Browsing by Author "Field, Stuart, committee member"
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Item Open Access 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 memberThis 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.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 Damping and switching in thin films and hetero-structures of magnetic materials and topological materials(Colorado State University. Libraries, 2020) Ding, Jinjun, author; Wu, Mingzhong, advisor; Camley, Robert, committee member; Field, Stuart, committee member; Roberts, Jacob, committee member; Shores, Matthew, committee memberYttrium iron garnet (Y3Fe5O12, YIG) materials have been widely used in microwave devices and have also shown high potential for magnonics and spintronics applications. This is because the fact that YIG materials have very low intrinsic damping and is electric insulating. 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 comparable to single-crystal YIG bulk materials. In this dissertation, the demonstration of using magnetron sputtering to grow high-quality polycrystalline nm-thick YIG films on gadolinium gallium garnet (Gd3Ga5O12, GGG) substrates is discussed in detail, which is of great technological significance as well as scientific research. The damping constant of the YIG films is the lowest among all the previous reports of nm-thick YIG films grown. Such demonstration of high-quality nm-thick YIG films proves the possibility of nanoscale patterning of YIG films and the future development of YIG-based nanoscale devices. Further, YIG thin films having a thickness of several nanometers and showing both strong perpendicular magnetic anisotropy (PMA) and low magnetic damping are realized in this dissertation. The phenomenon of spin pumping refers to the transfer of spins from precessional moments in a ferromagnet to a non-magnetic material. In a ferromagnetic/non-magnetic bi-layered system, spin pumping manifests itself as two distinct effects: (1) an enhancement in the damping in the ferromagnetic layer and (2) a pure spin current in the non-magnetic layer. This dissertation studies spin pumping effects in a ferromagnetic NiFe thin film associated with topological surface states (TSS) in a neighboring topological Dirac semimetal α-Sn thin film. Large damping enhancement due to the TSS of the Dirac semimetal alpha-Sn thin film is observed. Moreover, the spin current generated in the alpha-Sn film was utilized to switch a magnet through spin-orbit torque (SOT). The switching efficiency is comparable to that in topological insulators, which paves the way for the application of alpha-Sn thin films in future SOT-based magnetic memory. When a topological insulator (TI) is interfaced with a magnetic insulator (MI), it may host the anomalous Hall effect (AHE) and the quantum AHE associated with Berry-phase curvature in momentum space. This dissertation reports a bona fide topological Hall effect (THE) in a single magnetic phase TI/MI heterostructure (Bi2Se3/BaFe12O19) where the electrical transport is exclusively confined to the TI layer. Experimental observations are consistent with a THE originating from skyrmions in BaFe12O19 that are formed due to interfacial Dzyaloshinskii–Moriya interaction.Item Open Access Measuring dissolution rates and interfacial energetics of monolayer molybdenum disulfide electrodes in electrochemical systems(Colorado State University. Libraries, 2023) Toole, Justin R., author; Sambur, Justin, advisor; Henry, Chuck, committee member; Ackerson, Chris, committee member; Field, Stuart, committee memberMeeting carbon zero goals within the next few decades requires advances in energy conversion efficiency, and hydrogen fuel is believed to be a key part of the solution. Photoelectrochemical (PEC) devices can contribute to a renewable-based energy portfolio by directly producing storable chemical fuels. The electrode is a key component that determines what is thermodynamically and kinetically possible for a given PEC device. Unfortunately, semiconductor electrode efficiency can come at the cost of chemical stability. Also, the energetic description of an ultra-thin semiconductor electrode at the liquid interface is unclear. Here, we studied molybdenum disulfide (MoS2), a promising two-dimensional (2D) semiconductor, to improve understanding of interfacial energetics and electron transfer. The overarching hypothesis of this work is: if we quantitatively measure band energies of this 2D material, then we improve understanding of electron transfer efficiency and rates for involved chemical reactions. Knowledge from this research informs new ways to reduce solar energy conversion losses and may improve control over chemical reactions. Our experimental approach is to make in situ optical measurements while changing two key variables: (1) the electrode applied voltage (E), and (2) the liquid redox electrolyte environment (E0'). This thesis is organized into six chapters. Chapter 1 motivates semiconductor photoelectrochemistry as a viable approach for solar energy and chemical fuel production. Following the chronology of key scientific advances over the past few decades, Chapter 2 delves deeper into the established principles of semiconductor photoelectrochemistry, the unique properties of monolayer MoS2, and the current state of the field for making in situ optical measurements in an electrochemical cell. This chapter concludes with open questions that are addressed in Chapters 3 – 5. In Chapter 3, the stability of MoS2 is tested by literally pushing the semiconductor to its anodic decomposition limit. The crucial results are identification of the MoS2 dissolution onset potential (ED) and its thickness-dependent dissolution rates. Additional insights pertain to the long-term stability differences between monolayer and multilayer material. Chapter 4 includes the most noteworthy results wherein we develop a method to quantitatively measure the electronic band gap of monolayer MoS2 using a relatively simple optical setup. For the first time, we use an all-optical approach and many-body theory to report an abrupt change in potential-dependent band gap energies of monolayer MoS2 under electrochemical conditions. Chapter 5 summarizes preliminary work investigating how redox couples in the electrolyte may tune the optical signature of a monolayer MoS2 electrode. Finally, Chapter 6 concludes the thesis with suggestions for subsequent investigations available based on the expertise and resources within the Sambur group at Colorado State University.Item Open Access Modeling and controlling nanoscale patterns formed by bombardment with a broad ion beam(Colorado State University. Libraries, 2017) Harrison, Matthew Paul, author; Bradley, R. Mark, advisor; Gelfand, Martin, committee member; Shipman, Patrick, committee member; Field, Stuart, committee memberFor over half a century it has been known that bombarding a solid surface with a broad ion beam can produce periodic nanoscale structures. Given the virtually limitless promise of nanotechnology, the potential of ion bombardment to produce nanopatterned surfaces over large areas in a simple and economical way has attracted substantial interest. In the decades since its discovery, there has been a wealth of experimental and theoretical work examining the phenomenon in detail, with the eventual goal of using ion beam sputtering (IBS) to produce useful nanostructures. Despite the body of work, there are many open questions and unsurmounted challenges remain- ing. In this thesis, I present work that I have conducted in collaboration with my advisor, Mark Bradley, with whom I addressed some of these challenges. I show how we developed a formalism which connects information about single ion impacts to the evolution of a surface which sustains > 1016 such impacts per square centimeter. We have also produced theoretical results for the case of a binary material being bombarded while rotated azimuthally, with some unexpected findings. I also discuss some very exciting theoretical predictions for the case in which an elemental target is bombarded while the polar angle of ion incidence periodically changes. In this case we find the temporal driving can induce a surface pattern which is nearly perfectly periodic in the long time limit. I also discuss our work on using templated surfaces in conjunction with IBS to produce ii high quality blazed gratings and multilayer blazed gratings. This work is the subject of a current collaboration with Carmen Menoni and her students.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 Studies of tuning magnetic properties of ferromagnetic heterostructures(Colorado State University. Libraries, 2020) Lauzier, Joshua, author; de la Venta Granda, Jose, advisor; Buchanan, Kristen, committee member; Gelfand, Martin, committee member; Field, Stuart, committee member; Menoni, Carmen, committee memberThe magnetic properties of hybrid systems have increasingly become an area of intense focus in both fundamental research and technological application due to the inherent flexibility in material properties by mixing and matching various constituent components. One particularly interesting choice is hybrid heterostructures that consist of ferromagnetic (FM) materials and materials that undergo phase transitions, coupled via structural, electronic, and/or magnetic coupling. Two canonical examples of phase transition materials are vanadium dioxide (VO2) and iron rhodium (Fe50Rh50, abbreviated FeRh). Both materials undergo structural phase transitions (SPT). With increasing temperature, VO2 transitions from a low temperature monoclinic to high temperature rutile structure at 340 K. The SPT is concurrent with a 4-5 orders of magnitude metal to insulator transition (MIT) from a low temperature insulating phase to a high temperature metallic phase. Similarly, FeRh undergoes an isotropic 1% volume expansion at 370 K with increasing temperature. Coincident with the SPT, FeRh also undergoes a magnetic transition from a low temperature antiferromagnetic (AF) to a high temperature ferromagnetic (FM) phase, which is unusual for magnetic materials. The delicate nature of these transitions makes them sensitive to parameters such as stoichiometry, growth conditions, and external stimuli, which allows for high tunability of their respective phase transitions. In this thesis, we first show in Chapter 3 that the surface morphology and MIT properties of sputtered VO2 thin films can be tuned via deposition conditions such as deposition temperature and O2 flow rate during the sputtering process while maintaining the quality of the VO2 transition. Films grown at higher temperatures (>525 ℃) and low O2 flow rate show sub 2 nm surface roughness. Higher temperatures lead to a 'melted'-like surface morphology along with a 5 orders of magnitude MIT, comparable to single crystals. Choice of substrate allows another avenue to strongly tune both the morphology and the MIT characteristics while maintaining a strong VO2 transition due to lattice mismatch. In Chapter 4, we turn to a discussion of VO2/Ni bilayer structures, where the temperature induced VO2 SPT will impart a strain across the interface into the FM layer, which will then influence the magnetic properties via magnetoelastic coupling. Due to an inverse magnetostrictive effect the coercivity and magnetization of the FM layer can be strongly modified. Tuning the VO2 SPT via growth conditions or substrate choice then allows for tuning the coupled magnetic properties of the FM. For sufficiently smooth films, there is a strong enhancement in the coercivity localized close to their respective SPT Tc due to phase coexistence in the SPT material. This chapter is largely based on work previously published as "Coercivity enhancement in VO2/Ni bilayers due to interfacial stress" in Journal of Applied Physics.1 VO2/FM hybrid films also show a dependence on the growth conditions during the FM deposition, which is explored in Chapter 5. Films with the FM deposited above the VO2 phase transition critical temperature (Tc) show a high coercivity below Tc and a low coercivity above Tc, whereas films deposited below Tc show the opposite behavior. Films deposited below Tc also show an irreversibility in their magnetic properties the first time they are thermally cycled. A similar irreversibility is observed in the resistance vs. temperature (R vs. T) properties of bare VO2 films, and cracking as the VO2 crosses the SPT is proposed as a common mechanism. The plausibility of cracking as a mechanism is investigated via computational modeling of the R vs. T properties in a random resistor network, as well as probed directly via Atomic Force Microscopy (AFM). The work shown in this chapter has been previously published under the title "Magnetic irreversibility in VO2/Ni bilayers" in Journal of Physics: Condensed Matter.2 Sputtered FeRh/FM bilayer films show a similar sensitivity as the VO2/FM system to the growth conditions, with the coercivity below Tc tunable whether the FM is initially deposited above or below Tc. Above Tc, the magnetic FeRh phase adds an additional complication, dominating the magnetic response via exchange coupling. This effect is explored in FeRh/Ni bilayer systems in Chapter 6. Polarized neutron reflectometry (PNR) allows for depth dependent structural and magnetic characterization with nanometer resolution. PNR measurements show that the bilayer's magnetic behavior below Tc is likely driven by magnetoelastic effects due to the structural transition of the FeRh, rather than simple magnetic coupling or a pinned interfacial FM layer. The overall magnetic properties of the bilayers are therefore a product of both structural and magnetic coupling between the FeRh and the FM Ni layer. The results of this chapter have been previously published as "Using structural phase transitions to enhance the coercivity of ferromagnetic films" in Applied Physics Letters Materials.Item Open Access Tuning optoelectronic properties and understanding charge transport in nanocrystal thin films of earth abundant semiconducting materials(Colorado State University. Libraries, 2011) Riha, Shannon C., author; Parkinson, Bruce A., advisor; Prieto, Amy L., advisor; Elliott, C. Michael, committee member; Field, Stuart, committee member; Henry, Charles, committee member; Rappe, Anthony, committee memberWith the capability of producing nearly 600 TW annually, solar power is one renewable energy source with the potential to meet a large fraction of the world's burgeoning energy demand. To make solar technology cost-competitive with carbon-based fuels, cheaper devices need to be realized. Solution-processed solar cells from nanocrystal inks of earth abundant materials satisfy this requirement. Nonetheless, a major hurdle in commercializing such devices is poor charge transport through nanocrystal thin films. The efficiency of charge transport through nanocrystal thin films is strongly dependent on the quality of the nanocrystals, as well as their optoelectronic properties. Therefore, the first part of this dissertation is focused on synthesizing high quality nanocrystals of Cu2ZnSnS4, a promising earth abundant photovoltaic absorber material. The optoelectronic properties of the nanocrystals were tuned by altering the copper to zinc ratio, as well as by introducing selenium to create Cu2ZnSn(S1-xSex)4 solid solutions. Photoelectrochemical characterization was used to test the Cu2ZnSnS4 and Cu2ZnSn(S1-xSex)4 nanocrystal thin films. The results identify minority carrier diffusion and recombination via the redox shuttle as the major loss mechanisms hindering efficient charge transport through the nanocrystal thin films. One way to solve this issue is to sinter the nanocrystals together, creating large grains for efficient charge transport. Although this may be quick and effective, it can lead to the formation of structural defects, among other issues. To this end, using a different copper-based material, namely Cu2Se, and simple surface chemistry treatments, an alternative route to enhance charge transport through nanocrystals thin films is proposed.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.