Browsing by Author "Patton, Carl, committee member"
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Item Open Access Brillouin light scattering: a powerful tool for magnonics research(Colorado State University. Libraries, 2024) Swyt, Mitchell S., author; Buchanan, Kristen S., advisor; Patton, Carl, committee member; Menoni, Carmen, committee member; Field, Stuart, committee memberThe slow down in generation-over-generation improvement in CMOS based logic and storage devices has spurred recent exploration into magnonic devices, those based on propagating perturbations of magnetic order called magnons, or spin waves. These devices are of particular interest due to their chargeless, low-power operation, scalability to the nanoscale, and compatibility with existing CMOS technologies. By exploiting spin waves, information may be transferred and operated upon without electrical currents. Magnetic textures like Neel domain walls, chiral transitions between magnetic domains, or skyrmions, magnetic vortices, represent additional avenues in magnonics for data storage and logic devices. Magnonic crystals, artificial crystals made by modulating magnetic properties in a periodic fashion, are one example of magnonic devices that have seen recent interest. With applicability in logic and signal processing, study of how spin waves propagate through these crystals is a necessity in the pursuit of new crystal designs. Brillouin light scattering (BLS) spectroscopy, an inelastic light scattering technique, is a powerful tool in this pursuit, as it allows for the spatial and temporal mapping of spin wave propagation. In this thesis, we will discuss three studies of spin waves by BLS: a 1D magnonic crystal, a 2D magnonic crystal, and a study of the interfacial Dzyaloshinskii-Moriya interaction. First, time-resolved BLS was used to study the band gap formation in a 1D magnonic crystal. By mapping the propagation of spin wave pulses through the crystal, complex two dimensional interference patterns were observed. These patterns are ignored by the simple models used to understand the behavior of this crystal design, and we provide a model to calculate these patterns from the spin wave dispersion relation. The temporal development of interference that forms the basis for band gap formation in this system is also observed. Second, time-resolved BLS was used to study spin wave caustic beams in a 2D magnonic crystal. This crystal design represents a new regime in magnonic crystals, in which the patterning dimensions are much smaller than the spin wave wavelength and generate caustic beams. The formation of a narrow (3 MHz) wide rejection band is observed and the possible mechanisms, including edge effects and interference between caustic beams, are explored. Third, the temperature dependence of the interfacial Dzyaloshinskii-Moriya interaction (iDMI) is measured in a Pt/Co film for temperatures ranging from 15 K to room temperature. Previous studies have been reported for temperatures above room temperature and this study serves to test theory over a greater range of temperatures. The iDMI parameter was quantitatively measured by measuring the frequency difference for counter-propagating surface spin waves by BLS. These three studies demonstrate that BLS is a versatile and powerful tool in the field of magnonics.Item Open Access Nanometer-thick yttrium iron garnet film development and spintronics-related study(Colorado State University. Libraries, 2017) Chang, Houchen, author; Wu, Mingzhong, advisor; Celinski, Zbigniew, committee member; Field, Stuart, committee member; Marconi, Mario, committee member; Patton, Carl, committee memberIn the last decade, there has been a considerable interest in using yttrium iron garnet (Y3Fe5O12, YIG) materials for magnetic insulator-based spintronics studies. This interest derives from the fact that YIG materials have very low intrinsic damping. The development of YIG-based spintronics demands YIG films that have a thickness in the nanometer (nm) range and at the same time exhibit low damping similar to single-crystal YIG bulk materials. This dissertation reports comprehensive experimental studies on nm-thick YIG films by magnetron sputtering techniques. Optimization of sputtering control parameters and post-deposition annealing processes are discussed in detail. The feasibility of low-damping YIG nm-thick film growth via sputtering is demonstrated. A 22.3-nm-thick YIG film, for example, shows a Gilbert damping constant of less than 1.0 × 10-4. The demonstration is of great technological significance because sputtering is a thin film growth technique most widely used in industry. The spin Seebeck effect (SSE) refers to the generation of spin voltage in a ferromagnet (FM) due to a temperature gradient. The spin voltage can produce a pure spin current into a normal metal (NM) that is in contact with the FM. Various theoretical models have been proposed to interpret the SSE, although a complete understanding of the effect has not been realized yet. In this dissertation the study of the role of damping on the SSE in YIG thin films is conducted for the first time. With the thin film development method mentioned in the last paragraph, a series of YIG thin films showing very similar structural and static magnetic properties but rather different Gilbert damping values were prepared. A Pt capping layer was grown on each YIG film to probe the strength of the SSE. The experimental data show that the YIG films with a smaller intrinsic Gilbert damping shows a stronger SSE. The majority of the previous studies on YIG spintronics utilized YIG films that were grown on single-crystal Gd3Ga5O12 (GGG) substrates first and then capped with either a thin NM layer or a thin topological insulator (TI) layer. The use of the GGG substrates is crucial in terms of realizing high-quality YIG films, because GGG not only has a crystalline structure almost perfectly matching that of YIG but is also extremely stable at high temperature in oxygen that is the condition needed for YIG crystallization. The feasibility of growing high-quality YIG thin films on Pt thin films is explored in this dissertation. This work is of great significance because it enables the fabrication of sandwich-like NM/YIG/NM or NM/YIG/TI structures. Such tri-layered structures will facilitate various interesting fundamental studies as well as device developments. The demonstration of a magnon-mediated electric current drag phenomenon is presented as an example for such tri-layered structures.Item Open Access Nonlinear spin waves in magnetic thin films - foldover, dispersive shock waves, and spin pumping(Colorado State University. Libraries, 2016) Janantha, Pasdunkorale Arachchige Praveen, author; Wu, Mingzhong, advisor; Eykholt, Richard, committee member; Marconi, Mario, committee member; Patton, Carl, committee memberThree nonlinear phenomena of spin waves and the spin Seebeck effect in yttrium iron garnet (YIG)/Pt bi-layer structures are studied in this thesis and are reported in detail in Chapters 4-7. In the fourth chapter, the first observation of foldover effect of nonlinear eigenmodes in feedback ring systems is reported. The experiments made use of a system that consisted of a YIG thin film strip, which supported the propagation of forward volume spin waves, and a microwave amplifier, which amplified the signal from the output of the YIG strip and then fed it back to the input of the strip. The signal amplitude vs. frequency response in this ring system showed resonant peaks which resulted from ring eigenmodes. With an increase in the resonance amplitude, those resonant peaks evolved from symmetric peaks to asymmetric ones and then folded over to higher frequencies. The experimental observations were reproduced by theoretical calculations that took into account the nonlinearity-produced frequency shift of the traveling spin waves. The fifth chapter presents the first experimental observation of the formation of envelope dispersive shock wave (DSW) excitations from repulsive nonlinear spin waves. The experiments used a microwave step pulse to excite a spin-wave step pulse in a YIG thin film strip, in which the spin-wave amplitude increases rapidly. Under certain conditions, the spin-wave pulse evolved into a DSW excitation that consisted of a train of dark soliton-like dips with both the dip width and depth increasing from the front to the back and was terminated by a black soliton that had an almost zero intensity and a nearly 180 degree phase jump at its center. The sixth chapter reports on the spin pumping due to traveling spin waves. The experiment used a micron-thick YIG strip capped by a nanometer-thick Pt layer. The YIG film was biased by an in-plane magnetic field. The spin waves pumped spin currents into the Pt layer, and the later produced electrical voltages across the length of the Pt strip through the inverse spin Hall effect (ISHE). Several distinct pumping regimes were observed and were interpreted in the frame work of the nonlinear three-wave splitting processes of the spin waves. The seventh chapter presents the first experimental work on the roles of damping in the spin Seebeck effect (SSE). The experiments used YIG/Pt bi-layered structures where the YIG films exhibited very similar structural and static magnetic properties but very different damping. The data indicate that a decrease in the damping of the YIG film gives rise to an increase in the SSE coefficient, and this response shows quasi-linear behavior. The data also indicate that the SSE coefficient shows no notable dependences on the enhanced damping due to spin pumping.Item Open Access Spin currents and ferromagnetic resonance in magnetic thin films(Colorado State University. Libraries, 2017) Ellsworth, David, author; Wu, Mingzhong, advisor; Camley, Robert, committee member; Menoni, Carmen, committee member; Patton, Carl, committee member; Sites, James, committee memberSpin currents represent a new and exciting phenomenon. There is both a wealth of new physics to be discovered and understood, and many appealing devices which may result from this area of research. To fully realize the potential of this discipline it is necessary to develop new methods for realizing spin currents and explore new materials which may be suitable for spin current applications. Spin currents are an inherently dynamic phenomenon involving the transfer of angular momentum within and between different thin films. In order to understand and optimize such devices the dynamics of magnetization must be determined. This dissertation reports on novel approaches for spin current generation utilizing the magnetic insulators yttrium iron garnet (YIG) and M-type barium hexagonal ferrite (BaM). First, the light-induced spin Seebeck effect is reported for the first time in YIG. Additionally, the first measurement of the spin Seebeck effect without an external magnetic field is demonstrated. To accomplish this the self-biased BaM thin films are utilized. Second, a new method for the generation of spin currents is presented: the photo-spin-voltaic effect. In this new phenomenon, a spin current may be generated by photons in a non-magnetic metal that is in close proximity to a magnetic insulator. On exposure to light, there occurs a light induced, spin-dependent excitation of electrons in a few platinum layers near the metal/magnetic insulator interface. This excitation gives rise to a pure spin current which flows in the metal. This new effect is explored in detail and extensive measurements are carried out to confirm the photonic origin of the photo-spin-voltaic effect and exclude competing effects. In addition to the spin current measurements, magnetization dynamics were probed in thin films using ferromagnetic resonance (FMR). In order to determine the optimal material configuration for magnetic recording write heads, FMR measurements were used to perform damping studies on a set of FeCo samples with different numbers of lamination layers. The use of lamination layers has the potential to tune the damping in such films, while leaving the other magnetic properties unchanged. Finally, the sensitivity of the vector network analyzer FMR technique was improved. The use of field modulation and lock-in detection, along with the background subtraction of a Mach-Zehnder microwave interferometer working as a notch filter, is able to increase the sensitivity and lower the background noise of this measurement technique. This improved system opens the possibility of probing previously difficult samples with extremely low signals.Item Open Access Spin waves in magnetic thin films: new types of solitons and electrical control(Colorado State University. Libraries, 2017) Wang, Zihui, author; Wu, Mingzhong, advisor; Patton, Carl, committee member; Eykholt, Richard, committee member; Camley, Robert E., committee member; Marconi, Mario C., committee memberNew types of spin-wave solitons in magnetic thin films and the methods to control spin waves electrically are studied in this thesis. In the first part, the first observation of chaotic spin-wave solitons in yttrium iron garnet (YIG) thin film-based active feedback rings is presented. At some ring gain levels, one observes the self-generation of a single spin-wave soliton pulse in the ring. When the pulse circulates in the ring, its amplitude varies chaotically with time. The excitation of dark spin-wave envelope solitons in YIG thin film strips is also described. The formation of a pair of black solitons with a phase jump of 180° is observed for the first time. The excitation of bright solitons in the case of repulsive nonlinearity is also observed and is reproduced by a numerical simulation based on a high-order nonlinear Schrödinger equation. In the second part, the control of magnetization relaxation in ferromagnetic insulators via interfacial spin scattering is presented. In the experiments nanometer-thick YIG/Pt bi-layered structures are used, with the Pt layer biased by an electric voltage. The bias voltage produces a spin current across the Pt layer thickness due to the spin Hall effect. As this current scatters off the YIG surface, it exerts a torque on the YIG surface spins. This torque can reduce or increase the damping and thereby compress or broaden the ferromagnetic resonance linewidth of the YIG film, depending on the field/current configuration. The control of spin waves in a YIG thin film via interfacial spin scattering is also presented. In the experiments a 4.6-µm-thick YIG film strip with a 20-nm-thick Pt capping layer is used. A DC current pulse is applied to the Pt layer and produced a spin current across the Pt layer. As the spin current scatters off the YIG surface, it can either amplify or attenuate spin-wave pulses that travel in the YIG strip, depending on the current/field configuration.Item Open Access Synthesis and characterization of low-dimensional paramagnetic acetylide complexes(Colorado State University. Libraries, 2011) Hoffert, Wesley A., author; Shores, Matthew P., advisor; Anderson, Oren P., committee member; Prieto, Amy L., committee member; Bailey, Travis, committee member; Patton, Carl, committee memberTo view the abstract, please see the full text of the document.Item Open Access The effects of structural confinement and thermal profiles on propagating spin waves(Colorado State University. Libraries, 2018) Riley, Grant Alston, author; Buchanan, Kristen S., advisor; Neilson, James, committee member; Krueger, David, committee member; Patton, Carl, committee memberSpintronics is a growing field that relies on the spin degree of freedom in the form of spin currents instead of electronic charge to transmit and process information. There are many advantages to spin-based devices including scalability, a wide range of host materials including insulators, and almost no energy loss due to Joule heating. Spin angular momentum can be transmitted in the form of spin-polarized currents that flow through a metal, pure spin currents, or in the form of spin waves, disturbances in the magnetization state that can propagate and hence can carry energy. If such a spin-based paradigm is to be realized, there are many open questions that must be addressed. Two questions of particular importance are: how can short wavelength spin waves that are needed for information transmission be controllably generated? and once generated, how can these spin waves be modified and controlled? This thesis focusses on answering both of these questions through the investigation of spin waves in two different types of samples, patterned microstructures and thin continuous films, performed using Brillouin light scattering (BLS) spectroscopy. In the first experiment, the possibility of generating short wavelength spin waves by dynamically exciting a non-uniform magnetic state called the antivortex (AV) in a Permalloy microstructure is explored. Frequency scans were performed to identify a spectrum of high-frequency modes of the AV state. These modes were then individually mapped out by pumping at the frequency of the mode and performing spatially-resolved BLS scans. Comparing the experimental results with dispersion curves and micromagnetic simulations reveals that some of most prominent modes involve coupling of the AV dynamics to propagating spin waves in the adjacent nanowires highlighting the fact that the local magnetization state has a significant effect on the spin wave dynamics. Due to the natural way that an antivortex state can be incorporated into a nanowire network, this spin configuration may be useful as a means to generate or control spin waves for applications. In the second study we explore the possibility of modifying the propagation characteristics of both spin waves and spin caustic beams, which could be highly useful in spin-wave-based logic devices, using non-uniform thermal gradients up to 4.5 K/mm. These experiments were performed in a yttrium iron garnet (YIG) thin film - a model system for studying spin waves due to extremely low damping characteristics. An intricate diamond-shaped propagation pattern was observed and explained using the dispersion manifold for the YIG film and considering the range of wavevectors excited by the antenna. Significant modifications to the propagation characteristics such as beam angle, temporal pulse shape, mode profiles, and group velocity were observed as spin waves travelled into heated regions. These results will serve to broaden the understanding of how heat can be used to affect and control spin waves.Item Open Access Yttrium iron garnet nano films: epitaxial growth, damping, spin pumping, and magnetic proximity effect(Colorado State University. Libraries, 2014) Sun, Yiyan, author; Wu, Mingzhong, advisor; Patton, Carl, committee member; Field, Stuart, committee member; Reising, Steven, committee member; Celinski, Zbigniew, committee memberRecently, a new research field called magnetic insulator-based spintronics opened the door to a large amount of potential applications in the electronics industry. In this field, low-damping materials in the nanometer scale are critically needed for both fundamental studies, such as spin pumping, and device applications, such as spin-torque nano-oscillators. Yttrium iron garnet (YIG) materials are the best candidate among other materials. There is a critical demand for high-quality nanometer-thick YIG films. This dissertation reports experimental studies on YIG films with the thickness ranged from several nanometers to several hundreds of nanometers. Firstly, the feasibility of low-damping YIG nano films growth via pulsed laser deposition (PLD) techniques is demonstrated. A 5-nm-thick YIG film, for example, shows a peak-to-peak ferromagnetic resonance (FMR) linewidth of <10 Oe at 10 GHz. Optimization of PLD control parameters and post-deposition annealing processes and surface modification by ion beam etching for the realization of high-quality films are discussed in detail. The second main topic is on spin pumping and magnetic proximity effects in YIG nano films. Specifically, the dissertation touches on (1) the spin pumping efficiency of YIG nano films and (2) damping enhancement in YIG nano films due to Pt capping layers. Knowing the efficiency of spin angular momentum transfers across YIG/normal metal (NM) interfaces is critical to the use of YIG films for spintronics. Under subtopic (1), the spin transfer efficiency at YIG/NM interfaces is determined through the measurement of spin pumping-caused additional damping in YIG nano films. A fairly large portion of recent studies on YIG-based spintronics made use of a Pt capping layer either as a detector to measure spin currents or as a spin-current source. Work under subtopic (2), however, indicates that the growth of a Pt capping layer onto a YIG film can result in a significant damping enhancement in the YIG film. Fortunately, this damping can be completely suppressed simply by the addition of a thin Cu spacer in-between the YIG and Pt films. The interpretation of the observed damping enhancement in terms of the magnetic proximity effect in the Pt film is presented. The last topic addresses the growth of high-quality YIG thin films on metallic substrates. It is demonstrated that one can grow YIG thin films on Cu via the use of a protection layer of high entropy alloy nitrides. The YIG films showed a peak-to-peak FMR linewidth of about 1.1 Oe at 9.45 GHz. This work provides implications for the future development of YIG thin film-based monolithic devices for high frequency processing.