voltage scan rates which is commonly attributed to a “dead time” phenomenon which had remained elusive since it was first reported in the ‘60s. (iii) Assuming that the current is filamentary, Comsol simulations were performed and used to explain the observed dynamic properties of the current-voltage characteristics. Furthermore, the simulations suggest that filaments can interact with each other. (iv) The current-voltage characteristics have been studied as a function of temperature. The findings indicate that creation and annihilation of filaments is controlled by filling and neutralizing traps localized at the oxide/polymer interface. (v) Resistive switching was also studied in small-molecule OLEDs. It was shown that the degradation that leads to a loss of light output during operation is caused by the presence of a resistive switching layer. A diagnostic tool that predicts premature failure of OLEDs was devised and proposed. Resistive switching is a property of oxides. These layers can grow in a number of devices including, organic light emitting diodes (OLEDs), spin-valve transistors and photovoltaic devices fabricated in different types of material. Under strong electric fields the oxides can undergo dielectric breakdown and become resistive switching layers. Resistive switching strongly modifies the charge injection causing a number of deleterious effects and eventually device failure. In this respect the findings in this thesis are relevant to understand reliability issues in devices across a very broad field.As memórias resistivas baseiam-se na alteração da resistência elétrica de um material ou componente quando submetido a uma tensão elétrica. Este fenómeno deu origem a um novo elemento eletrónico que se passou a designar por “memristor” por sugestão de Leon Chua em 1971. [1] O “memristor” juntou-se assim aos componentes elétricos mais conhecidos, o condensador, a bobine e a resistência. Desde os anos 60 que a comutação resistiva tem sido observada numa variedade de materiais. No contexto desta tese os mais interessantes são por exemplo o SiOx, Al2O3, Ta2O5, ZrO2 e TiO2, onde a comutação resistiva é um processo eletrónico e não envolve uma mudança de fase do material. Os processos físicos envolvidos na comutação de resistência tem permanecido pouco claros. Os vários mecanismos propostos não tem merecido o consenso da comunidade científica. A ausência de um modelo físico tem impedido o desenvolvimento tecnológico destas memórias que têm assim progredido de forma empírica. Apesar da falta de conhecimento sobre os mecanismos físicos, as memórias resistivas oferecem um conjunto de vantagens sobre as tecnologias atuais. Isto despoletou uma intensa atividade de pesquisa quer no meio académico quer pela industria para comercializar este tipo de componente. As memórias resistivas combinam num só componente as vantagens de diversas tecnologias atuais. Podem ter a velocidade de acesso das memórias aleatórias de acesso dinâmico (DRAMs) com um custo muito inferior, com menor consumo de energia e sem necessidade de periodicamente fazer o restauro ou “refeshing”. Oferecem as características não-voláteis de uma memória do tipo “flash”, mas mais robustas, permitindo assim mais ciclos de leitura e escrita. Possibilitam uma elevada densidade e não sofrem dos problemas mecânicos dos discos duros associados com as cabeças de leitura. A comercialização deste tipo de memórias irá revolucionar as tecnologias de informação ao disponibilizar uma elevada capacidade de memória a baixo custo, em dimensões reduzidas e com muito baixo consumo de energia. As memórias resistivas também não precisam de alguma da eletrónica que acompanha os sistemas atuais, nomeadamente os sistemas de “cache”, reduzindo substancialmente os custos e a complexidade dos circuitos. O trabalho desenvolvido nesta tese foi focado nas propriedades elétricas das memórias resistivas com o objetivo de aumentar o nosso conhecimento sobre os mecanismos físicos e elétricos que controlam a comutação resistiva e a velocidade de acesso. As memórias estudadas nesta tese são estruturas do tipo metal-isolador-semicondutor (MIS). Foi usado óxido de alumínio e um polímero conjugado para a camada isolante e semicondutora respetivamente. Estas memórias comutam entre dois estados resistivos diferentes quando submetidas a voltagens definidas durante um certo período de tempo. Paralelamente, foi identificado que o processo físico que conduz a bi-estabilidade elétrica do óxido de alumínio é também responsável pela falha prematura de díodos emissores de luz orgânicos (OLEDs). A presença de óxido de alumínio nativo nos eletrodos dos OLEDs pode dar origem a transições resistivas que alteram o equilíbrio da injeção de portadores de carga e leva a degradação da eletroluminescência. Quer as memórias quer os díodos emissores de luz foram caraterizados usando técnicas elétricas e óticas. Medidas da resposta da corrente a degraus e/ou rampas de tensão permitiram avaliar a velocidade de comutação resistiva. Medidas da impedância no domínio da frequência foram usadas para estudar variações de carga nas interfaces da memória, e por último medidas do ruído elétrico complementadas com medidas óticas permitiram estudar flutuações na corrente causadas pela criação e aniquilação de pequenos caminhos condutores ou filamentos. Todas as medidas foram feitas num grande intervalo de temperatura e frequência. Esta tese contribui para o esclarecimento dos mecanismos físicos que originam comutações entre estados resistivos não-voláteis. As constantes de tempo que controlam o tempo de acesso à memória, isto é, o tempo para ler, escrever ou apagar foram também estudadas. Os resultados obtidos contribuíram para elucidar o mecanismo físico que determina o tempo de acesso. Estratégias para otimizar a rapidez deste tipo de memoria foram propostas. Foi identificado que a condução elétrica é não-homogénea. A corrente é transportada por filamentos. Foi possível quantificar as dimensões físicas e a densidade de corrente transportada por filamentos individuais. O estudo da dinâmica destes filamentos usando técnicas de análise de ruído elétrico permitiu concluir que os filamentos não são criados nem destruídos, mas sim ligados e desligados como interruptores. O mecanismo que liga os filamentos são buracos armadilhados na camada de óxido de alumínio. Quando os buracos são neutralizados por eletrões o filamento é desligado. Este resultado foi um dos contributos mais importantes para a área científica. A condução filamentar dá origem a um conjunto de observações não intuitivas. Concretamente, dá origem a uma dependência anómala da corrente elétrica com a temperatura. A corrente aumenta de forma discreta à medida que a temperatura diminui, isto porque o armazenamento de cargas em armadilhas a baixas temperaturas liga mais filamentos. Adicionalmente, a existência de condução filamentar dá origem a que a corrente elétrica diminua, quando as rampas de tensão rápidas são aplicadas sucessivamente. Os resultados desta tese também sugerem que filamentos de corrente vizinhos podem interatuar e dar origem a fenómenos correlacionados, quer durante o ligar, quer durante o desligar de filamentos. O campo elétrico associado a dois filamentos vizinhos induz um campo elétrico adicional na região intermédia que pode ligar um terceiro filamento. Se um filamento for desligado os filamentos na vizinhança terão mais probabilidade de ser desligados. Simulações usando o “COMSOL Multiphysics” parecem suportar a correlação destes fenómenos. A comutação resistiva é uma propriedade de óxidos binários. Este fenómeno pode ocorrer de forma não intencional, nomeadamente em díodos emissores de luz, células solares, válvulas de spin, transístores de efeito de campo e de uma forma geral, todos os componentes que usam elétrodos que oxidam. O conhecimento adquirido nesta tese é assim relevante para detetar e prevenir problemas de confiabilidade num conjunto vasto de componentes eletrónicos.Dutch Polymer Institute (DPI), Project No. 704, BISTABLE (resistive switching and OLED reliability) and by Fundação para Ciência e Tecnologia (FCT) through the Instituto de Telecomunicacões (IT)

Spin-dependent phenomena and device concepts explored in (Ga,Mn)As

Over the past two decades, the research of (Ga,Mn)As has led to a deeper understanding of relativistic spin-dependent phenomena in magnetic systems. It has also led to discoveries of new effects and demonstrations of unprecedented functionalities of experimental spintronic devices with general applicability to a wide range of materials. In this article we review the basic material properties that make (Ga,Mn)As a favorable test-bed system for spintronics research and discuss contributions of (Ga,Mn)As studies in the general context of the spin-dependent phenomena and device concepts. Special focus is on the spin-orbit coupling induced effects and the reviewed topics include the interaction of spin with electrical current, light, and heat.Comment: 47 pages, 41 figure

Synthesis of silicon nanocrystal memories by sputter deposition

In Silizium-Nanokristall-Speichern werden im Gate-Oxid eines Feldeffekttransistors eingebettete Silizium Nanokristalle genutzt, um Elektronen lokal zu speichern. Die gespeicherte Ladung bestimmt dann den Zustand der Speicherzelle. Ein wichtiger Aspekt in der Technologie dieser Speicher ist die Erzeugung der Nanokristalle mit einerwohldefinierten Größenverteilung und einem bestimmten Konzentrationsprofil im Gate-Oxid. In der vorliegenden Arbeit wurde dazu ein sehr flexibler Ansatz untersucht: die thermische Ausheilung von SiO2/SiOx (x < 2) Stapelschichten. Es wurde ein Sputterverfahren entwickelt, das die Abscheidung von SiO2 und SiOx Schichten beliebiger Zusammensetzung erlaubt. Die Bildung der Nanokristalle wurde in Abhängigkeit vom Ausheilregime und der SiOx Zusammensetzung charakterisiert, wobei unter anderem Methoden wie Photolumineszenz, Infrarot-Absorption, spektroskopische Ellipsometrie und Elektronenmikroskopie eingesetzt wurden. Anhand von MOS-Kondensatoren wurden die elektrischen Eigenschaften derart hergestellter Speicherzellen untersucht. Die Funktionalität der durch Sputterverfahren hergestellten Nanokristall-Speicher wurde erfolgreich nachgewiesen.In silicon nanocrystal memories, electronic charge is discretely stored in isolated silicon nanocrystals embedded in the gate oxide of a field effect transistor. The stored charge determines the state of the memory cell. One important aspect in the technology of silicon nanocrystal memories is the formation of nanocrystals near the SiO2-Si interface, since both, the size distribution and the depth profile of the area density of nanocrystals must be controlled. This work has focussed on the formation of gate oxide stacks with embedded nanocrystals using a very flexible approach: the thermal annealing of SiO2/SiOx (x < 2) stacks. A sputter deposition method allowing to deposit SiO2 and SiOx films of arbitrary composition has been developed and optimized. The formation of Si NC during thermal annealing of SiOX has been investigated experimentally as a function of SiOx composition and annealing regime using techniques such as photoluminescence, infrared absorption, spectral ellipsometry, and electron microscopy. To proof the concept, silicon nanocrystal memory capacitors have been prepared and characterized. The functionality of silicon nanocrystal memory devices based on sputtered gate oxide stacks has been successfully demonstrated

Van der Waals Heterostructures based on Two-dimensional Ferroelectric and Ferromagnetic Layers

Two-dimensional (2D) van der Waals (vdW) crystals provide a platform for studies of novel phenomena and promising applications beyond traditional systems. This PhD thesis focuses on vertical 2D vdW heterostructures, including ferroelectric semiconductor junctions (FSJs), p-n junction diodes, and magnetic tunnel junctions (MTJs). These have potential for non-volatile memories, ultraviolet (UV) photosensing and low-power electronics. The ferroelectric polarization of the vdW semiconductor α-In2Se3 in graphene/α-In2Se3/graphene FSJs was switched by the bias voltage, thus producing memristive effects in the transport characteristics. These can be modified by light due to screening of the polarization by photocreated carriers. The FSJs demonstrated a high photoresponsivity (up to ~ 10^6 A/W) and a relatively fast modulation (down to ~ 0.2 ms) of the photocurrent. The graphene/p-GaSe/n-In2Se3/graphene heterostructures were used to investigate novel mechanisms for the detection of UV light. The p-GaSe/n-In2Se3 type-II band alignment and the electric field at the vdW interfaces were found to be beneficial to suppress carrier recombination and enhance the UV-photoresponse. Finally, the Fe3GaTe2/WSe2/Fe3GaTe2 MTJs exhibited an ideal tunnelling behaviour with a tunnel magnetoresistance (TMR) signal as large as 85 % at room temperature, breaking through the bottleneck of previous vdW MTJs that worked only at low temperatures (T < 300 K). The findings of this work offer opportunities for further developments, including the optimization of device structures and their studies towards enhanced functionalities beyond the current state of the art

Synthesis of silicon nanocrystal memories by sputter deposition

In Silizium-Nanokristall-Speichern werden im Gate-Oxid eines Feldeffekttransistors eingebettete Silizium Nanokristalle genutzt, um Elektronen lokal zu speichern. Die gespeicherte Ladung bestimmt dann den Zustand der Speicherzelle. Ein wichtiger Aspekt in der Technologie dieser Speicher ist die Erzeugung der Nanokristalle mit einerwohldefinierten Größenverteilung und einem bestimmten Konzentrationsprofil im Gate-Oxid. In der vorliegenden Arbeit wurde dazu ein sehr flexibler Ansatz untersucht: die thermische Ausheilung von SiO2/SiOx (x &amp;lt; 2) Stapelschichten. Es wurde ein Sputterverfahren entwickelt, das die Abscheidung von SiO2 und SiOx Schichten beliebiger Zusammensetzung erlaubt. Die Bildung der Nanokristalle wurde in Abhängigkeit vom Ausheilregime und der SiOx Zusammensetzung charakterisiert, wobei unter anderem Methoden wie Photolumineszenz, Infrarot-Absorption, spektroskopische Ellipsometrie und Elektronenmikroskopie eingesetzt wurden. Anhand von MOS-Kondensatoren wurden die elektrischen Eigenschaften derart hergestellter Speicherzellen untersucht. Die Funktionalität der durch Sputterverfahren hergestellten Nanokristall-Speicher wurde erfolgreich nachgewiesen.In silicon nanocrystal memories, electronic charge is discretely stored in isolated silicon nanocrystals embedded in the gate oxide of a field effect transistor. The stored charge determines the state of the memory cell. One important aspect in the technology of silicon nanocrystal memories is the formation of nanocrystals near the SiO2-Si interface, since both, the size distribution and the depth profile of the area density of nanocrystals must be controlled. This work has focussed on the formation of gate oxide stacks with embedded nanocrystals using a very flexible approach: the thermal annealing of SiO2/SiOx (x &amp;lt; 2) stacks. A sputter deposition method allowing to deposit SiO2 and SiOx films of arbitrary composition has been developed and optimized. The formation of Si NC during thermal annealing of SiOX has been investigated experimentally as a function of SiOx composition and annealing regime using techniques such as photoluminescence, infrared absorption, spectral ellipsometry, and electron microscopy. To proof the concept, silicon nanocrystal memory capacitors have been prepared and characterized. The functionality of silicon nanocrystal memory devices based on sputtered gate oxide stacks has been successfully demonstrated