Excessive energy dissipation in CMOS devices during switching is the primary threat to continued downscaling of computing devices in accordance with Moore’s law. In the quest for alternatives to traditional transistor based electronics, nanomagnet-based computing [1, 2] is emerging as an attractive alternative since: (i) nanomagnets are intrinsically more energy-efficient than transistors due to the correlated switching of spins [3], and (ii) unlike transistors, magnets have no leakage and hence have no standby power dissipation. However, large energy dissipation in the clocking circuit appears to be a barrier to the realization of ultra low power logic devices with such nanomagnets. To alleviate this issue, we propose the use of a hybrid spintronics-straintronics or straintronic nanomagnetic logic (SML) paradigm. This uses a piezoelectric layer elastically coupled to an elliptically shaped magnetostrictive nanomagnetic layer for both logic [4-6] and memory [7-8] and other information processing [9-10] applications that could potentially be 2-3 orders of magnitude more energy efficient than current CMOS based devices. This dissertation focuses on studying the feasibility, performance and reliability of such nanomagnetic logic circuits by simulating the nanoscale magnetization dynamics of dipole coupled nanomagnets clocked by stress. Specifically, the topics addressed are: 1. Theoretical study of multiferroic nanomagnetic arrays laid out in specific geometric patterns to implement a “logic wire” for unidirectional information propagation and a universal logic gate [4-6]. 2. Monte Carlo simulations of the magnetization trajectories in a simple system of dipole coupled nanomagnets and NAND gate described by the Landau-Lifshitz-Gilbert (LLG) equations simulated in the presence of random thermal noise to understand the dynamics switching error [11, 12] in such devices. 3. Arriving at a lower bound for energy dissipation as a function of switching error [13] for a practical nanomagnetic logic scheme. 4. Clocking of nanomagnetic logic with surface acoustic waves (SAW) to drastically decrease the lithographic burden needed to contact each multiferroic nanomagnet while maintaining pipelined information processing. 5. Nanomagnets with four (or higher states) implemented with shape engineering. Two types of magnet that encode four states: (i) diamond, and (ii) concave nanomagnets are studied for coherence of the switching process.

Magnetic interlayer coupling is one of the central phenomena in spintronics. It has been predicted that the sign of interlayer coupling can be manipulated by electric fields, instead of electric currents, thereby offering a promising low energy magnetization switching mechanism. Here we present the experimental demonstration of voltage-controlled interlayer coupling in a new perpendicular magnetic tunnel junction system with a GdOx tunnel barrier, where a large perpendicular magnetic anisotropy and a sizable tunnelling magnetoresistance have been achieved at room temperature. Owing to the interfacial nature of the magnetism, the ability to move oxygen vacancies within the barrier, and a large proximity-induced magnetization of GdOx, both the magnitude and the sign of the interlayer coupling in these junctions can be directly controlled by voltage. These results pave a new path towards achieving energy-efficient magnetization switching by controlling interlayer coupling.

The <i>spin</i> of an electron has for long excited researchers both with respect to its fundamental physics and technological applications. Consequently, the traditional field driven switching of ferromagnets gave way for more scalable current driven switching based on the well-known spin transfer torque phenomenon. However, in the quest for better energy-efficiency, the manipulation of electron spin through pure voltage driven or voltage-assisted mechanisms are being intensely explored. In this research, we demonstrate that the very physics and the characteristics of such voltage driven devices enable interesting possibilities with respect to memory, neuromorphic and logic applications. We rely on the recent experimental demonstrations of two novel voltage effects on nano-magnets - the voltage controlled magnetic anisotropy (VCMA) and the pure voltage driven magneto-electric (ME) effect. Specifically, we propose in-situ, in-memory, vector logic operations by exploiting the voltage asymmetry and precessional switching dynamics of the VCMA effect to construct 'stateful' logic gates. Stateful logic are those in which the same device acts as a storage element and compute engine, simultaneously. In addition, we show that the pure voltage driven mono-domain switching and domain-wall motion of nano-magnets through the ME effect can be leveraged to construct neuro-mimetic devices exhibiting leaky-integrate-fire dynamics of biological neurons and as well as non-volatile synaptic elements. Further, we propose a voltage driven logic-device using the ME switching and demonstrate that the proposed logic-device can be used to construct a complete cascadable logic family including XNOR, IMP (implication), NAND and NOR gates. Additionally, we present an energy and area efficient content addressable memory using a logic compatible ME-XNOR device. The presented research shows that voltage driven switching can augment the very functionality and widen the application scope of spin based devices and circuits.

Dilute magnetic semiconductors (DMSs) are attractive. They are candidate materials for applications in novel spintronic devices. Because of the giant Zeemaneect in the paramagnetic state, a magnetic eld can be used to manipulate the spin and charge of carriers in DMSs. One possibility is to exploit the nonhomogeneous magnetic elds due to superconductors. In this thesis, the heterostructures of the planar DMS and superconductors in dierent geometries and superconducting states are investigated to understand the electronic structure of electrons in the DMS. The combination of a superconducting disk in the Meissner state and the planar DMS is studied using both simple and realistic models of the magnetic eld associated with the disk. The giant Zeeman interaction is found to substantially inuence the energies of magnetically conned states in the adjacent DMS. In the simple model eld, the giant Zeeman energy acts as an extra conning potential and results in spin dependent electron states exhibiting dierent spatial distributions, while the more realistic model eld results in conned states exhibiting a variety of mixed spin characters. The hybrid of a superconducting lm in a superconducting vortex state and the DMS is then explored. The concentrated magnetic eld due to an isolated vortex is shown to trap strongly spin polarised electron states. In the case of an Abrikosov lattice of vortices, interactions between vortex-bound states result in a band structure which can be controlled by the magnitude of an external uniform magnetic eld. It is found that the numerical band structures obtained using a basis of Landau states dier from those previously reported, leading to the development of a tight-binding theory to conrm their corrections. Another hybrid investigated is a square superconductor above the DMS. In this case, the arrangement of vortices is distorted by the boundary of the sample, leading to the possibility of multivortex state and/or giant vortex states. It is discovered that the magnetic eld due to the former state induces \molecular" electron states in the DMS, while that due to the latter state induces electron states with increased spatial distribution. Tight-binding theory is again used to describe the observed energy levels and the interactions between electron states induced by the magnetic elds due to separated vortices in the multivortex state.

This thesis presents the results from investigations of ultrafast magnetisation dynamics driven by pure spin currents. Spin orbit coupling in heavy metal layers - such as tungsten, tantalum or platinum - allows for the generation of pure spin currents, whereby spin up and spin down electrons move in opposite directions. Hence, a flow of angular momentum can be controlled through the manipulation of charge current through a heavy metal layer. When a spin current is injected into a ferromagnet, a torque is exerted on its magnetisation, with the potential to induce a wide variety of ultrafast dynamics. The experimental investigation of these phenomena employed a variety of high-frequency electrical techniques and time-resolved scanning Kerr microscopy (TRSKM) methods. In addition, various simulative and analytical approaches were used to gain insight into the underlying mechanisms. Spin Hall nano-oscillators (SHNOs) have recently been shown to support a tunable GHz spin wave `bullet’ under injection of direct current (DC), making it an exciting candidate for microwave communication applications. This thesis will show how TRKSM can be used to measure the torques within these devices, revealing that radio frequency (RF) current does not possess the same distribution as the DC. The competition between self-inductance and focusing within the device geometry results in a modified distribution of spin current. Further TRSKM measurements show the modified torque landscape to promote the mobility of the `bullet' within the magnetic layer. Devices that exploit spin currents for magnetisation reversal have received interest from academia and industry for their potential use as memory elements. The perpendicular magnetic anisotropy present in Ta/CoFeB/MgO leads to lower write currents and higher thermal stability. However, ultrafast processes have not been previously observed in such devices. TRSKM measurements of Hall bar devices were compared with a macrospin model to understand the underlying torques, and to investigate the conditions required to promote switching. Square elements built from the same stack structure exhibited contrasting static and dynamic behaviour. Pulsed currents drove differing dynamics at the edge and center of the device, while enabling the realignment of magnetic domains. The domains themselves could be driven directly by the spin current leading to domain wall dynamics. Measurements with a bipolar electrical pulse demonstrated that meta-stable switching can be achieved in micron-scale elements.

Graphene, an atomically thin material consisting of a hexagonal, highly packed carbon lattice, is of great interests in its magnetic properties. These interests can be categorized in several fields: graphene-based magnetic materials and their applications, large diamagnetism of graphene, and the heterostructures of graphene and other two dimensional materials. In the first aspect, magnetic moments can be in theory introduced to graphene by minimizing its size or introducing structural defects, leading to a very light magnetic material. Furthermore, weak spin-orbital interaction, and long spin relaxation length make graphene promising for spintronics. The first part of this thesis addressed our experimental investigation in defect-induced magnetism of graphene. Non-interacted spins of graphene have been observed by intentionally introducing vacancies and adatoms through ion-irradiation and fluorination, respectively. The defect concentration or the magnetic moments introduced in this thesis cannot provide enough interaction for magnetic coupling. Furthermore, the spins induced by vacancies and adatoms can be controlled through shifting the Fermi energy of graphene using molecular doping, where the adatoms were alternatively introduced by annealing in the inert environment. The paramagnetic responses in graphene induced by vacancy-type defects can only be diverted to half of its maximum, while those induced by sp3 defects can be almost completely suppressed. This difference is supposed that vacancy-type defects induced two localized states (pie and sigma). Only the latter states, which is also the only states induced by sp3 defects, involves in the suppression of magnetic moments at the maximum doping achieved in this thesis. The observation through high resolution transmission electron microscope (HR-TEM) provides more information to the hypothesis of the previous magnetic findings. Reconstructed single vacancy is the majority of defects discovered in proton-irradiated graphene. This result verifies the defect-induced magnetic findings in our results, as well as the electronic properties of defected graphene in the literatures. On the other hand, the diamagnetic susceptibility of neutral graphene is suggested to be larger than that of graphite, and vanish rapidly as a delta-like function when graphene is doped. In our result, surprisingly, the diamagnetic susceptibility varies little when the Fermi level is less than 0.3 eV, in contrast with the theory. When the Fermi energy is higher than 0.3 eV, susceptibility then reduces significantly as the trend of graphite. The little variation in susceptibility near the Dirac point is probably attributed to the spatial confinement of graphene nanoflakes, which are the composition of graphene laminates. In the end of this thesis, we discuss the magnetic properties in one of the other two dimensional materials, molybdenum disulfide (MoS2). It is a potential material for graphene-based heterostructure applications. The magnetic moments in MoS2 are shown to be induced by either edges or vacancies, which are introduced by sonication or proton-irradiation, respectively, similar to the suggestions by theories. However, no significant ferromagnetic finding has been found in all of our cases.

Organic semiconductors exhibit exceptionally long spin lifetimes, and recent observations of the inverse spin hall effect as well as micrometer spin diffusion lengths in conjugated polymers have spiked interest in employing such carbon-based materials in spintronics applications. The charge transport and photophysics of organic semiconductors have been intensely studied for optoelectronic applications, revealing subtle relationships between molecular geometry, morphology and physical properties. Similar structure-property relationships remain mostly unknown for spin dynamics, where the charge carrier spins couple to their environment through hyperfine (HFI) and spin-orbit interactions (SOC). HFIs provide a pathway for spin relaxation while SOC plays a dual role in such materials: it couples the spin to its angular momentum and therefore enables both spin-to-charge conversion and spin relaxation. Understanding the molecular SOC, and finding a means to tune its strength, therefore is fundamentally important for materials design and selection. However, quantifying SOC strengths indirectly through spin relaxation effects has proven difficult due to competing relaxation mechanisms. We initially present a systematic study of the g-tensor shift in molecular semiconductors and establish it as a probe for the SOC strength in a series of high mobility molecular semiconductors. The results demonstrate a rich variability of molecular g-shifts with the effective SOC, depending on subtle aspects of molecular composition and structure. We then correlate the above g-shifts to spin-lattice relaxation times over four orders of magnitude, from 200 µs to 0.15 µs, for isolated molecules in solution and relate our findings to the spin relaxation mechanisms that are likely to be relevant in solid state systems. Isolated molecules provide an ideal model system to investigate a spin's interactions with its environment but device applications commonly employ thin films. The second half of this thesis investigates polaron spin lifetimes in field effect transistors with high-mobility conjugated polymers as active layers. We use field-induced electron spin resonance measurements to demonstrate that spin relaxation is governed by the charges' hopping motion at low temperatures while Elliott-Yafet-like relaxation due to short-range, rapid spin density dynamics likely dominates high temperature spin lifetimes. Such a microscopic relaxation mechanism is highly sensitive to the local conformation of polymer backbones and we show its dependence on the degree of crystallinity in a polymer film.

Spintronics reaches beyond typical charge-based information storage technologies by utilizing an addressable degree of freedom for electron manipulation, the electron spin polarization. With mounting experimental data and improved theoretical understanding of spin manipulation, spintronics has become a potential alternative to charge-based technologies. However, for a long time, spintronics was not thought to be feasible without the ability to electrostatically control spin conductance at room temperature. Only recently, graphene, a 2D honeycomb crystalline allotrope of carbon only one atom thick, was identified because of its predicted, long spin coherence length and experimentally realized electrostatic gate tunability. However, there exist several challenges with graphene spintronics implementation including weak spin-orbit coupling that provides excellent spin transfer yet prevents charge to spin current conversion, and a conductivity mismatch due to the large difference in carrier density between graphene and a ferromagnet (FM) that must be mitigated by use of a tunnel barrier contact. Additionally, the usage of graphene produced via CVD methods amenable to semiconductor industry in conjunction with graphene spin valve fabrication must be explored in order to promote implementation of graphene-based spintronics. Despite advances in the area of graphene-based spintronics, there is a lack of understanding regarding the coupling of industry-amenable techniques for both graphene synthesis and lateral spin valve fabrication. In order to make any impact on the application of graphene spintronics in industry, it is critical to demonstrate wafer-scale graphene spin devices enabled by wafer-scale graphene synthesis, which utilizes thin film, wafer-supported CVD growth methods. In this work, high-quality graphene was synthesized using a vertical cold-wall furnace and catalyst confinement on both SiO2/Si and C-plane sapphire wafers and the implementation of the as-grown graphene for fabrication of graphene-based non-local spin valves was examined. Optimized CVD graphene was demonstrated to have ID/G ≈ 0.04 and I2D/G ≈ 2.3 across a 2" diameter graphene film with excellent continuity and uniformity. Since high-quality, large-area, and continuous CVD graphene was grown, it enabled the fabrication of large device arrays with 40 individually addressable non-local spin valves exhibiting 83% yield. Using these arrays, the effects of channel width and length, ferromagnetic-tunnel barrier width, tunnel barrier thickness, and level of oxidation for Ti-based tunnel barrier contacts were elucidated. Non-local, in-plane magnetic sweeps resulted in high signal-to-noise ratios with measured ΔRNL across the as-fabricated arrays as high as 12 Ω with channel lengths up to 2 µm. In addition to in-plane magnetic field spin signal values, vertical magnetic field precession Hanle effect measurements were conducted. From this, spin transport properties were extracted including: spin polarization efficiency, coherence lifetime, and coherence distance. The evaluation of industry-amenable production methods of both high-quality graphene and lateral graphene non-local spin valves are the first steps toward promoting the feasibility of graphene as a lateral spin transport interconnect material in future spintronics applications. By addressing issues using a holistic approach, from graphene synthesis to spin transport implementation, it is possible to begin assessment of the challenges involved for graphene spintronics.

The motion of domain walls due to the spin torque generated by coherent carrier transport is of considerable interest for the development of spintronic devices. We model the charge and spin transport through domain walls in ferromagnetic semiconductors for various systems. With an appropriate model Hamiltonian for the spin– dependent potential, we calculate wavefunctions inside the domain walls which are then used to calculate transmission and reflection coefficients, which are then in turn used to calculate current and spin torque. Starting with a simple approximation for the change in magnetization inside the domain wall, and ending with a sophisticated transfer matrix method, we model the common π wall, the less–studied 2π wall, and a system of two π walls separated by a variable distance. We uncover an interesting width dependence on the transport properties of the domain wall. 2π walls in particular, have definitive maximums in resistance and spin torque for certain domain wall widths that can be seen as a function of the spin mistracking in the system — when the spins are either passing straight through the domain wall (narrow walls) or adiabatically following the magnetization (wide walls), the resistance is low as transmission is high. In the intermediate region, there is room for the spins to rotate their magnetization, but not necessarily all the way through a 360 degree rotation, leading to reflection and resistance. We also calculate that there are widths for which the total velocity of a 2π wall is greater than that of a same–sized π wall. In the double–wall system, we model how the system reacts to changes in the separation of the domain walls. When the domain walls are far apart, they act as a spin–selective resonant double barrier, with sharp resonance peaks in the transmission profile. Brought closer and closer together, the number and sharpness of the peaks decrease, the spectrum smooths out, and the domain walls brought together have a transmission spectrum with many of the similar features from the 2π wall. Looking at the individual walls, we find an interesting interaction that has three distinct regimes: 1) repulsion, where the left wall moves to the left and the right wall to the right; 2) motion together, where the two walls both move to the right, even at the same velocity for one special value of separation; and 3) attraction, where the left wall moves to the right and the right wall moves to the left. This speaks to a kind of natural equilibrium distance between the domain walls. This is of major interest for device purposes as it means that stacks of domain walls could be self–correcting in their motions along a track. Much experimental work needs to be done to make this a reality, however.

New semiconductor materials may enable next-generation â spintronicâ devices which exploit both the spin and charge of an electron for data processing, storage, and transfer. The realization of such devices would benefit greatly from room temperature ferromagnetic dilute magnetic semiconductors. Theoretical predictions have suggested that room temperature ferromagnetism may be possible in the wide bandgap semiconductors GaMnN and ZnMnO, though the existing models require input from the growth of high-quality materials. This work focuses on an experimental effort to develop high-quality materials in both of these wide bandgap materials systems. ZnMnO and ZnCoO single crystals have been grown by a modified melt growth technique. X-ray diffraction was used to examine the structural quality and demonstrate the single crystal character of these devices. Substitutional transition metal incorporation has been verified by optical transmission and electron paramagnetic resonance measurements. No indications of ferromagnetic hysteresis are observed from the bulk single crystal samples, and temperature dependent magnetization studies demonstrate a dominant antiferromagnetic exchange interaction. Efforts to introduce ferromagnetic ordering were only successful through processing techniques which significantly degraded the material quality. GaMnN thin films were grown by metalorganic chemical vapor deposition. Good crystalline quality and a consistent growth mode with Mn incorporation were verified by several independent characterization techniques. Substitutional incorporation of Mn on the Ga lattice site was confirmed by electron paramagnetic resonance. Mn acted as a deep acceptor in GaN. Nevertheless, ferromagnetic hysteresis was observed in the GaMnN films. The apparent strength of the magnetization correlated with the relative ratio of trivalent to divalent Mn. Valence state control through codoping with additional donors such as silicon was observed. Additional studies on GaFeN also showed a magnetic hysteresis. A comparison with implanted samples showed that the common origin to the apparent strong ferromagnetic hysteresis related to contribution from Mn substitutional ions. The observed magnetic hysteresis is due to the formation of Mn-rich regions during the growth process. This work demonstrated that the original intrinsic models for room temperature ferromagnetism in the wide bandgap semiconductors do not hold and the room temperature ferromagnetism in these materials results from extrinsic contributions.