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As "big-data" becomes the norm the demand for improved, energy-efficient storage and computation is valuable and never satisfied. Magnets and their properties are highly favourable for this problem and it has driven research into novel architectures and the resulting effects. Scanning-probe microscopy (SPM) is ideal for visualising physical properties of nanoscale systems as SPM is cheap, modular, and possess high spatial resolution (~ 20 nm). Instrument modifications and image processing means complex architectures can be investigated. This thesis uses these techniques to explore nanomagnetic systems with promise in novel computation. First, a patterned magnetic probe was designed for customised magnetic force microscopy (MFM) measurements and evaluated against commercial equivalents. The novel probes revealed advantageous properties for imaging heterogeneous samples. Subsequently, a novel SPM method was developed for characterising spin textures in a nanowire with high magnetic susceptibility by inducing spin caloritronic effects with a joule-heated SPM probe. Both methods give precedent for characterising challenging magnetic samples for applications including magnetic storage. Following this, local behaviours in artificial spin ice (ASI) are investigated by advanced MFM and supplementary techniques. ASI are 2D systems that exhibit collective dynamics and emergent monopoles, which have application to reconfigurable magnonics and probabilistic computation. Here, the formation of non-Ising states in a novel ASI lattice were shown to form and randomly distribute across the array deterministically and their frequency was highly tunable. The strayfield of many frustrated vertices was examined by quantitative-MFM and electron holography, which revealed that even periodic configurations were energetically disparate and influences the non-Ising state properties. Finally, artificial defects in an ASI lattice were shown to influence neighbouring vertices by injecting monopoles into the lattice. Their characteristics are compared to those that form independently by in situ MFM and Lorentz microscopy. The described effects have implications for devices aimed towards energy-efficient storage and computation.

The stability and origin of magnetic remanence in rocks and meteorites carried by ferromagnetic grains with nonuniform magnetization are not completely understood. This research aims to provide experimental data at the micromagnetic level through domain observations with submicron resolution of magnetic microstructures in small particles of synthetic intermediate titanomagnetite (Chapters 4 and 5) and in the Chelyabinsk meteorite (Chapter 6) using Magnetic Force Microscopy (MFM). To better interpret magnetic microstructures of three-dimensional particles from two dimensional MFM-images, crystallographic data from electron backscatter diffraction and numerical micromagnetic modeling were obtained. MFM-imaging was also combined with electron diffraction x-ray spectroscopy (using electron microscopy or electron microprobe) to identify the composition of magnetic phases investigated in this work. This research provided evidence for the existence of magnetic vortex states in micron-sized intermediate titanomagnetite particles exhibiting a range of available local energy minimum states at room temperature such as single vortex and multivortex states. The experimental results agree well with micromagnetic modeling conducted in this study. Moreover, this research demonstrated a high temperature stability of single vortex state and transdomain transitions between multivortex states and single vortex state with increasing temperatures up to the Curie temperature. Our findings suggest that intermediate titanomagnetite grains in the single vortex state can be a source of stable and reliable paleomagnetic records. In addition, high temperature results obtained in this study provide constraints for micromagnetic models and theory of thermoremanent magnetization in vortex states. MFM investigation of Chelyabinsk meteorite revealed the presence of submicron inclusions of magnetite and Cr-magnetite that are potentially stable carriers of paleomagnetic record in this meteorite but have not been identified by bulk magnetic characterization. This work has demonstrated that the integration of MFM-imaging with high-resolution, non-magnetic characterization techniques helps to provide meaningful conclusions about domain state and magnetic mineralogy of the main carriers of paleomagnetic records in rocks and meteorites.

The interfacial and magnetic properties of the metal-rich phases and reconstructions of MnxNy and GaN are investigated. Thin films of the two most metal-rich phases of MnxNy (ε and ζ) are grown on MgO(001) using a custom-built ultra high vacuum molecular beam epitaxy growth system. By the same means, thin films of GaN with the two most metal-rich reconstructions [c(6×12) and psuedo-1×1+1⁄12] are grown on GaN(0001) and Al2O3(0001). The interfacial properties of these materials such as surface structure and local density of states are investigated in situ with low temperature scanning tunneling microscopy and reflection high energy electron diffraction. The bulk structure of these thin films are investigated using x-ray diffraction. Measurements of the morphology and magnetism of these thin films are made ex situ using atomic/magnetic force microscopy, spin-polarized scanning tunneling microscopy, scanning electron microscopy, vibrating sample magnetometry, and superconducting quantum interference device magnetometry techniques. Measurements of the chemical composition of the samples are made using back scattered electron scanning electron microscopy, energy dispersive x-ray spectroscopy, and Rutherford backscattering spectrometry.These techniques reveal that growth temperature heavily influences the quality of the ε-Mn4N grown. A nucleation temperature below 480 °C is observed to result in the growth of substantial antiferromagnetic η-Mn3N2 grains alongside ferrimagnetic ε-Mn4N grains. The most significant component of perpendicular magnetic anisotropy in ε-Mn4N thin films (9 to 300 nm thick) grown on MgO(001) is attributed to the shape induced partial overlap of Ising domains out-of-plane. Spin-polarized scanning tunneling microscopy and magnetic force microscopy measurements are consistent with the interpretation that these ε-Mn4N thin films form single material out-of-plane spin valves due to this out-of-plane overlapping of Ising domains.The ζ-Mn10N thin films, which are found to possess a regular surface corrugation along the [100]-direction, grown on MgO(001) are shown to be γ-type ζ-phase. Conflicting evidence for both zero and non-zero net-magnetization is suggestive of the presence of both antiferromagnetic and ferromagnetic properties in ζ-phase MnxNy thin films.The interfacial structure of the Ga-rich reconstructions of GaN are investigated. The ground state of the psuedo-1×1+1⁄12 surface reconstruction on GaN(0001) is found to consist of a 9×12 structure. The distribution of Ga at the surface of GaN thin films deposited under Ga saturation on Al2O3(0001) is found to consist of an equilibrium between Ga gas on the c(6×12) reconstruction of GaN(0001̅) and on the psuedo-1×1+1⁄12 reconstruction of GaN(0001) as well as liquid Ga droplets. A side by side comparison of c(6×12) and psuedo-1×1+1⁄12 reconstructions shows contrast between the local density of states of the two reconstructions at multiple voltages between 0 and 3 V.

Magnetic force microscopy studies in varying temperature and applied external magnetic field of magnetic thin films of LaCoO₃ under strain and single crystal UMn₂Ge₂ have been performed. In the case of LaCoO₃ thin films the aim is an understanding of the response of the magnetic microstructure to different signs and degrees of strain and a further attempt to distinguish the effect of defects from strain-induced effects. In UMn₂Ge₂ the magnetic microstructure is imaged for the first time and signatures of a possible phase transition at 150 K and crystalline anisotropy are explored. The first portion of this dissertation focuses on the synthesis methods used to produce the samples investigated and the critical role of synthesis in producing high-quality samples. This is followed by a discussion of characterization techniques used to obtain local and global magnetic and structural characteristics, with particular emphasis on magnetic force microscopy including noise characteristics and a discussion of achieving a high force gradient sensitivity by optimizing the fiber-optic interferometer used for cantilever deflection detection. Design elements and features of the multi-mode variable-temperature atomic force microscope used to obtain magnetic force microscopy images are presented and results for LaCoO₃ and UMn₂Ge₂ are discussed.

In recent years, current data storage and semiconductors have begun to reach their fundamental limits. In current hard disk drive based data storage technology the size of the magnetic bit that data is stored on has begun to reach it size limits due to the superparamagnetic limit. In semiconductor technologies, devices are beginning to reach their fundamental limits due to the current leakage effect in small devices. If these technologies will continue with the current trends of packing more data/devices into the same area, new technologies need to be developed. In the following pages, two new technologies will be proposed for increasing the areal density of magnetic data storage technologies. After that, three novel approaches to creating logic cells on patterned magnetic media at much higher areal densities than current semiconductor technologies can support. First a study of Co/Pd multilayers as a candidate material for the next generation of magnetic media for a number of applications will be described. Following that, a patterned, three layer, magnetic media made up of magnetic multilayers will be fabricated and tested to show the feasibility of using it for the basis of next generation data storage technologies. Then, three novel approaches to magnetic logic gates using magnetic multilayers based media will be designed, fabricated, and tested using state of the art fabrication and analysis techniques. Each of the proposed magnetic logic cells will consist of a patterned media made up on of two, three, or four stacked magnetic multilayers media. The two layer structures are fabricated to be implemented on current hard disk drives. The three and four stacked structures can be implemented in a number of ways, from hard disk drives to magnetic quantum-dot cellular automata. The latter would allow these logic cells to be integrated with semiconductor technologies to create a hybrid device that utilizes the best aspects of each technology.

The high degree of modification through chemical substitution afforded by the perovskite crystal structure and its related counterparts allows a systematic study of structure-property relationships critical to understand the wide variety of exotic phenomena observed in these materials where the spin, charge, orbital, and lattice degrees of freedom are highly correlated. From the multiple phenomena observed in these materials, which includes multiferroicity, catalytic activity, and high temperature superconductivity, this study is concerned with a material that displays colossal magnetoresistance (CMR), La₂₋₂xSr₁₊₂xMn₂O₇; this is a naturally bilayered manganite that exhibits CMR at a paramagnetic to ferromagnetic transition that coincides with an insulator to metal transition. The strong correlation between different degrees of freedom in the material leads to considerable variation in its magnetic properties due to doping even in the small range studied of 0.32 [less than or equal to] x [less than or equal to] 0.4, where the easy axis of magnetization changes from the c-axis to the ab plane. Magnetic force microscopy (MFM) was used for this part of the work, to visualize the local variation of the out of plane (c-axis) magnetization or magnetic microstructure of La₂₋₂xSr₁₊₂xMn₂O₇ for 0.32 [less than or equal to] x [less than or equal to] 0.4 at the exposed ab surface and its evolution due to an applied magnetic field at 4 K. For the x = 0.32 composition, which is close to the out of plane to in plane magnetization transition, a strong preferred magnetization direction within the ab plane or magnetocrystalline anisotropy was observed. The stray magnetic field of the MFM tip perturbs the magnetic microstructure of low coercivity materials like diluted magnetic semiconductors, making it unsuitable for the study of such materials. For this reason, as part of this project a scanning Hall probe microscope (SHPM), a magnetic imaging technique complementary to MFM that uses a Hall sensor that provides a magnetically non-invasive calibrated measurement of the stray fields at the surface of a sample with good resolution (~ 1 [micrometer]), was designed. The construction of a compact cryogenic variable-temperature (77 - 300 K) SHPM, highlighting its features, is described.

In recent years, both synthetic as well as naturally occurring superparamagnetic nanoparticles (SPNs) have become increasingly important in biomedicine. For instance, iron deposits in many pathological tissues are known to contain an accumulation of the superparamagnetic protein, ferritin. Additionally, man-made SPNs have found biomedical applications ranging from cell-tagging in vitro to contrast agents for in vivo diagnostic imaging. Despite the widespread use and occurrence of SPNs, detection and characterization of their magnetic properties, especially at the single-particle level and/or in biological samples, remains a challenge. Magnetic signals arising from SPNs can be complicated by factors such as spatial distribution, magnetic anisotropy, particle aggregation and magnetic dipolar interaction, thereby confounding their analysis. Techniques that can detect SPNs at the single particle level are therefore highly desirable. The goal of this thesis was to develop an analytical microscopy technique, namely magnetic force microscopy (MFM), to detect and spatially localize synthetic and natural SPNs for biomedical applications. We aimed to (1) increase MFM sensitivity to detect SPNs at the single-particle level and (2) quantify and spatially localize iron-ligated proteins (ferritin) in vitro and in biological samples using MFM.Two approaches were employed to improve MFM sensitivity. First, we showed how exploitation of magnetic anisotropy could produce a higher, more uniform MFM signal from single SPNs. Second, we showed how an increase in probe magnetic moment increased both the magnitude and range up to which the MFM signal could be detected from a single SPN. We further showed how MFM could enable accurate quantitative estimation of ferritin content in ferritin-apoferritin mixtures. Finally, we demonstrated how MFM could be used to detect iron/ferritin in serum and animal tissue with spatial resolution and sensitivity surpassing that obtained using conventional biochemical assays. We envisage these advancements will allow MFM to serve as a novel biosensing technique to spatially localize iron/ferritin in small aliquots of clinical samples (i.e. serum) and in tissue biopsies at the ultra-sensitive and ultra-structural level. We also discuss how future work incorporating our advancements could lead to the development of a novel indirect MFM technique, which could enable high-throughput analysis of SPNs for biomedical applications.

Spintronics is a burgeoning field that endeavors to use the spin of an electron for information processing. Remarkable progress has been made in understanding injection of spin-polarized electron populations/currents into semiconductors, which are typically non-magnetic, as well as manipulation and detection of said spins. This dissertation presents a combination of numerical analysis and experiments aimed at understanding and developing imaging tools that utilize the direct coupling of spins with the spatially varying magnetic field of a micromagnetic probe. This coupling produces a spin detection technique which relies on the resulting magnetic force on a cantilever to which the magnetic probe is attached. While extensively used in materials research, such an imaging tool has lagged as a characterization tool for spintronics. This is because of the challenges associated with measuring extremely small forces from these samples, and a lack of understanding of the effects of the inhomogeneous vector field from the probe on the sample spin density. I have developed a numerical framework, based on solving the spin-transport equation, for understanding behavior of spins in the presence of spatially varying parameters. Based on analysis of the analytically solvable case of spin in a uniform magnetic field, but with multiple vector components, I have identified a crucial quantity, θB. This captures the competition between parallel and perpendicular field components dictating the collective spin precession behavior in these spin ensembles. The numerical simulations show that in the proximity of the micromagnet the Hanle response is broadening due to the field component parallel to the injected spin direction. This result also sheds light on the effect of magnetic injectors used in spintronics devices. Our simulations show that the Hanle response will also be artificially broadened in this case due to the magnetic field from the injector. Such broadening is the most likely cause for the larger than expected Hanle halfwidths measured in recent experiments, and should be taken into account while extracting spin lifetime from such data. Insights from numerical analysis have led us to propose scanned magnetic perturbation imaging for spatially mapping spin properties. This technique enhances the imaging capabilities of existing detection methods by encoding local information in the field gradients from a micromagnet. The spin density is governed by a convolution of local spin properties and the local magnetic field that perturbs the spins. By scanning this local perturbation a contrast in the globally detected signal, resulting from sample inhomogeneity, can be seen. On the experimental side I have measured the broadening of the Hanle response due to a micromagnetic tip, using a globally integrated spin photoluminescence signal from optically excited spins in n-GaAs epitaxial membranes. I have also conducted sensitive force detection experiments to measure the magnetic forces from the spins in these samples. I have measured small forces of the order femtonewtons, similar to that predicted from simulations. The Hanle response of this force signal behaves as expected. This may be the first force detection of non-equilibrium itinerant spin-polarized electrons in paramagnetic semiconductors.

This dissertation is divided into two parts. In the first, the general paradigm of scanned probe microscopy is outlined with a focus on atomic force microscopy and a few of its variations. Magnetic force microscopy is covered in detail as it forms the basis of the second part of this dissertation. The core elements and extra features of the instrument are described with attention paid to the upgrades made by the author. In the second part of this dissertation, background information on perovskite oxides and the inverse spinel system, magnetite, is given. Magnetic force microscopy studies were done on three thin film systems and are detailed. In the first study, ferromagnetic manganite films were subjected to discontinuous changes in strain due to structural transitions in their barium titanate substrates. The resulting effect on the magnetic domains was observed. In the second study, the ferromagnetism of a tensile-strained LaCoO₃ film was studied across temperatures from 4.3 K to 90 K and applied fields up to [mu]₀H=1.1 T. Finally, the properties of antiphase domains in magnetite films of varying film strain due to transition metal buffer layers was probed by imaging the magnetic domains which are pinned to the antiphase boundaries.

Magnetoelectricity (ME) is a physical property that results from an exchange between polar (electric dipole) and spin (magnetic dipole) subsystem: i.e., a change in polarization (P) with application of magnetic field (H), or a change in magnetization (M) with applied electric field (E). Magnetoelectricity can be found both in single phase and composite materials. Compared with single phase multiferroic materials, composite multiferroics have higher ME effects. Through a strictive interaction between the piezoelectricity of the ferroelectric phase and the magnetostriction of the ferromagnetic phase, said multiferroic composites are capable of producing relatively large ME coefficients. This Dissertation focused on the deposition and characterization of two-phase composite magnetoelectric thin films. First, single phase ferroelectric thin films were studied to improve the multiferroic properties of the composite thin films. Then structural, ferroelectric, ferromagnetic, and magnetoelectric properties of composite thin films were researched. Finally, regular nano-array composite films were deposited and characterized. First, for single phase ferroelectric thin films, the phase stability was controlled by epitaxial engineering. Because ferroelectric properties are strongly related to their crystal structure, it is necessary to study the crystal structures in single phase ferroelectric thin films. Through constraint of the substrates, the phase stability of the ferroelectric thin films were able to be altered. Epitaxial thin-layers of Pb(Fe1/2Nb1/2)O3 (or PFN) grown on (001), (110), and (111) SrTiO3 substrates are tetragonal, orthorhombic, and rhombohedral respectively. The larger constraint stress induces higher piezoelectric constants in tetragonal PFN thin film. Epitaxial thin-layers of Pb(Zr0.52Ti0.48)O3 (or PZT) grown on (001), (110), and (111) SrTiO3 substrates are tetragonal, monoclinic C, and rhombohedral respectively. Enhanced ferroelectric properties were found in the low symmetry monoclinic phase. A triclinic phase in BFO was observed when it was deposited on tilted (001) STO substrates by selecting low symmetry (or interim) orientations of single crystal substrates. Then, in two phase composite magnetoelectric thin films, the morphology stability was controlled by epitaxial engineering. Because multiferroic properties are strongly related to the nano-structures of the composite thin films, it is necessary to research the nano-structures in composite thin films. Nano-belt structures were observed in both BaTiO3-CoFe2O4 and BiFeO3-CoFe2O4 systems: by changing the orientation of substrates or annealing condition, the nano-pillar structure could be changed into nano-belts structure. By doing so, the anisotropy of ferromagnetic properties changes accordingly. The multi-ferroic properties and magnetoelectric properties or (001), (110) and (111) self-assembled BiFeO3-CoFe2O4 nano-composite thin film were also measured. Finally, the regular CoFe2O4-BiFeO3 nano-array composite was deposited by pulsed laser deposition patterned using a focused ion beam. Top and cross-section views of the composite thin film showed an ordered CoFe2O4 nano-array embedded in a BiFeO3 matrix. Multiferroic and magnetoelectric properties were measured by piezoresponse force microscopy and magnetic force microscopy. Results show (i) switching of the magnetization in ferromagnetic CoFe2O4 and of the polarization in ferroelectric BiFeO3 phases under external magnetic and electric field respectively, and (ii) changes of the magnetization of CoFe2O4 by applying an electric field to the BiFeO3 phase.

The data storage industry recently substituted its five-decade-old technology (longitudinal magnetic recording), which reached its fundamental limitations due to thermal instabilities in the recording media, with perpendicular magnetic recording. Nonetheless, the lifetime of this newly adopted technology and other next generation alternatives including heat-assisted magnetic recording (HAMR) and bit patterned media (BPM) is comparably short. In order to defer the superparamagnetic limit substantially beyond 1 Tbits/in^2, it may be necessary to stack information in a third (vertical) dimension. This vertical stacking underlies the concept of multilevel (ML) three-dimensional (3D) magnetic recording and memory. In 3D magnetic devices, information is recorded, not only on one surface (as in all modern 2D applications), but in the entire 3D bulk of the recording media. As a result, substantially larger amount of data could be recorded on the same surface area (as compared to any 2D alternative). This study addresses design configuration, media fabrication, and characterization techniques for a particular ML 3D magnetic recording device. A newly developed ML 3D magnetic recording media enables selective reading and writing of up to 4 distinct signal levels, in comparison to only two signals in conventional 2D-based magnetic recording media. It is believed that at least over six layers (or over 2^6 = 64 signal levels) could be independently accessed in a ML 3D magnetic recording device. Gallium ion implantation as a potential method to alter the magnetic properties of 3D media is also discussed. Furthermore, with every emerging technology, both fabrication systems and characterization techniques must be developed. One of the most crucial characterization tools, which enable the direct visualization of the smallest bit of magnetic information, is Magnetic Force Microscopy (MFM). MFM is thus an integral part of the development and characterization of ML 3D magnetic recording devices. Nevertheless, the best lateral resolution conventional state-of-the-art probes can produce under ambient conditions is limited to about ~ 25 nm. In this study, newly developed fabrication techniques for MFM probes are presented. Specifically, plateau probes to enhance the overall capabilities of MFM and high lateral resolution (below 10 nm in ambient conditions) multi-domain MFM probe. The enhancement in MFM capabilities has the potential to facilitate the development of HAMR, BPM, and ML 3D magnetic systems.