The 2D band exhibits almost no change between different substrates which reclaims Raman as an

140 Pages·2015·4.23 MB·English

by  Emil Song

UCLA
UCLA Electronic Theses and Dissertations
Title
Spin-orbitronics: Electrical control of magnets via spin-orbit interaction
Permalink
https://escholarship.org/uc/item/9v5049t4
Author
Upadhyaya, Pramey
Publication Date
2015

Peer reviewed|Thesis/dissertation
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University of California
UNIVERSITY OF CALIFORNIA
Los Angeles

Spin-orbitronics: Electrical control of
magnets via spin-orbit interaction

A dissertation submitted in partial satisfaction of the
requirements for the degree Doctor of Philosophy
in Electrical Engineering

by

Pramey Upadhyaya

2015
© Copyright by
Pramey Upadhyaya
2015
ABSTRACT OF THE DISSERTATION

Spin-orbitronics: Electrical control of
magnets via spin-orbit interaction

by

Pramey Upadhyaya
Doctor of Philosophy in Electrical Engineering
University of California, Los Angeles, 2015
Professor Kang Lung Wang, Chair

Relativistic effects, having far reaching consequences for advancing our fundamental
understanding of the nature, have so far mostly played an academic role in solid-state systems.
For example, electrons moving in atomic orbitals close to the speed of light acquire a relativistic
shift in energy via the so-called spin-orbit interaction (SOI). More recently, the ability to
engineer this relativistic SOI in magnetic system, has shown potential to extend the reach of
relativity into technological applications by providing an energy-efficient electrical knob to
control magnetic order via torques, now referred to as the spin-orbit torques (SOT). In this
dissertation, this “spin-orbitronic” control of magnets interfaced with heavy elements, which in
turn possess a high SOI, is presented. Starting from a symmetry-based phenomenology, the role
of reduced symmetries leading to the identification of two flavors of SOT: current-induced and
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voltage-induced is highlighted. Focusing first on magnetic-memory-type applications, theoretical
proposal and experimental demonstration of new SOT resulting from breaking additional lateral
structural symmetry is then presented, which allows for the removal of power hungry external
magnetic fields for switching magnets. Subsequently, the required switching currents are reduced
by nearly three orders of magnitude via demonstration of extremely efficient SOT in topological
insulator-based magnets with engineered inversion asymmetry. Next, motivated by going beyond
memory applications, the role of SOT to create and manipulate magnetic solitons, i.e. particle-
like magnetic configurations capable of storing and transporting non-volatile information is
presented. This includes: (a) experimental demonstration of a scheme for current-induced
creation and manipulation of such solitons utilizing inhomogeneous SOT, (b) theoretical
possibility of manipulating these solitons via more energy efficient electric-field-induced SOT.
Finally, excitation of magnetization dynamics in insulating magnets, for transporting information
by Joule heating free “pure spin currents”, is of particular importance for low power requirement.
Consequently, an optical scheme demonstrating current-induced SOT in insulating magnets is
developed, followed by a proof of principle demonstration of motion of solitons via these pure
spin currents.

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The dissertation of Pramey Upadhyaya is approved.
Yaroslav Tserkovnyak
Oscar M. Stafsudd
Kang Lung Wang, Committee Chair

University of California, Los Angeles
2015

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Dedicated to my family,
specially to the loving memory
of my grandfather, Somnath Upadhyaya

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TABLE OF CONTENTS

CHAPTER 1: INTRODUCTION 1
1.1 PARADIGM SHIFT IN INFORMATION PROCESSING: TOWARDS "GREENER" TECHNOLOGY 1
1.2 SPINTRONICS: PROMISE OF MAGNETS FOR GREENER TECHNOLOGY 3
1.3 ELECTRICAL CONTROL OF MAGNETS: MECHANISMS 5
1.4 SPIN-ORBITRONICS 9
1.4.1 SPIN-ORBIT INTERACTION (SOI) AS THE COUPLING MECHANISM 9
1.4.2 MARRYING SOI AND MAGNETS : MATERIAL SYSTEMS 11
1.4.3 NOVEL OPPORTUNITES OPENED BY SOI 1 2
1.5 ROLE OF SYMMETRIES AND TYPES OF SOT 1 4
CHAPTER 2: MAGNETIZATION SWITCHING BY SPIN ORBIT TORQUE 19
2.1 MOTIVATION 19
2.2 EXTERNAL MAGNETIC FIELD FREE SWITCHING 20
2.2.1 LATERAL SYMMETRY BREAKING-INDUCED NOVEL SOT 21

2.2.2 EXPERIMENTAL DEMONSTRATION 25

2.2.3 POSSIBLE MICROSCOPIC ORIGIN OF NOVEL TORQUE 30

2.2.4 CURRENT-INDUCED SWITCHING IN THE ABSENCE OF EXTERNAL FIELDS 34

2.3 REDUCING SWITCHING CURRENT DENSITY : SOT IN TOPOLOGICAL-INSULATOR BASED MAGNETS 36
2.3.1 MATERIAL SYSTEM: ASYMMETRIC MAGNETIC TOPOLOGICAL INSULATORS 38
2.3.2 CURRENT-INDUCED MAGNETIZATION SWITCHING 39
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CHAPTER 3: SOT-INDUCED CREATION AND MOTION OF MAGNETIC SOLITONS 43
3.1 MOTIVATION 44
3.2 BLOWING MAGNETIC SKYRMION BUBBLES 46
3.2.1 EXPERIMENT: TRANSFORMING CHIRAL STRIPE DOMAINS INTO SKYRMIONS 48
3.2.2 CAPTURING THE TRANSFORMATION PROCESS 51
3.2.3 EFFECT OF TOPOLOGY ON DYNAMICS 53
3.2.4 OUTLOOK FOR SKYRMIONICS 56
3.3 ELECTRIC-FIELD GUIDING OF SKYRMIONS 57
3.3.1 ELECTRIC FIELD EFFECT: STATICS 59

3.2.2 ELECTRIC FIELD EFFECT: DYNAMICS 61

3.2.3 SKYRMIONICS BEYOND RACETRACK 69

3.4 ELECTRIC-FIELD-INDUCED DOMAIN-WALL MOTION 72
3.4.1 STABILIZING A DOMAIN WALL: EQUILIBRIUM CONFIGURATIONS 73
3.4.2 MICROMAGNETIC SIMULATIONS 77
3.4.3 ANALYTICAL MODEL 79
3.4.4 APPLICATIONS: ELECTRIC-FIELD-INDUCED DEPINNING AND CHIRALITY SWITCHING 82
CHAPTER 4: SOT IN MAGNETIC INSULATORS & MAGNON-INDUCED SOLITON MOTION 87
4.1 MOTIVATION 87
4.2 OPTICAL PROBE FOR SOT 88
4.3 SPIN-ORBIT FIELDS IN INSULATING YIG/PT 91
4.4 THERMAL MAGNONS-INDUCED DOMAIN-WALL MOTION 95
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4.4.1 EXPERIMENTAL SETUP AND RESULTS 97
4.4.2 "NON-LOCAL GEOMETRY": CONSISTENCY CHECK FOR MAGNON-DRIVEN DOMAIN-WALL MOTION 100
4.5 COMPARISON BETWEEN METALLIC AND INSULATING SYSTEM 104
CHAPTER 5: CONCLUSIONS 105
REFERENCES 109

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