Spin Currents and Spin Orbit Torques in Ferromagnets and

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Spin Currents and Spin Orbit Torques in Ferromagnets and

Transcript Of Spin Currents and Spin Orbit Torques in Ferromagnets and

Spin Currents and Spin Orbit Torques in Ferromagnets and Antiferromagnets
by
Yu-Ming Hung
A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Physics New York University May, 2017
Prof. Andrew D. Kent

c Yu-Ming Hung All Rights Reserved, 2017

Dedication
To my wife, for her love and support.
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Acknowledgements
This thesis would not have been possible without the help, guidance, and endless patience of many individuals. First, I am extremely grateful to my thesis adviser, Prof. Andrew D. Kent. He has led me into the research fields of magnetism and spintronics by instructing my research with great patience. I was able to overcome a lot of difficulties in experiments due to his encouragement and support. Under his leadership, it is a great pleasure to work in Kent lab where the academia atmosphere is comfortable for scientific discussions.
I am thankful to talented post-docs during my research work. I thank Drs. Dirk Backes, Gabriel Chaves, Ferran Macia, Christian Hahn, Bartek Kardasz, Debangsu Roy, Volker Sluka, and Li Ye for sharing their expertise and skills that are indispensable for me in gaining research experience. I am particular thankful to Dr. Georg Wolf for his patience and unselfishness to guide me in the process of learning research skills especially in magnetic simulation, probe station construction, and computer programming.
I am also grateful to many group members in Kent lab for their support and help: Drs. Daniel Gopman, Huanlong Liu, Daniele Pinna, and Pradeep Subedi and Eason Chen, Jinting Hang, and Marion Lavanant. I would like to give a special thanks to Laura Rehm who helped me in the sample fabrication and experimental projects. We share both the failure and success in experiments and kept our morale
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up at work. Lastly, it is a great privilege working with significant scientists in the field
of magnetism and spintronics. I thank Dr. Hendrik Ohldag for hosting me at Stanford Synchrotron Radiation Laboratory and more importantly, for his inspiration in experimental work. Prof. Lior Klein at Bar-Ilan University has been a great collaborator and mentor providing insightful comments and suggestions in my simulation work. I am fortunate to work with Dr. Suzanne G.E. te Velthuis and Dr. Wanjun Jiang who host and instruct me to conduct experiments at Argonne National Laboratory.
My thesis research was funded by the Institute for Nanoelectronics Discovery and Exploration (INDEX), a funded center of Nanoelectronics Research Initiative (NRI), a Semiconductor Research Corporation (SRC) program sponsored by NERC and NIST. I also acknowledge support in part from the National Science Foundation through projects NSF-DMR-1309202 and NSF-DMR-1610416.
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Abstract
This thesis focuses on the interactions of spin currents and materials with magnetic order, e.g., ferromagnetic and antiferromagnetic thin films. The spin current is generated in two ways. First by spin-polarized conduction-electrons associated with the spin Hall effect in heavy metals (HMs) and, second, by exciting spinwaves in ferrimagnetic insulators using a microwave frequency magnetic field.
A conduction-electron spin current can be generated by spin-orbit coupling in a heavy non-magnetic metal and transfer its spin angular momentum to a ferromagnet, providing a means of reversing the magnetization of perpendicularly magnetized ultrathin films with currents that flow in the plane of the layers. The torques on the magnetization are known as spin-orbit torques (SOT). In the first part of my thesis project I investigated and contrasted the quasistatic (slowly swept current) and pulsed current-induced switching characteristics of micrometer scale Hall crosses consisting of very thin (<1 nm) perpendicularly magnetized CoFeB layers on β-Ta. While complete magnetization reversal occurs at a threshold current density in the quasistatic case, pulses with short duration (≤10 ns) and larger amplitude (≃10 times the quasistatic threshold current) lead to only partial magnetization reversal and domain formation. The partial reversal is associated with the limited time for reversed domain expansion during the pulse.
The second part of my thesis project studies and considers applications of
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SOT-driven domain wall (DW) motion in a perpendicularly magnetized ultrathin ferromagnet sandwiched between a heavy metal and an oxide. My experiment results demonstrate that the DW motion can be explained by a combination of the spin Hall effect, which generates a SOT, and Dzyaloshinskii–Moriya interaction, which stabilizes chiral N´eel-type DW. Based on SOT-driven DW motion and magnetic coupling between electrically isolated ferromagnetic elements, I proposed a new type of spin logic devices. I then demonstrate the device operation by using micromagnetic modeling which involves studying the magnetic coupling induced by fringe fields from chiral DWs in perpendicularly magnetized nanowires.
The last part of my thesis project reports spin transport and spin-Hall magnetoresistance (SMR) in yttrium iron garnet Y3Fe5O12 (YIG)/NiO/Pt trilayers with varied NiO thickness. To characterize the spin transport through NiO we excite ferromagnetic resonance in YIG with a microwave frequency magnetic field and detect the voltage associated with the inverse spin-Hall effect (ISHE) in the Pt layer. The ISHE signal is found to decay exponentially with the NiO thickness with a characteristic decay length of 3.9 nm. However, in contrast to the ISHE response, as the NiO thickness increases the SMR signal goes towards zero abruptly at a NiO thickness of 4 nm, highlighting the different length scales associated with the spin-transport in NiO and SMR in such trilayers.
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Table of Contents

Dedication

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Acknowledgements

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Abstract

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Table of Contents

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List of Figures

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List of Tables

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1 Introduction

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1.1 Spin-orbit Effect and Spin Current . . . . . . . . . . . . . . . . . . 1

1.1.1 Rashba Effect . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.1.2 Spin Hall Effect . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2 Spin-orbit Torque . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.2.1 Single Domain Model . . . . . . . . . . . . . . . . . . . . . . 10

1.2.2 Landau-Lifshitz-Gilbert Equation . . . . . . . . . . . . . . . 21

1.3 Spin-orbit Torque Driven Magnetization Reversal . . . . . . . . . . 22

1.3.1 Anti-damping Switching . . . . . . . . . . . . . . . . . . . . 22

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1.3.2 Effective Field Switching . . . . . . . . . . . . . . . . . . . . 23 1.4 Spin-orbit Torque Driven Domain Wall Motion . . . . . . . . . . . . 27
1.4.1 Dzyaloshinskii-Moriya Interaction . . . . . . . . . . . . . . . 29 1.4.2 Domain Wall Motion . . . . . . . . . . . . . . . . . . . . . . 29 1.5 Spin-wave Spin Current . . . . . . . . . . . . . . . . . . . . . . . . 31 1.5.1 Ferromagnetic Resonance . . . . . . . . . . . . . . . . . . . 32 1.5.2 Inverse Spin Hall Effect . . . . . . . . . . . . . . . . . . . . 33 1.5.3 Spin-Hall Magnetoresistance . . . . . . . . . . . . . . . . . . 34 1.6 An Overview of My Thesis . . . . . . . . . . . . . . . . . . . . . . . 36

2 Experimental Methods

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2.1 Sample Fabrication Process . . . . . . . . . . . . . . . . . . . . . . 40

2.1.1 Thin Film Deposition . . . . . . . . . . . . . . . . . . . . . . 40

2.1.2 Device Patterning and Etching . . . . . . . . . . . . . . . . 45

2.2 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

2.2.1 Magneto-optical Kerr Effect . . . . . . . . . . . . . . . . . . 46

2.2.2 Ferromagnetic Resonance . . . . . . . . . . . . . . . . . . . 50

2.2.3 Magnetotransport Measurements . . . . . . . . . . . . . . . 54

2.2.3.1 Dipole Electromagnet . . . . . . . . . . . . . . . . 56

2.2.3.2 Manual Probe Station . . . . . . . . . . . . . . . . 57

2.3 Software for Automated Experiments . . . . . . . . . . . . . . . . . 60

3 Current-induced Switching with Spin-orbit Torques in Ultrathin

Films with Perpendicular Magnetic Anisotropy

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3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

3.1.1 Perpendicular Magnetic Anisotropy in Ultrathin Films . . . 65

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3.1.2 Experimental Method . . . . . . . . . . . . . . . . . . . . . 71 3.2 Quasistatic Current-induced Magnetization Switching . . . . . . . . 74 3.3 Pulsed Current-induced Magnetization Switching . . . . . . . . . . 77 3.4 Micromagnetic Simulations . . . . . . . . . . . . . . . . . . . . . . . 79 3.5 Discussion and Summary . . . . . . . . . . . . . . . . . . . . . . . . 83

4 Domain Wall Fringe Field Coupled Spin Logic

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4.1 Device Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

4.2 Device Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

4.2.1 Device Characteristics . . . . . . . . . . . . . . . . . . . . . 90

4.2.2 Coupled Magnets . . . . . . . . . . . . . . . . . . . . . . . . 94

4.3 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . 98

4.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

5 Spin Transport in Antiferromagnetic NiO and Magnetoresistance

in Y3Fe5O12/NiO/Pt Structures

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5.1 Experimental Methods . . . . . . . . . . . . . . . . . . . . . . . . . 106

5.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 108

6 Summary

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6.1 Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

List of Publications and Conference Contributions

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Bibliography

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