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New Directions for Magnetic Memory : Alternative Switching Mechanisms for Magnetic Random-Access Memory New directions for magnetic memory : alternative switching mechanisms for magnetic random-access memory Citation for published version (APA): van den Brink, A. (2016). New directions for magnetic memory : alternative switching mechanisms for magnetic random-access memory. Technische Universiteit Eindhoven. Document status and date: Published: 19/09/2016 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal. If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement: www.tue.nl/taverne Take down policy If you believe that this document breaches copyright please contact us at: [email protected] providing details and we will investigate your claim. Download date: 25. Sep. 2021 New Directions for Magnetic Memory Alternative switching mechanisms for magnetic random-access memory PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de rector magnificus prof.dr.ir. F.P.T. Baaijens, voor een commissie aangewezen door het College voor Promoties, in het openbaar te verdedigen op maandag 19 september 2016 om 16:00 uur door Arie van den Brink geboren te Nijkerk Dit proefschrift is goedgekeurd door de promotoren en de samenstelling van de promotiecommissie is als volgt: voorzitter: prof.dr.ir. G.M.W. Kroesen 1e promotor: prof.dr.ir. H.J.M. Swagten 2e promotor: prof.dr. B. Koopmans leden: prof.dr. C.H. Marrows (University of Leeds) prof.dr. G.S.D. Beach (Massachusetts Institute of Technology) prof.dr. R. Coehoorn prof.dr. R.A. Duine (Universiteit Utrecht) adviseur: dr. M. Manfrini (imec) Het onderzoek dat in dit proefschrift wordt beschreven is uitgevoerd in overeenstemming met de TU/e Gedragscode Wetenschapsbeoefening. New Directions for Magnetic Memory Alternative switching mechanisms for magnetic random-access memory A catalogue record is available from the Eindhoven University of Technology Library ISBN: 978-90-386-4131-7 This thesis is part of NanoNextNL, a micro and nanotechnology innovation programme of the Dutch government and 130 partners from academia and industry. More information on www.nanonextnl.nl Copyright © 2016 Arie van den Brink Contents 1 Introduction1 1.1 Contemporary memory technologies . 1 1.2 Magnetic Random Access Memory . 5 1.3 The scaling challenge . 11 1.4 Alternative switching mechanisms for magnetic memory . 12 1.5 This thesis . 13 2 Background 15 2.1 Basic concepts . 15 2.2 Magnetic Anisotropy . 17 2.3 Electric-field effect . 20 2.4 Spin-Transfer Torque . 24 2.5 Spin-Hall Effect . 25 2.6 Exchange Bias . 30 3 Methods 35 3.1 Sample preparation . 35 3.2 Analysis of magnetic properties . 38 3.3 Simulation of macrospin dynamics . 41 4 Large electric field effect in a Pt/Co/AlOx/Pt junction 49 4.1 Introduction . 50 4.2 Methods . 50 4.3 Results . 51 4.4 Discussion . 53 4.5 Summary . 55 i ii 5 Spin-Hall-assisted magnetic random access memory 57 5.1 Introduction . 58 5.2 Methods . 59 5.3 Results . 59 5.4 Discussion . 62 5.5 Summary . 63 5.6 Supplementary Information . 64 6 Orthogonal exchange bias 73 6.1 Introduction . 73 6.2 Methods . 76 6.3 Results . 77 6.4 Discussion . 88 6.5 Summary . 91 7 Field-free switching by spin-Hall effect and exchange bias 93 7.1 Introduction . 94 7.2 Methods . 94 7.3 Results . 96 7.4 Discussion . 101 7.5 Summary . 103 7.6 Supplementary Information . 103 A Numerical implementation of the LLG equation 119 B Temperature in a macrospin system 123 C Measuring orthogonal exchange bias using MOKE 125 Summary 129 Curriculum Vitae 131 Publications 133 Dankwoord 135 Bibliography 137 1 Introduction In this thesis, several novel switching mechanisms for magnetic random-access memory (MRAM) are investigated. This introductory chapter discusses the in- volved physics at a basic level, and provides a contemporary context to underline the technological relevance of the presented work. First, a brief overview of his- toric and contemporary computer memories is presented. This serves to illustrate the opportunities and challenges that exist in MRAM development today, involv- ing a trade-off between cost, density, and performance. Subsequently, the physical concepts involved in reading, writing and storing information in MRAM cells are introduced. These concepts are by now well-established, and reveal an inherent challenge in creating ever smaller memory cells while maintaining stability and durability. In this thesis, this challenge is addressed by exploring novel switch- ing mechanisms, which will be briefly introduced here. Finally, an overview of the remainder of this thesis is presented. 1.1 Contemporary memory technologies Memory is a vital part of any computer, complementing logic operations with the ability to buffer intermediate results and store input and output data. Since the early days of computing, two distinct types of computer memory have been used: quickly accessible volatile memory and slower non-volatile memory which retains information in the power-off state. The earliest electronic computers used various types of vacuum tubes for volatile storage, storing information in the form of electric charge. Non-volatile memory came in the form of paper sheets, until magnetic alternatives were developed. These early magnetic memories stored information in 1 2 Chapter 1. Introduction Table 1.1: Comparison of key memory characteristics in existing and emerging random- access memory technologies. Technology abbreviations are explained in the text. `End.' is short for endurance (write cycles before breakdown), `R/W' denotes read/write time. Adapted from Ref. [1]. Technology End. R/W (ns) Density Other SRAM 1 < 1 Low Volatile, standby power DRAM 1 30 Medium Volatile, standby power Flash (NOR) 105 100 / 103 Medium High write voltage / power Flash (NAND) 105 100 / 106 High High write voltage / power FeRAM 1014 30 Low Destructive read-out RRAM 109 1 - 100 High Complex mechanism PCM 109 10 / 100 High Operating T < 125 ◦C STT-MRAM 1 2 - 30 Medium Low read signal a ferromagnetic coating on a large cylinder (drum memory) or in tiny magnetic rings (core memory), laying the foundation for the modern hard-disk drive (HDD) and random-access memory (RAM), respectively. Despite the success of particular memory types in certain decades, a truly univer- sal memory has never been discovered. Rather, today's memory market comprises a myriad of established and upcoming technologies1{6, competing for specific ap- plications in a trade-off between cost, density, and performance. To introduce the reader to this field, a short overview of contemporary technologies is presented in this section. Currently, computers make use of Static and Dynamic Random Access Mem- ory (SRAM and DRAM) for fast, volatile data storage. Non-volatile storage, on the other hand, is performed by magnetic hard-disk drives (HDDs) and, to an increasing degree, Flash memory. Emerging technologies include Ferroelec- tric RAM (FeRAM), Resistive RAM (RRAM), Phase-Change Memory (PCM) and Spin-Transfer Torque Magnetic RAM (STT-MRAM). An overview of performance characteristics of these technologies is presented in Table 1.1, and a brief discussion of each memory type is included below. It will become apparent that STT-MRAM offers a unique combination of desirable properties, making it very attractive for near-future applications. 1.1.1 Static random-access memory Where speed is more important than density and cost, such as in processor cache memory, today's computers employ Static Random Access Memory (SRAM). This 1.1. Contemporary memory technologies 3 a) SRAM b) DRAM c) Flash WL VDD Capacitor Control Gate BL WL Floating Gate + + n+ n+ n n p-Si p-Si BL BL Figure 1.1: Contemporary random-access memory architectures. (a) Static Random Access Memory (SRAM) cells contain four transistors in a flip-flop arrangement, passing or blocking a supply voltage VDD to a bit line (BL) and inverse bit line (BL). A cell is addressed by a word line (WL) using two more transistors. (b) Dynamic Random Access Memory (DRAM) cells consist of a transistor (depicted here as a gated p-n-p doped silicon channel) addressing a capacitor. (c) Flash memory cells are transistors with a floating gate, which is surrounded by insulating material and can be charged using a control gate. Adapted from Ref. [4]. technology is based on a six transistor flip-flop circuit, where four transistors store a binary state and two transistors control the access to the cell during write and read operations (Figure 1.1a).
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