Developing Variation Aware Simulation Tools, Models, and Designs for STT-RAM

DEVELOPING VARIATION AWARE SIMULATION TOOLS, MODELS, AND DESIGNS FOR STT-RAM Enes Eken, PhD University of Pittsburgh, 2017 In recent years, we have been witnessing the rise of spin-transfer torque random access memory (STT-RAM) technology. There are a couple of reasons which explain why STT-RAM has attracted a great deal of attention. Although conventional memory technologies like SRAM, DRAM and Flash memories are commonly used in the modern computer industry, they have major shortcomings, such as high leakage current, high power consumption and volatility. Although these drawbacks could have been overlooked in the past, they have become major concerns. Its characteristics, including low-power consumption, fast read-write access time and non-volatility make STT-RAM a promising candidate to solve the problems of other memory technologies. However, like all other memory technologies, STT-RAM has some problems such as long switching time and large programming energy of Magnetic Tunneling Junction (MTJ) which are waiting to be solved. In order to solve these long switching time and large programming energy problems, Spin-Hall Effect (SHE) assisted STT-RAM structure (SHE-RAM) has been recently invented. In this work, I propose two possible SHE-RAM designs from the aspects of two different write access operations, namely, High Density SHE-RAM and Disturbance Free SHE-RAM, respectively. In addition to the SHE-RAM designs, I will also propose a simulation tool for STT-RAMs. As an early-stage modeling tool, NVSim has been widely adopted for simulations of emerging nonvolatile memory technologies in computer architecture research, including STT-RAM, ReRAM, PCM, etc. I will introduce a new member of NVSim family – NVSim-VXs, which enables statistical simulation of STT-RAM for write performance, errors, and energy consumption

Study of spin-dependent transport phenomena in magnetic tunneling systems?

Ph.DDOCTOR OF PHILOSOPH

Generation of Spin Currents for Spintronic Logic Applications

University of Minnesota Ph.D. dissertation. April 2016. Major: Electrical Engineering. Advisor: Jian-Ping Wang. 1 computer file (PDF); ix, 116 pages.Current complementary metal oxide semiconductor (CMOS) technologies currently suffer drawbacks such as increased power consumption and device variability with scaling as well as volatility. In order to further advance computation technologies in the future, new and alternative devices are being explored to overcome these limitations. One promising approach is spintronic devices in which information is stored and computed based on the spin of electrons rather than the absence or presence of charge such as in CMOS. Spintronics offers many possible benefits including fast operational speed, low power consumption, and nonvolatility. This dissertation explores methods of generating spin polarized currents for the operation of logic devices and the fabrication of these devices for logic applications. The first device explored is a non-local lateral spin valve which can be used to generate a pure spin current and is the basic building block for the concept of all-spin logic. A unique top-down fabrication approach for lateral spin valves is created and demonstrated. Sub 100nm Co nanopillar devices are fabricated on a Cu channel using a top down approach that allows the entire material stack to be deposited initially under vacuum as opposed to devices fabricated using shadow beam lithography or lift-off techniques for ferromagnetic strips. A non-local signal is measured in these devices which indicates the top-down approach can successfully be used for integration of these devices. This demonstration is essential for these devices to be successfully implemented and scaled in computer applications at the industrial level. . In the second part of the dissertation, my research on spin Hall effect devices and the application of these devices for a spin Hall majority gate logic device are presented. The spin Hall effect is explored in bulk perpendicular TbFeCo/Ta devices which lays the groundwork for the following experiments. Then, a composite spin Hall structure is developed in order to switch perpendicular magnetization using the spin Hall effect without the need for an externally applied field. To demonstrate the ability to tune the material properties of a spin Hall channel, studies are also presented on a variety of multilayer spin Hall devices. Last, a three-input MTJ device is proposed for a spin orbit torque combined with spin transfer torque majority gate. Three MTJ devices are fabricated on Ta and three distinct switching states are shown corresponding to switching of the individual input elements. Additionally, simulation work is presented to verify the concept of the majority gate

Towards Oxide Electronics:a Roadmap

At the end of a rush lasting over half a century, in which CMOS technology has been experiencing a constant and breathtaking increase of device speed and density, Moore's law is approaching the insurmountable barrier given by the ultimate atomic nature of matter. A major challenge for 21st century scientists is finding novel strategies, concepts and materials for replacing silicon-based CMOS semiconductor technologies and guaranteeing a continued and steady technological progress in next decades. Among the materials classes candidate to contribute to this momentous challenge, oxide films and heterostructures are a particularly appealing hunting ground. The vastity, intended in pure chemical terms, of this class of compounds, the complexity of their correlated behaviour, and the wealth of functional properties they display, has already made these systems the subject of choice, worldwide, of a strongly networked, dynamic and interdisciplinary research community. Oxide science and technology has been the target of a wide four-year project, named Towards Oxide-Based Electronics (TO-BE), that has been recently running in Europe and has involved as participants several hundred scientists from 29 EU countries. In this review and perspective paper, published as a final deliverable of the TO-BE Action, the opportunities of oxides as future electronic materials for Information and Communication Technologies ICT and Energy are discussed. The paper is organized as a set of contributions, all selected and ordered as individual building blocks of a wider general scheme. After a brief preface by the editors and an introductory contribution, two sections follow. The first is mainly devoted to providing a perspective on the latest theoretical and experimental methods that are employed to investigate oxides and to produce oxide-based films, heterostructures and devices. In the second, all contributions are dedicated to different specific fields of applications of oxide thin films and heterostructures, in sectors as data storage and computing, optics and plasmonics, magnonics, energy conversion and harvesting, and power electronics

層間交換結合を有する磁性多層膜における電流誘起磁化スイッチング

Tohoku University加藤秀実課

Engineering Magnetic and Topological Properties in Epitaxial Heusler Compounds

Commercially viable spintronic devices require magnetic contacts with high electrical conductivity, high spin polarization, low Gilbert damping, and perpendicular magnetic anisotropy. The contact must also be amenable to thin film growth techniques to allow device scalability. Until now, this combination of properties had yet to be obtained in a single material. The exquisite control over crystal growth conditions and elemental composition imparted by molecular beam epitaxy can be leveraged to tune magnetic and electronic material properties closer to the ideal set desired by device researchers.Ferromagnetic metals composed of elements with low atomic weight are commonly used for spintronics, but the industry standard CoFeB does not possess high spin polarization, and its perpendicular magnetic anisotropy depends on film thickness, limiting its versatility. On the other hand, Heusler compounds are a class of over 1000 ternary intermetallic materials with highly variable magnetic and electronic properties. The Heusler compound Co2MnSi is well known as a half-metal with 100% spin polarization at the Fermi level, making it an ideal source of spin-polarized current. However, Co2MnSi does not possess perpendicular magnetic anisotropy. In this work, the magnetic anisotropy of Heusler compounds is engineered by breaking their cubic crystal symmetry. This can be accomplished by growing tetragonal crystal structures with the unique axis aligned out-of-plane, or by engineering superlattices composed of alternating layers of dissimilar Heusler compounds. In both cases, the resulting perpendicular magnetic anisotropy does not depend on film thickness, making the materials attractive for a broad range of spintronic device applications. Additionally, the Heusler compound superlattices studied here are composed of Co2MnAl and Fe2MnAl, which combine their electronic structures to produce 95% spin polarization as measured by spin-resolved photoemission spectroscopy. This combines two important magnetic properties never before seen in a single material system. The growth, structural, electronic and magnetic properties of the engineered films will be presented.Finally, Co2TiGe is explored as a candidate of an exotic class of topological materials known as Weyl semimetals. These systems possess a unique band structure that arises due to broken time-reversal symmetry resulting from the internal magnetization. Electrons with energy and momentum near so-called Weyl points have zero effective mass and a discrete chiral charge, making them analogous to the elusive Weyl fermion. The signatures of Weyl semimetallicity in Co2TiGe are probed using magnetotransport and synchrotron-based angle-resolved photoemission spectroscopy