Introduction to Magnetic Random-Access Memory

Magnetic random-access memory (MRAM) is poised to replace traditional computer memory based on complementary metal-oxide semiconductors (CMOS). MRAM will surpass all other types of memory devices in terms of nonvolatility, low energy dissipation, fast switching speed, radiation hardness, and durability. Although toggle-MRAM is currently a commercial product, it is clear that future developments in MRAM will be based on spin-transfer torque, which makes use of electrons' spin angular momentum instead of their charge. MRAM will require an amalgamation of magnetics and microelectronics technologies. However, researchers and developers in magnetics and in microelectronics attend different technical conferences, publish in different journals, use different tools, and have different backgrounds in condensed-matter physics, electrical engineering, and materials science.
This book is an introduction to MRAM for microelectronics engineers written by specialists in magnetic materials and devices. It presents the basic phenomena involved in MRAM, the materials and film stacks being used, the basic principles of the various types of MRAM (toggle and spin-transfer torque; magnetized in-plane or perpendicular-to-plane), the back-end magnetic technology, and recent developments toward logic-in-memory architectures. It helps bridge the cultural gap between the microelectronics and magnetics communities.

Auteur

Bernard Dieny has conducted research in magnetism for 30 years. He played a key role in the pioneering work on spin-valves at IBM Almaden Research Center in 1990-1991. In 2001, he co-founded SPINTEC in Grenoble, France, a public research laboratory devoted to spin-electronic phenomena and components. Dieny is co-inventor of 70 patents and has co-authored more than 340 scientific publications. He received an outstanding achievement award from IBM in 1992 for the development of spin-valves, the European Descartes Prize for Research in 2006, and two Advanced Research Grants from the European Research Council in 2009 and 2015. He is co-founder of two companies, one dedicated to magnetic random-access memory, Crocus Technology, the other to the design of hybrid CMOS/magnetic circuits, EVADERIS. In 2011 he was elected Fellow of the Institute of Electrical and Electronics Engineers.

Ronald B. Goldfarb was leader of the Magnetics Group at the National Institute of Standards and Technology in Boulder, Colorado, USA, from 2000 to 2015. He has published over 60 papers, book chapters, and encyclopedia articles in the areas of magnetic measurements, superconductor characterization, and instrumentation. In 2004 he was elected Fellow of the Institute of Electrical and Electronics Engineers (IEEE). From 1995 to 2004 he was editor in chief of IEEE Transactions on Magnetics. He is the founder and chief editor of IEEE Magnetics Letters, established in 2010. He received the IEEE Magnetics Society Distinguished Service Award in 2016. Kyung-Jin Lee is a professor in the Department of Materials Science and Engineering, and an adjunct professor of the KU-KIST Graduate School of Converging Science and Technology, at Korea University. Before joining the university, he worked for Samsung Advanced Institute of Technology in the areas of magnetic recording and magnetic random-access memory. His current research is focused on understanding the underlying physics of current-induced magnetic excitations and exploring new spintronic devices utilizing spin-transfer torque. He is co-inventor of 20 patents and has more than 100 scientific publications in the areas of magnetic random-access memory, spin-transfer torque, and spin-orbit torques. He received an outstanding patent award from the Korea Patent Office in 2005 and an award for Excellent Research on Basic Science from the Korean government in 2010. In 2013 he was recognized by the National Academy of Engineering of Korea as a leading scientist in spintronics, "one of the top 100 technologies of the future."

Contenu

About the Editors xi

Preface A Perspective on Nonvolatile Magnetic Memory Technology xiii

**Chapter 1 Basic Spintronic Transport Phenomena 1
Nicolas Locatelli and Vincent Cros

1.1 Giant Magnetoresistance 2

1.1.1 Basics of Electronic Transport in Magnetic Materials 2

1.1.2 A Simple Model to Describe GMR: The Two-Current Model 5

1.1.3 Discovery of GMR and Early GMR Developments 7

1.1.4 Main Applications of GMR 8

1.2 Tunneling Magnetoresistance 9

1.2.1 Basics of Quantum Mechanical Tunneling 10

1.2.2 First Approach to Tunnel Magnetoresistance: Jullière's Model 11

1.2.3 The Slonczewski Model 14

1.2.3.1 The Model 14

1.2.3.2 Experimental Observations 15

1.2.3.3 About the TMR Angular Dependence 15

1.2.4 More Complex Models: The Spin Filtering Effect 16

1.2.4.1 Incoherent Tunneling Through an Amorphous (Al2O3) Barrier 16

1.2.4.2 Coherent Tunneling Through a Crystalline MgO Barrier 17

1.2.5 Bias Dependence of Tunnel Magnetotransport 19

1.3 The Spin-Transfer Phenomenon 20

1.3.1 The Concept and Origin of the Spin-Transfer Effect 20

1.3.1.1 The In-Plane Torque 20

1.3.1.2 The Out-of-Plane Torque 23

1.3.2 Spin-Transfer-Induced Magnetization Dynamics 23

1.3.2.1 A Simple Analogy 24

1.3.2.2 Toward MRAM Based on Spin-Transfer Torque 25

1.3.3 Main Events Concerning Spin-Transfer Advances 26

References 27

**Chapter 2 Magnetic Properties of Materials for Mram 29
Shinji Yuasa

2.1 Magnetic Tunnel Junctions for MRAM 29

2.2 Magnetic Materials and Magnetic Properties 31

2.2.1 Ferromagnet and Antiferromagnet 31

2.2.2 Demagnetizing Field and Shape Anisotropy 33

2.2.3 Magnetocrystalline Anisotropy, Interface Magnetic Anisotropy, and Perpendicular Magnetic Anisotropy 35

2.2.4 Exchange Bias 36

2.2.5 Interlayer Exchange Coupling and Synthetic Antiferromagnetic Structure 37

2.2.6 Spin-Valve Structure 38

2.3 Basic Materials and Magnetotransport Properties 39

2.3.1 Metallic Nonmagnetic Spacer for GMR Spin-Valve 39

2.3.2 Magnetic Tunnel Junction with Amorphous AlO Tunnel Barrier 41

2.3.3 Magnetic Tunnel Junction with Crystalline MgO(0 0 1) Tunnel Barrier 44

2.3.3.1 Epitaxial MTJ with a Single-Crystal MgO(0 0 1) Barrier 44

2.3.3.2 CoFeB/MgO/CoFeB MTJ with a (0 0 1)-Textured MgO Barrier for Device Applications 46

2.3.3.3 Device Applications of MgO-Based MTJs 48

References 51

**Chapter 3 Micromagnetism Applied to Magnetic Nanostructures 55
Liliana D. Buda-Prejbeanu

3.1 Micromagnetic Theory: From Basic Concepts Toward the Equations 55

3.1.1 Free Energy of a Magnetic System 56

3.1.1.1 Exchange Energy 56

3.1.1.2 Magnetocrystalline Anisotropy Energy 57

3.1.1.3 Demagnetizing Energy 57

3.1.1.4 Zeeman Energy 60

3.1.2 Magnetically Stable State and Equilibrium Equations 61

3.1.3 Equations of Magnetization Motion 62

3.1.4 Length Scales in Micromagnetism 63

3.1.5 Modification Related to Spin-Transfer Torque Phenomena and SpinOrbit Coupling 64

3.1.6 Thermal Fluctuations 65

3.1.7 Numerical Micromagnetism 66

3.2 Micromagnetic Configurations in Magnetic Circular Dots 67

3.3 STT-Induced Magnetization Switching: Comparison of Macrospin and Micromagnetism 70

3.4 Example of Magnetization Precessional STT Switching: Role of Dipolar Coupling 73

References 76

Chapter 4 Magnetization Dynamics 79
William E. Bailey

4.1 LandauLifshitzGilbert Equation 79

4.1.1 Introduction 79

4.1.2 Variables in the Equation 80

4.1.3 The Equation 81

4.1.3.1 Precessional Term 82

4.1.3.2 Relaxation Term 83

4.2 Small-Angle Magnetization Dynamics 84 4.2...