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Dissertation Brillouin Light Scattering Study of Linear and Nonlinear Spin Dissertation Brillouin Light Scattering Study of Linear and Nonlinear Spin Waves in Continuous and Patterned Magnetic Thin Films Submitted by Hau-Jian Jason Liu Department of Physics In partial fulfillment of the requirements For the Degree of Doctor of Philosophy Colorado State University Fort Collins, Colorado Summer 2014 Doctoral Committee: Advisor: Kristen S. Buchanan Martin P. Gelfand Pavel Kabos James R. Neilson Copyright by Hau-Jian Jason Liu 2014 All Rights Reserved Abstract Brillouin Light Scattering Study of Linear and Nonlinear Spin Waves in Continuous and Patterned Magnetic Thin Films This thesis focuses on the use of the Brillouin light scattering (BLS) technique to measure spin waves or magnons in thin films. BLS is an experimental technique that measures the inelastically scattered light from photon-magnon interactions. Broadly, three different exper- iments are presented in this thesis: the measurements of spin wave properties in iron cobalt (FeCo), yttrium iron garnet (YIG), and microstructures involving Permalloy (Ni80Fe20) and cobalt nickel (CoNi). First, conventional backward scattering BLS was used to measure the spin waves in a set of Fe65Co35 films that were provided by Seagate Technologies. By fitting the spin wave frequencies that were measured as a function of the external magnetic field and film thickness, the quantum mechanical parameter responsible for short range order, known as the exchange parameter, was determined. Second, nonlinear spin waves were mea- sured in YIG using conventional forward scattering BLS with time resolution. Two nonlinear three wave processes were observed, namely, the three magnon splitting and confluence. The nonlinear power threshold, the saturation magnetization, and the film thickness were deter- mined independently using network analyzer measurements. The spin wave group velocities were determined from the space- and time-resolved BLS data and compared to calculations from the dispersion relations. Back calculations showed the location where the three magnon splitting process took place. Lastly, spin waves in Permalloy and CoNi microstrips were mea- sured using a recently developed micro-BLS. The micro-BLS, with a spatial resolution of 250 nm, allows for measuring the effects on the lateral confinement of spin waves in microstrips. The confinement of spin waves led to modifications to the dispersion relations, which were ii compared against the spin wave frequencies obtained from the micro-BLS. The Permalloy experiments shows non-reciprocity in surface spin wave modes with opposite wavevectors and provides a quantitative measure of the difference in excitation efficiency between the surface spin wave and the backward volume spin wave modes. Measurements were also conducted in the Permalloy microstrips at zero external magnetic field, showing evidence that propagating spin waves can be observed by exploiting the effects of shape anisotropy. Finally, preliminary measurements were done on CoNi microstrips with perpendicular anisotropy. A magnetic signal was detected, however further investigation will be needed to determine the exact origin of the observed signal and to definitively answer the question as to whether or not BLS can be used to measure spin waves in perpendicularly magnetized films. Overall, the experiments and results presented in this thesis show that BLS is a useful tool for measuring spin wave properties in magnetic thin films. iii Acknowledgements The work that lead to the completion of this thesis would not have been possible without the support of many individuals that I have had the privilege and honor of knowing and working with. I would like to thank my advisor, Kristen, for giving me the opportunity to work on a world-class experiment in the field of magnetics. Her guidance and mentoring has opened doors for me I never thought I would ever walk through. I would also like to thank my friends, colleagues, and mentors at CSU. My fellow graduate students Tim, Ben, Alex, Joel, Grant, and Praveen have provided me a forum to discuss ideas and concepts that I have struggled with. These discussions, although sometimes heated, have been helpful in preparing the work that went into this thesis. Leif has provided the formatting for this thesis to meet the graduate school requirements. Carl and Mingzhong have been mentors who have provided valuable advice throughout my graduate studies. Wendy has kept me on track administratively since I started graduate school, and I believe I would not be where I am today without her. I am grateful for the moral support of my family and dearest friends: Nichelle and Mena. My dog, Awesome, deserves a treat for providing me with the necessary distractions from my studies when I needed it the most. Lastly, a special thanks to Doc Morris whom deserves my deepest gratitude. He has been an inspiration to me with his dedication to teaching and his passion for physics. iv Table of Contents Abstract . ii Acknowledgements . iv List of Tables . ix List of Figures . x List of Symbols . xiii List of Acronyms. xv Chapter 1. Introduction . 1 1.1. Motivation . 1 1.2. Objectives . 2 1.3. Thesis Structure and Organization . 2 Chapter 2. Magnetization Dynamics . 5 2.1. Introduction . 5 2.2. Magnetostatic Energies . 7 2.3. Uniform Precession and Ferromagnetic Resonance . 9 2.4. Spin Waves . 10 2.4.1. Spin Wave Modes. 11 2.4.2. Dispersion Relations . 13 2.5. Conclusions . 18 Chapter 3. Experimental Setup of the Conventional and Micro-Brillouin Light Scattering Apparatus . 19 v 3.1. Introduction . 19 3.2. Light Scattering . 20 3.2.1. Raman Scattering . 21 3.2.2. Brillouin Scattering . 22 3.2.3. Magneto-Optic Kerr Effect. 22 3.3. Brillouin Light Scattering Apparatus . 24 3.3.1. Fabry-Perot Interferometer . 24 3.3.2. Conventional BLS . 29 3.3.3. Micro-BLS. 32 3.4. Conclusions . 36 Chapter 4. Exchange Parameter of FeCo Alloys . 37 4.1. Introduction . 37 4.2. Sample and Experimental Setup . 38 4.3. Experimental Results . 40 4.3.1. External Magnetic Field-Dependent BLS Spectra . 40 4.3.2. Thickness-Dependent BLS Spectra . 41 4.4. Dispersion Relation Fits and Discussion . 42 4.5. Conclusions . 45 Chapter 5. Time-Evolution of Nonlinear Spin Wave Processes in Yttrium Iron Garnet Thin Films. 47 5.1. Introduction . 47 5.1.1. Dispersion Relation and Conservation Laws . 48 5.2. Experimental Setup . 50 vi 5.3. Experimental Results . 53 5.3.1. Network Analyzer Measurements . 53 5.3.2. Power- and Pulse Width-Dependent Measurements. 57 5.3.3. One-Dimensional Scan . 61 5.4. Discussion . 65 5.5. Conclusions . ..
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