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Browsing by Author "Buchanan, Kristen S., advisor"

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    Brillouin light scattering spectroscopy of phonons, magnons, and magnetoelastic waves
    (Colorado State University. Libraries, 2022) Nygren, Katherine Elise, author; Buchanan, Kristen S., advisor; Field, Stuart, committee member; Brewer, Samuel, committee member; Shores, Matthew, committee member
    This thesis discusses three projects that involve the propagation of waves through the utilization of an optical measurement technique known as Brillouin light scattering (BLS) spectroscopy. BLS spectroscopy measurements were completed using a six pass tandem Fabry-Pérot interferometer to detect light that has inelastically scattered from vibrational, spin, or magnetoelastic waves in a sample. This measurement method is noncontact, so wires do not need to be connected to the sample, nondamaging (unless the laser power is too high, and only for sensitive samples), and can detect nonlinear signals. The first project uses an antenna called an interdigital transducer to produce a surface acoustic wave. This wave travels across a piezoelectric substrate and couples to a spin wave in a nickel film. The coupled wave known as a magnetoelastic wave is then studied using BLS as a function of the external applied magnetic field. These results are used to help us understand how the magnetic resonance contributes to the coupled wave. Further BLS measurements as a function of distance across the nickel film are used to calculate a decay length of the magnetoelastic wave two orders of magnitude larger than the decay length for a pure spin wave in nickel. Second, we explore a device using a thin film of an organic ferrimagnet called vanadium tetracyanoethylene (VTCNE) that is magnetic at room temperature and has low damping, which rivals damping in high quality YIG films commonly used in microwave applications. Because VTCNE is oxygen sensitive it is encapsulated between two pieces of glass using an epoxy. The encapsulation does not change the damping, however due to magnetostriction, the strain of the epoxy may change the magnetic properties of the film. To understand how the epoxy strain can effect this device and others with similar encapsulation, we study thermal phonons in the encapsulation materials using Brillouin light scattering. The thermal phonon measurements along with phonon simulations allow us to calculate both the wave speeds and the elastic properties of the materials. These calculated properties can then be used to model future VTCNE devices. The final major project uses BLS spectroscopy to study spin waves in a Y-shaped structure of an iron nickel alloy. Using an in-plane externally applied magnetic field and an antenna across the top of the Y, we excite magnons in each arm of the Y, which then propagate into the base of the Y. BLS measurements are taken in each arm and the base of the Y, as a function of the driving frequency, and a 2D spatial map of the spin waves in the Y-structure was obtained to gain additional information on the modes that propagate past the junction of the Y. The BLS data in conjunction with simulations, demonstrate an indirect way to efficiently excite Damon-Eshbach spin waves as well as convert low wavevector spin waves in the arms of the Y into higher wavevector spin waves as they propagate into the base of the Y. The wavevector conversion and more efficient method of generating Damon-Eshbach spin waves are tools that can be exploited in magnonic device designs. Three additional spin wave projects are also discussed briefly. The projects include a yttrium iron garnet (YIG) confined structure, a VO2 film with a metal-insulator-transition near room temperature, and a heavy metal-ferrimagnet-heavy metal sample that should have a strong interfacial Dzyaloshinskii-Moriya interaction.
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    Brillouin light scattering study of linear and nonlinear spin waves in continuous and patterned magnetic thin films
    (Colorado State University. Libraries, 2014) Liu, Hau-Jian Jason, author; Buchanan, Kristen S., advisor; Gelfand, Martin P., committee member; Kabos, Pavel, committee member; Neilson, James R., committee member
    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 experiments 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 measured 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 determined 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 measured 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 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.
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    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 member
    The 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.
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    Investigation of the dynamics of magnetic vortices and antivortices using micromagnetic simulations
    (Colorado State University. Libraries, 2017) Asmat-Uceda, Martin Antonio, author; Buchanan, Kristen S., advisor; Gelfand, Martin P., committee member; Wu, Mingzhong, committee member; Shores, Matthew, committee member
    This thesis is focused on investigating the dynamic properties of spin textures in patterned magnetic structures by using micromagnetic simulations. These textures become particularly relevant at sub-micron length scales where the interplay between magnetostatic and exchange energy leads to unique properties that are of great interest both from a fundamental perspective and for the development of new technologies. Two different systems, a magnetic antivortex (AV) stabilized in the intersection of perpendicular microwires, and three interacting vortices in an equilateral arrangement, were considered for this study. For the first system, the AV, the formation process and the excitation spectra were investigated. Since the AV is a metastable state, the design of a host structure capable of stabilizing it requires careful consideration and it is desirable to have general guidelines that could help to optimize the AV formation rate. The role of the shape anisotropy and the field dependence of the AV formation process is discussed in detail. Micromagnetic simulations along with magneto-optical Kerr effect and magnetic force microscopy measurements demonstrated that the asymmetry in the structure can be used to promote the formation of such AV's and that regions with lower shape anisotropy lead the reversal process, while simulations of the dynamic response show that when the system is excited with in-plane and out-of-plane external magnetic fields, normal modes with azimuthal and radial characteristics are found, respectively, in addition to the low frequency gyrotropic mode. The modes are influenced by the spin texture in the intersection, which offers additional possibilities for manipulating spin waves (SW). For the second system, three interacting vortices are simulated and compared with a simple analytical model that considers only dipolar interactions. It was found that when a fitting parameter is introduced to the model, the main features of the simulations are captured better than more complex models, which suggest that this simple framework can be used to accurately model more complex vortex networks.
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    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 member
    Spintronics 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.