Browsing by Author "Wu, Mingzhong, advisor"

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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 member
Yttrium 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.
Open Access
Damping mechanisms in magnetic recording materials & microwave-assisted magnetization reversal
(Colorado State University. Libraries, 2014) Lu, Lei, author; Wu, Mingzhong, advisor; Gelfand, Martin P., committee member; Kabos, Pavel, committee member; Marconi, Mario C., committee member; Patton, Carl E., committee member
Understanding the damping of magnetization precession in magnetic recording materials is of both fundamental and practical significance. From the practical perspective, the relaxation processes not only set a natural limit to the time of magnetization switching which determines recording data rates, but also play critical roles in advanced magnetic recording techniques such as microwave-assisted magnetic recording and two-dimensional magnetic recording. Experimental and theoretical studies of magnon-electron scattering and two-magnon scattering (TMS) contributions to magnetization relaxations in magnetic recording head and media materials were conducted for the first time in this dissertation. The accuracy of ferromagnetic resonance (FMR) measurements was increased by the use of vector network analyzer (VNA) FMR techniques. Working equations of the grain-to-grain TMS and grain boundary TMS processes were developed based on the TMS models of Krivosik and Mo, and were applied to understand the relaxation mechanisms in various recording-related thin film materials. The dependences of the FMR behavior and relaxation rates on the external field orientation, the microwave frequency, and the temperature were investigated experimentally in the following three domains: the exchange-coupled composite media, the free layers of tunnel magneto-resistance readers, and FeCo alloy films for future writers. The theoretical models were used to analyze the experimental data and to understand the relaxation mechanisms. Microwave-assisted magnetization reversal (MAMR) is considered as a promising mechanism for further increasing the recording area density and pushing it beyond the super-paramagnetic limit. The MAMR operation was demonstrated with a 700-Gbit/in2 perpendicular media sample in this thesis study. For microwaves with frequencies close to the FMR frequency of the media, MAMR was observed for microwave power higher than a certain threshold. For microwaves with certain high power, MAMR was observed for a broad microwave frequency range which covers the FMR frequency and is centered below the FMR frequency.
Open Access
Fabrication and analysis of vanadium oxides and vanadium oxide based magnetic hybrid structures
(Colorado State University. Libraries, 2021) Sutton, Logan, author; Wu, Mingzhong, advisor; de la Venta, Jose, committee member; Ross, Kathryn, committee member; Prieto, Amy, committee member
Vanadium oxide films and vanadium oxide-based magnetic hybrid structures are fabricated using various techniques and studied optically, electrically, structurally, and magnetically for their potential applications into magnetic recording, room temperature refrigeration, and optical switches. The different types of behavior seen in the transitions of the vanadium oxide class of compounds can be altered and optimized according to desirable qualities for these applications. Several different techniques were used for the fabrication of vanadium oxide-ferromagnetic (FM) composites with the goal of causing magnetic coupling and the optimization of coupling between the vanadium oxide compound and the FM compound. The ball milling process was used as the primary step in formation of the composites, but was shown to be ineffective at causing coupling between the compounds if used alone. The addition of a sintering process was shown to successfully couple V2O3 and Ni, with an optimization of the process determined to be primarily dependent on temperature. Optimized composites showed up to 56% changes in coercivity at the transition temperature of the V2O3. VO2 based composites were unable to be coupled due to problems with the reduction and oxidation of the compounds involved, and a lack of diffusion. A sol-gel technique for the fabrication of VO2 layers was optimized for large transitional properties and refined for reproducibility. Magnetic hybrid structures formed from the sol-gel fabricated films were shown to have comparable properties to their sputtered counterparts. W doped films fabricated using the sol-gel technique, when compared to doping using a sputtering technique, were demonstrated to allow for larger control over the ideal doping range. Doping was shown to have negligible effect on the morphology of the films, but produced several W based impurities. Although doping produced expected shifts and decreases in the transitional electrical transport properties, there were also unexplained shifts in the absolute resistance for higher doping. Magnetic hybrid structures based on doped films still produced large changes in the magnetic properties of the FM layer, but these changes were shifted to lower temperatures and reduced. Transmission and reflection of VO2 films fabricated using different techniques were shown to have different qualitative and quantitative behaviors at different optical wavelengths of incidence. Most films were shown to have downward switching in both the transmission and reflection at the transition, however thinner films sometimes showed upward switching in the transmission. Downward bumps caused by interference were seen in the reflection at 980 nm, as well as at 635 nm for two other films. The model that was developed to try to reproduce this behavior is successful for 60% of the films, and able to reproduce all of the qualitative behaviors described. However the trends in the fitted refractive index do not help elucidate what physical mechanism is responsible for the differences seen between samples.
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 member
In 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.
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 member
Three 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.
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 member
Spin 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.
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 member
New 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.
Open Access
Studies of magnetization dynamics in magnetic recording media and patterned yttrium iron garnet films
(Colorado State University. Libraries, 2018) Richardson, Daniel, author; Wu, Mingzhong, advisor; de la Venta Granda, Jose, committee member; Kabos, Pavel, committee member; Krueger, David, committee member; Marconi, Mario, committee member
Exchange coupling and damping are studied in magnetic media materials for applications in current perpendicular magnetic recording (PMR) technology as well as future heat assisted magnetic recording (HAMR) media technology. Damping and exchange coupling are directly related to magnetization switching time in writing operation and the signal-to-noise ratio in reading, both critical to the performance of hard disk drives. Intergranular exchange is studied in current PMR media to see how exchange is altered in the presence of SiO2 based segregant. By varying the segregant by as much as 30%, there is strong tunability of the exchange field between the grains. The damping in future FePt-based HAMR media is studied near the curie temperature (725 K) of FePt where the writing stage in the recording media takes place. The trends of ferromagnetic resonance (FMR) linewidth varying with the sample temperature, the volume fraction of carbon in the media, and the angle of the external field indicate that the overall damping includes strong contributions from intrinsic magnon-electron scattering as well as extrinsic two-magnon scattering between the grains. Interlayer exchange coupling and damping were studied in magnetic layered systems consisting of a soft ferromagnetic transition metal or alloy layer and a hard FePt layer at room and elevated temperatures. It was found that exchange coupling and damping are strongly dependent on temperature, the soft layer thickness, and the choice of material of the soft layer. Spin waves are studied in the linear and non-linear regimes using magnonic crystals consisting of yttrium iron garnet (YIG) thin film strips with periodic etched lines or periodic metallic lines deposited on top of the YIG strip, as well as YIG strips with randomly spaced metallic lines deposited on top. The various media provide ways of controlling the dispersion by altering the interference of the spin waves, allowing for a wide range of interesting phenomenon to be observed. Spin-wave fractals are observed for the first time in a YIG strip with periodic etched lines. The etched lines serve as position dependent potentials to increase dispersion in the YIG strip large enough for fractal formation in the nonlinear regime. This is also the first time fractals of any type that have been observed without the formation of time-domain solitons. Spin-wave localization is observed in the linear regime for the first time in YIG strips with randomly spaced metallic lines where the metallic lines serve as potential barriers for causing spin wave interference. Magnonic crystals consisting of YIG strips with periodically spaced metallic lines are used to compare a standing wave state with the localized state. The localized state is much stronger and much more confined to a smaller physical space than the standing wave state.
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 member
Recently, 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.