Magnetization reversal and dynamic behaviour of patterned ferromagnetic nanostructures

MAGNETIZATION REVERSAL AND DYNAMIC BEHAVIOR OF PATTERNED FERROMAGNETIC NANOSTRUCTURES SHIMON NATIONAL UNIVERSITY OF SINGAPORE 2014 MAGNETIZATION REVERSAL AND DYNAMIC BEHAVIOR OF PATTERNED FERROMAGNETIC NANOSTRUCTURES SHIMON (M. Eng., Massachusetts Institute of Technology) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN ADVANCED MATERIALS FOR MICRO- AND NANO-SYSTEMS (AMM&NS) SINGAPORE-MIT ALLIANCE NATIONAL UNIVERSITY OF SINGAPORE 2014 DECLARATION I hereby declare that this thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously Shimon 20 June 2014 O Lord, our Lord, how majestic is your name in all the earth! You have set your glory above the heavens. … When I look at your heavens, the work of your fingers, the moon and the stars, which you have set in place, what is man that you are mindful of him, and the son of man that you care for him? Yet you have made him a little lower than the heavenly beings and crowned him with glory and honor. … O Lord, our Lord, how majestic is your name in all the earth! (Psalm of David) Acknowledgements I would like to thank God whom I know in Lord Jesus Christ for giving me the opportunity to pursue and complete my doctoral study in NUS. I wish to thank several people who have helped and supported me throughout my PhD. Firstly, I would like to thank my main thesis advisor Prof. Adekunle O. Adeyeye for his continuous guidance, motivation and advices throughout my PhD study. His exemplary disciplines and work ethics have inspired me to better each day both in work and personal life. Secondly, I would like to thank my thesis co-advisor Prof. Caroline A. Ross for her critical assessments on my research works, encouragement and the numerous conference calls after office hours. I would like to thank both my thesis advisors for their time and dedication to review, comment, modify and proofread numerous drafts of this thesis and all my previous papers manuscripts. It is a great honor to have known and worked with them. I would like to thank current and past members of Prof. Adeyeye’s group: I would like to thank Dr. Navab Singh for providing the deep ultraviolet resist templates used in this thesis. I would like to thank Dr. Debashish Tripathy and Dr. Shikha Jain for their training and guidance in the first year of my PhD and for their friendship till now. I would like to thank my two immediate seniors in the group, my lunch buddy, Dr. Liu Xinming and the ‘tech expert’, Dr. Ding Junjia for the great time we shared in and outside the lab, for the unwavering support, for all the trainings, help and most importantly for sharing not only tons of i scientific knowledge but also countless goodies and foodies throughout these four years. I would like to thank people of ISML for making ISML a wonderful place to research, have fun and make friends. I would like to thank Ms. Loh Fong Leong and Ms. Xiao Yun for their technical and procurement support throughout my PhD. I would like to thank Singapore-MIT Alliance for the funding and its staff: Mr. Neo Choon Siong, Ms. Nurdiana binte Housman, Ms. Shirley Jong Mey Jing and Ms. Hong Yanling. I would also like to thank Mr. Praveen Deorani of ISML for his expertise in 3D OOMMF script and Linux, Dr. Mark D. Mascaro for developing OOMMFTools software and his technical support on it, Mr. Abdul Jalil bin Din of PCB lab and Ms. Eunice Wong of ECE department. I would like to thank my parent, my elder brother, my aunt and my late grandparent for their continuous motivation and prayer throughout my PhD. I would also like to thank all my thoughtful families and friends who have sent well wishes, motivated or helped in a way or another during my PhD study. Soli Deo Gloria! ii Table of Contents Acknowledgements i Table of Contents . iii Summary vii List of Figures x List of Symbols and Abbreviations xix Statement of Originality . xxii CHAPTER 1. Introduction . 1.1. Background . 1.2. Motivation . 1.2.1. Magnetic disks . 1.2.2. Magnetic rings . 1.2.3. Bi-component nanostructures 1.3. Focus of Thesis 1.4. Organization of Thesis CHAPTER 2. Theoretical Background . 10 2.1. Introduction . 10 2.2. Micromagnetic Energies 10 2.2.1. Exchange energy . 11 2.2.2. Magnetostatic energy 12 2.2.3. Magnetocrystalline anisotropy energy 12 2.2.4. Zeeman energy 13 2.2.5. Interplay between energy terms and domain formation 13 2.3. Magnetization reversal of circular ferromagnetic disks 15 2.4. Magnetization reversal of ferromagnetic rings . 17 2.5. Ferromagnetic Resonance . 19 2.6. Brillouin Light Scattering 22 iii 2.7. Planar Hall Effect 25 2.8. Summary . 27 CHAPTER 3. Experimental and Simulation Techniques . 28 3.1. Introduction . 28 3.2. Pattern Fabrication Techniques . 28 3.2.1. Ultraviolet lithography 28 3.2.2. KrF deep ultraviolet lithography . 30 3.2.3. Electron beam lithography 32 3.3. Materials Deposition Techniques 33 3.3.1. Electron beam evaporation and sputter deposition 34 3.3.2. Angle deposition and selective etching . 36 3.3.3. Lift-off . 38 3.4. Characterization Techniques . 39 3.4.1. Scanning electron microscopy . 39 3.4.2. Scanning probe microscopy 42 3.4.3. Magneto-optic Kerr effect spectroscopy . 44 3.4.4. Vibrating sample magnetometer . 46 3.4.5. Ferromagnetic resonance spectroscopy . 47 3.4.6. Brillouin light scattering spectroscopy 49 3.4.7. Planar Hall Effect measurement 52 3.5. Micromagnetic Simulation 53 3.5.1. Quasistatic simulation . 55 3.5.2. Dynamic simulation 56 3.6. Summary . 58 CHAPTER 4. Static and Dynamic Behavior Comparison between Rectangular and Circular NiFe Thin Film Rings 59 4.1. Introduction . 59 4.2. Static behavior . 61 4.2.1. Reversal mechanisms 62 4.2.2. Switching field comparison . 70 4.2.3. Effect of ring thickness 72 iv 4.3. Dynamic behavior . 77 4.3.1. Arrays with inter-ring separation of 550 nm . 78 4.3.2. Interacting ring arrays . 83 4.4. Summary . 92 CHAPTER 5. Reversal Mechanisms of Coupled bi-Component Magnetic Nanostructures . 94 5.1. Introduction . 94 5.2. Fabrication . 95 5.3. Bi-component disks . 99 5.4. Bi-component rectangular rings and ring/wires 105 5.5. Summary . 115 CHAPTER 6. Vortex Dynamics in Thickness-Modulated NiFe Disks . 117 6.1. Introduction . 117 6.2. Fabrication . 118 6.3. Static behavior . 121 6.3.1. Reversal mechanism 121 6.3.2. Control of vortex chirality and propagation 124 6.4. Dynamic behavior . 127 6.5. Effect of interlayer magnetostatic interaction . 132 6.6. Vortex chirality detection for memory storage application . 137 6.7. Summary . 141 CHAPTER 7. Simultaneous Control of Vortex Chirality and Polarity in Thickness-Modulated [CoPd]n/Ti/NiFe Disks 143 7.1. Introduction . 143 7.2. Fabrication . 144 7.3. Static behavior . 146 7.3.1. Roles of [CoPd]n underlayer 146 7.3.2. Simultaneous control of vortex chirality and polarity . 154 7.4. Brillouin light scattering studies 157 7.4.1. BLS thermal spectra 157 v 7.4.2. 2D μ-BLS intensity mapping . 160 7.5. Summary . 166 CHAPTER 8. Conclusion . 168 8.1. Overview . 168 8.2. Summary of results 168 8.3. Future works 172 APPENDIX A. MOKE Loops of Rectangular Rings Measured at Various Happ angles . 174 APPENDIX B. Smit-Beljers Resonance Formulation . 175 APPENDIX C. 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Rep., vol. 10, p. 113, 1955. 205 [...]... understanding the static and highfrequency magnetic behavior of a wide range of patterned ferromagnetic nanostructures has been motivated by the prospect of their utilization as high density memory elements, domain wall logic, spin wave guide, magnonic crystal and microwave filters In this thesis, the static magnetization reversal process and dynamic behavior of various patterned ferromagnetic nanostructures. .. Focus of Thesis In this thesis, a detailed study of reversal mechanism and dynamic behavior of ferromagnetic nanostructures will be presented The main body of the thesis is divided into four parts The first part focuses on comparison of static and dynamic behaviors between rectangular and circular thin film rings The second part deals with the fabrication and characterization of bi-component nanostructures. .. bicomponent nanostructures The reversal mechanism of the resulting magnetic nanostructures have also been investigated and modeled [3] G Shimon, A O Adeyeye and C A Ross, Reversal mechanisms of coupled bi-component magnetic nanostructures , Appl Phys Lett 101, 083112 (2012) • Systematic control of vortex chirality and propagation using thickness modulation in Ni80Fe20 disks Detection of vortex propagation and. .. representation of dynamic equation of motion: (a) without and (b) with damping term 54 Fig 3-17 Dynamic magnetization response (MZ/MS) of a circular ring after a week pulse field is applied in (a) time-domain, (b) frequency domain (c-d) Plots showing the time, duration and amplitude of pulse field 57 Fig 4-1 SEM micrographs showing arrays of isolated (s=3 μm) rectangular rings and circular rings of w=350... (rad.s-1) xxi Statement of Originality The author claims the following aspects of this thesis to be original contributions to scientific knowledge • A systematic comparison of static and dynamic behavior between rectangular and circular thin film magnetic rings as a function of ring thickness and interring spacing [1] G Shimon, A O Adeyeye and C A Ross, “Comparative study of magnetization reversal process... in a vortex state and (b) Cartesian and polar coordinate systems 176 Fig B-2 Plot of 2D FMR spectra showing the mode A splitting in rectangular ring and circular ring as derived using Smit-Beljers formulation 181 Fig C-1 Micromagnetic simulations showing the modification of magnetization reversal of bi-component disk using the combination of NiFe/Fe, Ni/Fe and NiFe/Ni for lens and crescent regions... reversal mechanism and dynamic behavior when compared to circular rings This understanding is useful in providing model structure to study DWs pinning and propagation, spin wave confinement and the effect of shape anisotropy on dynamic behavior of ring arrays Furthermore, a comprehensive investigation and comparison of rings’ behavior as a function of their dimension, geometry and inter-ring spacing... angles θ=0° and θ=30° for t=40nm 71 Fig 4-7 MOKE loops as a function of film thickness for both rectangular and circular rings of w=350 nm 72 Fig 4-8 Plot of Hs1 and Hs2 as a function of film thickness for (a) rectangular and (b) circular rings, extracted from MOKE measurements 73 Fig 4-9 Magnetic configurations at remanence of circular and rectangular rings for (a) t=20 nm and (b)... shows the plot of first derivative of the M-H loop in the up-sweep direction 15 Fig 2-3 Typical magnetization reversal process of circular ring 17 Fig 2-4 Simulated spin configurations showing (a) two types of 180° DWs in an onion state and (b) two types of vortex chirality in a vortex state 18 Fig 2-5 Schematic diagram showing magnetization precession under Happ and perpendicular... Shimon, V Ravichandar, A O Adeyeye and C A Ross, “Simultaneous control of vortex polarity and chirality in thicknessmodulated [CoPd] n/Ti/Ni80Fe20 disks”, Appl Phys Lett 105, 152408 (2014) xxiii CHAPTER 1 Introduction 1.1 Background Patterned ferromagnetic nanostructures have been the focus of attention not only for fundamental magnetic studies but also because of their potential in a wide range of applications . MAGNETIZATION REVERSAL AND DYNAMIC BEHAVIOR OF PATTERNED FERROMAGNETIC NANOSTRUCTURES SHIMON NATIONAL UNIVERSITY OF SINGAPORE 2014 MAGNETIZATION REVERSAL. between energy terms and domain formation 13 2.3. Magnetization reversal of circular ferromagnetic disks 15 2.4. Magnetization reversal of ferromagnetic rings 17 2.5. Ferromagnetic Resonance. REVERSAL AND DYNAMIC BEHAVIOR OF PATTERNED FERROMAGNETIC NANOSTRUCTURES SHIMON (M. Eng., Massachusetts Institute of Technology) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY