The rise of neuromorphic computing and iteration of new memory call for energy-efficient magnetization manipulation by spin and a better understanding of fundamental magnetotransport properties. Over the past decades, most works on the spin source focused on polycrystalline and amorphous materials. In the first part of this dissertation, we investigate the spin Hall effect in epitaxial Pt (200), (220), and (111). A tunable spin current generation can be achieved in Pt (220). The charge-to-spin conversion efficiency can be significantly enhanced along specific crystallographic orientation by utilizing the anisotropic resistivity Our work highlights the future work on enhanced SHE arising from crystalline orientation dependence which will boost the energy-efficient spintronic devices.

On the other hand, tunable charge-to-spin conversion can be achieved by utilizing the materials with magnetic phase transition, which motivated us to study the 3d transition metal-Pt3 alloy and the rare earth material (Ho). Prior to further spintronics study, the fundamental magnetotransport properties in those materials need to be investigated. We successfully fabricate the chemically ordered epitaxial CrPt3 and FePt3. The large anomalous Hall effect measured on CrPt3 is in good agreement with the theoretical prediction. Moreover, by probing the exchange bias via electrical transport, it helps us quantitatively investigate the stability of antiferromagnetic nature in FePt3 under external perturbations.

Furthermore, there are still intriguing properties remaining in "old" materials which lead to the magnetotransport study on hcp Ho and hcp Co. Our discovery of six-fold angular dependent magnetoresistance and step-like planar Hall effect in Ho gives rise to the strong magnetostriction coupled magnetoresistance. Remarkably, although predicted years ago, the strong c-axis dependent anomalous Hall conductivity in hcp Co lacks experimental support, which is demonstrated in this dissertation. The magnetotransport study on hcp Co offers a comprehensive picture of anisotropic anomalous Hall effect and magnetoresistance in ferromagnets with in-plane uniaxial magnetic anisotropy.

In the field of magnetics, there are many devices that exhibit either behavior in the microwave regime or response to stimuli from microwave sources. The ultimate goal of this work is to study these devices and characterize these behaviors. In support of this goal, a myriad tasks are required including and not limited to device fabrication, test fixture fabrication and machining, characterization and testing software programming, and automated data processing. Specialized RF equipment will be utilized to measure high frequency signals including high frequency fixtures, probes, RF cables, vector network analyzers, and spectrum analyzers. The results of these characterization measurements include the finding of high frequency oscillation modes in perpendicular spin valve nanopillars, the discovery of a magnetically tunable ring oscillators, characterization of spin torque nano oscillators in in-plane magnetic tunnel junction devices, a study of a cross coupled spin torque nano oscillator, and device characterization using temperature dependent ferromagnetic resonance.

Patterned magnetic nano-structures are under extensive research due to their interesting emergent physics and promising applications in high-density magnetic data storage, through magnetic logic to bio-magnetic functionality. Bit-patterned media is an example of such structures which is a leading candidate to reach magnetic densities which cannot be achieved by conventional magnetic media.

Patterned arrays of complex heterostructures such as exchange-coupled composites are studied in this thesis as a potential for next generation of magnetic recording media. Exchange-coupled composites have shown new functionality and performance advantages in magnetic recording and bit patterned media provide unique capability to implement such architectures. Due to unique resonant properties of such structures, their possible application in spin transfer torque memory and microwave assisted switching is also studied.

This dissertation is divided into seven chapters. The first chapter covers the history of magnetic recording, the need to increase magnetic storage density, and the challenges in the field. The second chapter introduces basic concepts of magnetism. The third chapter explains the fabrication methods for thin films and various lithographic techniques that were used to pattern the devices under study for this thesis. The fourth chapter introduces the exchanged coupled system with the structure of [Co/Pd] / Fe / [Co/Pd], where the thickness of Fe is varied, and presents the magnetic properties of such structures using conventional magnetometers. The fifth chapter goes beyond what is learned in the fourth chapter and utilizes polarized neutron reflectometry to study the vertical exchange coupling and reversal mechanism in patterned structures with such structure. The sixth chapter explores the dynamic properties of the patterned samples, and their reversal mechanism under microwave field. The final chapter summarizes the results and describes the prospects for future applications of these structures.

Control and manipulation of magnetic textures is promising for the development of next-generation data storage, memory and processing technologies. Towards this goal, domain wall manipulation in two materials systems are presented here and thoroughly evaluated. Domain walls in ferrimagnetic Cobalt-Terbium alloys and multilayers are created, moved and stabilized via thermal gradients and a static magnetic field and exploit the unique properties of the system across the magnetic compensation point. The response of the systems to thermal gradients is observed via Kerr microscopy and used to determine the positioning of domain walls within patterned devices. Magnetic skyrmions are discovered in thin-film multilayered stacks using an Pt/Co/Os/Pt heterostructures where the thin Osmium layer is used to break interfacial symmetry and enhance the Dzyaloshinskii-Moriya interaction. The resulting skyrmions are manipulated using temperature, magnetic field, and electric current, and special attention is paid to their motion and nucleation behavior. Skyrmions are observed to be formed by low applied currents from nucleation sites and by collapse of stripe textures. Patterned wires allow for the observation of skyrmion nucleation behavior in free space, as well as defect sites, and real-time Kerr microscopy imaging is presented of skyrmion and stripe dynamics. These systems are evaluated from a perspective of their growth, patterning, measurement, and the novel behavior of the magnetic textures.

This dissertation explores the nanofabrication and characterization of anisotropic metallic nanomaterials. Employing both bottom-up and top-down fabrication techniques, the chemistry and crystal structure of metals were exploited to create an assortment of anisotropic nanomaterials ranging from gold nanocrystals to nickel nanowires to CoFeB/MgO thin films. We investigate the role of anisotropy on the physical properties in these nanomaterials. The anisotropy arises from the combined effects of the shape, crystal structure and symmetry breaking at interfaces. The effect of anisotropy on properties such as strain, magnetic anisotropy and micromagnetic configuration were investigated. A variety of characterization techniques ranging from imaging, simulations and magneto-transport measurements were employed. We first study the strain distribution within anisotropic nanocrystals associated with its shape. We further study the interplay between the nanocrystals and the substrate on the strain distribution. We then study the relation between the strain, magneto-crystalline anisotropy and shape on the magnetic response of Ni nanostructures, including magnetic vortex configurations in Ni nano-cube and nano-wires. This interplay leads to complex domain patterns and could enable novel bio-magnetic applications and magnetic-superconducting devices. Lastly we explore ion-irradiation as a tool to modify the interfaces of ultra thin films with which we can tailor the magnetic anisotropy.

The first part of this dissertation mainly focus on thin film solid oxide fuel cells. In recent years, solid oxide fuel cells are becoming more popular as a green energy source of electricity due to their high efficiency and low carbon emission. However, high operation temperature and materials costs are the main issues impeding the application of this technique. In this presentation, by applying magnetron sputtering technique to fabricate thin film solid oxide fuel cell, we are able to operate the cells under much lower temperature with superior performance under hydrogen or hydrocarbon fuels. Different composite nano-porous cathodes and anodes are fabricated and characterized, all showing reliable, reproducible and scalable results. The second part of the dissertation focus on fabrication and characterizations of spintronic devices, including Si-doped single crystalline Ni nanowires, spin-triplet spintronic and SOT-MRAM devices. Spintronics has been widely applied in data storage applications such as hard disk drives, STT-MRAMs, and SOT-MRAMs, etc. Recently, development of spintronics theories and experiments has allowed it to expand to broader applications, such as domain wall MRAMs, neuromorphic computing and quantum computing. In this section, we will introduce the growth of Si-doped single crystalline Ni nanowires, the effects from AMR, OMR and domain wall MR at low magnetic fields and electron-magnon interaction at high magnetic fields. We studied superconducting triplet spin valve devices. In general, singlet Cooper pairs generated from superconducting layers cannot survive in magnetic layer. However, if we introduce an inhomogeneous moments between these two layers, triplets Cooper pair appeared and can survive much longer than singlet Cooper pair in ferromagnetic materials. In our study, we fabricated epitaxial Ni and Co superconducting spin-triplet spin valves to study this effect. By rotating the soft NiFe layer to break the linearity of magnetizations, inhomogeneous moments can be generated. Magnetization configurations are characterized by polarized neutron reflectometer. By measuring the change of superconducting critical temperature at different angles of external field, we observed that the behavior of spin valve is highly sensitive to the thickness of the inhomogeneous magnetic layers. Lastly, the SOT-MRAM project during the internship in IBM is presented. SOT-MRAM is a potential second generation MRAM compared to STT-MRAM due to its higher efficiency, stability and lower switching current. We fabricated single SOT-MRAM device using $\beta$-W as spin current source. Ion beam etching process, the most critical fabrication process, has been studied. Shapes and aspect ratio effects on nanopillars are discussed. We also introduced thin Hf interlayers in the heterostructure to further decrease the critical switching current. Simulations proves an enhancement in perpendicular anisotropy will lead to this reduction in critical switching current.

In this dissertation, we investigate the stabilization and dynamics of chiral domain walls and skyrmions in several ferromagnet/heavy metal thin-film multilayers. In each material system, the ferromagnet/heavy metal interfaces were engineered to give rise to an interfacial Dzyaloshinskii-Moriya interaction (iDMI) – a spin-orbit coupling mediated effect known to promote the formation of chiral magnetic textures. Throughout several comprehensive studies (presented herein as separate chapters), we explore the stabilization and dynamics of skyrmions in three distinct types of material systems: Thin-films close to experiencing a spin-reorientation transition, ultrathin films with low ferromagnetic exchange and high perpendicular magnetic anisotropy, and thick multilayers with large perpendicular magnetic anisotropy. Through experimental and theoretical studies, we explore the processes by which skyrmions can be created and moved in response to both quasi-static and transient electrical currents, magnetic fields, and temperature changes. In all cases, the systematic variation of material properties and experimental conditions to better understand and control the mechanisms by which skyrmions are stabilized is a key underlying theme.

In another thrust, we have studied the field-driven dynamics of chiral domain walls and have characterized a new effect in which magnetic stripe domains with chiral domain walls unidirectionally expand in response to applied magnetic fields, with growth symmetries that cannot be understood from the static energy frameworks typically employed to understand the impact of the interfacial Dzyaloshinskii-Moriya interaction in thin-film systems. Using analytical models, we have found that the in-plane torques generated by perpendicular magnetic fields stabilize steady-state domain wall magnetization profiles that are highly asymmetric in elastic energy, resulting in abnormal growth behaviors that are in line with the experimental findings. Building on this understanding of the field-driven dynamics of stripe domains, we expand this understanding to the growth directionalities observed when stripe domain motion is induced by spin-orbit torque.

The dissertation explores in detail the formation of dipole stabilized skyrmions and skyrmion lattices that we discovered in amorphous Fe/Gd multilayers. The dipole skyrmion phase exists in amorphous Fe/Gd films which is commonly associated as a classical magnetic bubble material. However, as we discuss throughout the thesis, there are clear distinctions between both magnetic textures in topology, magnetic energies required to favor their formation and their response under external perturbations. Bubble domains typically form in films where the uniaxial anisotropy Ku exceeds the demagnetization anisotropy Kd or have a Q = Ku/Kd > 1 whereas the dipole skyrmions and skyrmion lattices we report exist in thin films with Q < 1.

Over the course of the thesis, we present comprehensive studies as individual chapters describing the magnetic, ferromagnetic resonance and transport properties associated with the dipole skyrmions. For each of these studies, we first identified the dipole skyrmion phase using real and reciprocal space imaging techniques and then quantified the respective properties of these films. We then established a one-to-one correlation between magnetic, ferromagnetic resonance and transport measurements and the domain morphologies observed thus allowing us to assign unique attributes to the dipole skyrmion phase. With aid of numerical simulations, we are able to further validate our experimental observations and investigate additional magnetic information that is not easily accessible in experiments.

Overall, dipole skyrmions possess interesting physics that are comparable and complements the physics observed in skyrmions stabilized by the Dzyaloshinskii–Moriya interaction. The sub-100nm features exists over a broad range of temperatures and magnetic fields which can further tuned by modifying the film composition or thickness. The dipole skyrmion phase exists in a material parameter space that had previously not been investigated by neither bubble domain nor skyrmion scientific communities. Last, our results show that a lot of the interesting physics resulting from these textures appears to be a result of topology and not the physical mechanism that forms these chiral cylindrical-like textures.