Valley-Spin Hall Effect-based Nonvolatile Memory with Exchange-Coupling-Enabled Electrical Isolation of Read and Write Paths

Valley-spin hall (VSH) effect in monolayer WSe2 has been shown to exhibit highly beneficial features for nonvolatile memory (NVM) design. Key advantages of VSH-based magnetic random-access memory (VSH-MRAM) over spin orbit torque (SOT)-MRAM include access transistor-less compact bit-cell and low power switching of perpendicular magnetic anisotropy (PMA) magnets. Nevertheless, large device resistance in the read path (RS) due to low mobility of WSe2 and Schottky contacts deteriorates sense margin, offsetting the benefits of VSH-MRAM. To address this limitation, we propose another flavor of VSH-based MRAM that (while inheriting most of the benefits of VSH-MRAM) achieves lower RS in the read path by electrically isolating the read and write terminals. This is enabled by coupling VSH with electrically-isolated but magnetically-coupled PMA magnets via interlayer exchange-coupling. Designing the proposed devices using object oriented micro magnetic framework (OOMMF) simulation, we ensure the robustness of the exchange-coupled PMA system under process variations. To maintain a compact memory footprint, we share the read access transistor across multiple bit-cells. Compared to the existing VSH-MRAMs, our design achieves 39%-42% and 36%-46% reduction in read time and energy, respectively, along with 1.1X-1.3X larger sense margin at a comparable area. This comes at the cost of 1.7X and 2.0X increase in write time and energy, respectively. Thus, the proposed design is suitable for applications in which reads are more dominant than writes

XNOR-VSH: A Valley-Spin Hall Effect-based Compact and Energy-Efficient Synaptic Crossbar Array for Binary Neural Networks

Binary neural networks (BNNs) have shown an immense promise for resource-constrained edge artificial intelligence (AI) platforms as their binarized weights and inputs can significantly reduce the compute, storage and communication costs. Several works have explored XNOR-based BNNs using SRAMs and nonvolatile memories (NVMs). However, these designs typically need two bit-cells to encode signed weights leading to an area overhead. In this paper, we address this issue by proposing a compact and low power in-memory computing (IMC) of XNOR-based dot products featuring signed weight encoding in a single bit-cell. Our approach utilizes valley-spin Hall (VSH) effect in monolayer tungsten di-selenide to design an XNOR bit-cell (named 'XNOR-VSH') with differential storage and access-transistor-less topology. We co-optimize the proposed VSH device and a memory array to enable robust in-memory dot product computations between signed binary inputs and signed binary weights with sense margin (SM) > 1 micro-amps. Our results show that the proposed XNOR-VSH array achieves 4.8% ~ 9.0% and 37% ~ 63% lower IMC latency and energy, respectively, with 4 % ~ 64% smaller area compared to spin-transfer-torque (STT)-MRAM and spin-orbit-torque (SOT)-MRAM based XNOR-arrays

Spin-orbit proximity in van der Waals heterostructures

165 p.In this thesis, graphene/transition metal dichalcogenides van der Waals hetero-structures were used to develop a device that combines Hall probes with ferromagnetic electrodes. With it, the spin Hall effect in graphene induced by spin-orbit coupling proximity with MoS2 and WSe2 could unambiguously be demonstrated. The Hanle precession of the non-local resistance not only gives convincing experimental proof but also allows the quantification of the spin transport and the spin-to-charge conversion. The fact that both occur in different parts of the same material gives rise to a high efficiency for the voltage output up to room temperature. Additionally, the control by applying a gate voltage was shown in graphene proximitized with WSe2, enabling a record efficiency measured of around 40 nm. Additionally, in a graphene/WSe2 lateral spin valve, coherent, electrically controllable spin precession in the absence of an external magnetic field was achieved, even in the diffusive regime

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

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

Probing and tuning the electronic properties of low dimensional van der Waals materials

177 p.The investigation on the physical properties of new materials is of fundamental importance to gain understanding and knowledge on systems and phenomena which one day may be exploited for revolutionary technological applications. In this regard, probing and tuning the electronic transport properties of low-dimensional materials might represent one of the routes that can satisfy the requirements of modern electronics/spintronics advancements. Following the common thread of investigating and manipulating the transport properties of low dimensional and low symmetrical systems, this thesis will be divided in two main parts. In the first part molecular functionalization is exploited to tune the intrinsic physical properties of two van der Waals materials: a superconductor and a ferromagnet. The second part focuses on the study of the charge to spin interconversion mechanisms in low symmetry systems. In particular, the study of the magnetoelectrical properties of Tellurium nanowires revealed a tight relationship between spin related phenomena and the symmetry breaking in such a chiral system

Electrical control of spin and valley in spin-orbit coupled graphene multilayers

Electrical control of magnetism has been a major techonogical pursuit of the spintronics community, owing to its far-reaching implications for data storage and transmission. Here, we propose and analyze a new mechanism for electrical switching of isospin, using chiral-stacked graphene multilayers, such as bernal bilayer graphene or rhombohedral trilayer graphene, encapsulated by transition metal dichalcogenide (TMD) substrates. Leveraging the proximity-induced spin-orbit coupling from the TMD, we demonstrate electrical switching of correlation-induced spin and/or valley polarization, by reversing a perpendicular displacement field or the chemical potential. We substantiate our proposal with both analytical arguments and self-consistent Hartree-Fock numerics. Finally, we illustrate how the relative alignment of the TMDs, together with the top and bottom gate voltages, can be used to selectively switch distinct isospin flavors, putting forward correlated van der Waals heterostructures as a promising platform for spintronics and valleytronics.Comment: 5 pages, 4 figures. (v2) Significant re-write correcting the magnetic moment, SM adde

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

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