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The antiferromagnetic coupling between a rare-earth (RE) and a tran- sition metal (TM) ferromagnet can be exploited to engineer normal state and superconducting functional devices. RE/TM ferromagnetic multi- layers were previously used as spin-mixers to generate spin-triplet su- percurrents. This was possible due to magnetic inhomogeneity present in the devices, however the precise nature of the inhomogeneity was not understood. Here we present a comprehensive study of the Ni/Gd/Ni system using a powerful element-specific measurement technique: x-ray magnetic circular dichroism. In order to analyse the experimental results we present a novel model based on the Stoner-Wohlfarth model, which shows that significant inhomogeneity exists at the Ni/Gd interfaces due to the competition between the exchange energies within the system and the Zeeman energy of the applied magnetic field. The experiment and model together provide a complete overview of the Ni/Gd/Ni system due to the breadth of temperatures and thicknesses studied. The knowledge gained from this work is then applied to designing and test- ing new spin valves based on the intrinsic inhomogeneity at the RE/TM interface, and both Ni/Gd- and Gd/Ho-based devices show reversible magnetic switching behaviour which alters the superconducting critical temperature.

This thesis contains four studies of the effects of disorder and randomness on strongly correlated quantum phases of matter. Starting with an itinerant ferromagnet, I first use an order-by-disorder approach to show that adding quenched charged disorder to the model generates new quantum fluctuations in the vicinity of the quantum critical point which lead to the formation of a novel magnetic phase known as a helical glass. Switching to bosons, I then employ a momentum-shell renormalisation group analysis of disordered lattice gases of bosons where I show that disorder breaks ergodicity in a non-trivial way, leading to unexpected glassy freezing effects. This work was carried out in the context of ultracold atomic gases, however the same physics can be realised in dimerised quantum antiferromagnets. By mapping the antiferromagnetic model onto a hard-core lattice gas of bosons, I go on to show the importance of the non-ergodic effects to the thermodynamics of the model and find evidence for an unusual glassy phase known as a Mott glass not previously thought to exist in this model. Finally, I use a mean-field numerical approach to simulate current generation quantum gas microscopes and demonstrate the feasibility of a novel measurement scheme designed to measure the Edwards-Anderson order parameter, a quantity which describes the degree of ergodicity breaking and which has never before been experimentally measured in any strongly correlated quantum system. Together, these works show that the addition of disorder into strongly interacting quantum systems can lead to qualitatively new behaviour, triggering the formation of new phases and new physics, rather than simply leading to small quantitative changes to the physics of the clean system. They provide new insights into the underlying physics of the models and make direct connection with experimental systems which can be used to test the results presented here.

This thesis presents the experimental study of superconducting nanowires under the influence of ferromagnetic nano-structures. Placing superconducting and ferromagnetic materials in contact with one another causes their long range orders to compete. This manifests as the leakage of superconducting properties into the ferromagnet and the suppression of superconductivity in the superconductor near the interface, known as the proximity and inverse proximity eects, respectively. The experiments presented in this thesis aim to show that the inverse proximity eect is sensitive to the magnetization of the ferromagnet, specically that the suppression is weaker if the ferromagnet has an inhomogeneous magnetization. To do this, the magnetic vortex state in sub-micron nickel disks and L-shape domain wall traps were used as the inhomogeneous magnetizations. The magnetization in the nickel disks and L-shape domain wall traps were investigated using magnetic force microscopy (MFM), in situ MFM, magnetotransport, and modelling. Aluminium nanowires were deposited over the ferromagnets and low temperature transport measurements of the hybrid structures were performed. It is found that the superconductivity in the nanowire above the disks is suppressed, creating an SNS junction. The critical current is shown to be sensitive to the magnetic history of the disks. The critical current of the entire nanowire is found to be dependent on the properties of the hybrid junction. This long-range influence has a thermal origin due to Joule heating in the hybrid junction as demonstrated by use of heat sink structures and Andreev loop interferometers. Replacing the disk with Lshape domain wall traps shows that the suppression of superconductivity is weakest when a domain wall is placed beneath the nanowire. Lastly, comparison to theory indicates the junction length is proportional to temperature. The results presented demonstrate previously unknown complexity in the behaviour of so-called proximity junctions and a step toward magnetically controlled superconducting circuitry.

This thesis will discuss the utilization of ferromagnets for the generation of spin currents via various spin Hall like effects. The global magnetic order in a ferromagnetic material breaks the requisite symmetries to allow for the generation of spin currents with arbitrary polarizations which could prove useful for magnetic information storage technology. We first discuss some of the theoretical background of the various microscopic origins which can lead to spin Hall currents in ferromagnets as well as the theory behind some of the the experimental techniques used in this thesis. We then describe the measurement of a non-reorientable component of the spin Hall effect generated by a ferromagnetic Ni-Fe alloy using the spin torque ferromagnetic resonance technique, as well as modifications that we needed to make to the analysis of these measurements to account for coupling between magnetic layers driven by dynamic spin pumping. We conclude with a discussion of the materials problems which must be solved to utilize the spin anomalous Hall effect to switch out-of-plane polarized ferromagnetic layers as well as our attempts to realize such material systems.

Over 20 years ago, Blagoev et. al. predicted an s-wave pairing instability in a ferromagnetic Fermi liquid (FFL) as a consequence of spin fluctuations [5]. Shortly after, it was discovered that, when magnetic interactions in the ferromagnetic superconductor UGe2 dominate, quasiparticles with parallel spin form pairs in odd-parity orbitals; i.e., a form of spin-triplet p-wave superconductivity emerges, in contrast to Blagoev et. al.'s prediction [6]. In this work, we return to this issue by introducing the effects of a gapped amplitude (or "Higgs") mode on the vertex corrections and subsequent form of Cooper pairing. As the Higgs mode only propagates in the presence of a finite spin current, such an amplitude mode results in strong momentum-dependence in the many-body vertex. This results in the emergence of an unconventional form of superconductivity mediated by unconventional low-energy modes in a weak itinerant ferromagnet.

This thesis advances the theory of spintronics by exploring ways in which collective spin dynamics can be manipulated in magnetic materials, with an emphasis on sytems which can exhibit low dissipation spin currents and nontrivial response to external driving.In Chapter 1, we provide an introductory review of mesoscopic ferromagnetic dynamics. The Landau-Lifshitz equation is motivated and derived, with a brief discussion on its application in the presence of dissipative forces and external driving. Then we discuss the notion of spin superfluidity in easy plane ferromagnets and compare this to traditional Ginzberg-Landau superconductivity.In Chapter 2, we further extend the analogy between ferromagnetism and superconductivity, and apply this anology to the study of the ferromagnet with strong coherent easy-plane anisotropy and weak in-plane coherent anisotropy. The low bias non-equilibrium phase diagram is mapped, and the potential for producing applications with superconductor-based circuit functionality at elevated temperatures is discussed.In Chapter 3, the spin superfluid is studied in strong driving regimes in which the anaology to superconductivity begins to break down. Multiple non-equilibrium phases are discovered which are associated with the onset of non-linear effects, such as choatic oscillations and stationary soliton formation, near the spin torque injection region. Using numerical simulations and analytical modeling, we observe a robustness in spin superfluid transport and its high bias phase diagram despite the presence of symmetry breaking dipole-dipole interactions, finite size effects, and dissipation.In Chapter 4, we turn to microscopic magnetic lattices, in particular weakly coupled arrays of quantum spin chains with fermionizable, via a Jordan-Wigner transformation, Hamiltonians. Using mean field theory, we find models which may exhibit dissipationless spin currents closely analogous to that of superconductivity and the quantum Hall effect.In Chapter 5, we study the manipulation of magnetic domain walls on a wire with mechanical waves. We show how ferromagnetic and anitferromagnetic domain walls can be driven by circularly and linearly polarized waves, respectively. We note the potential for applications using mechanical waves as a means for manipulating magnetic solitons in insulators.For Chapter 6, in collaboration with an experimental study on a cleaved edge overgrowth sample, we analyze the chiral edge states of the quantum Hall effect and their tunneling properties. Our modeling provides a theoretical fooundation from which the experimental method of momentum resolved spectroscopy can provide insight into how the quantum Hall effect evolves with changing magnetic field. Spin splitting of the chiral edge states is observed in the presence of a strong in-plane magnetic field.We close with Chapter 7 giving an outlook for future work which could build off of the research presented in this thesis.

In this thesis, we present a theoretical investigation of the proximity effects in ferromagnet/superconductor heterostructures with inhomogeneous magnetization, including ferromagnet/ferromagnet/superconductor (F1F2S) trilayers and conical-ferromagnet/superconductor bilayers. We numerically obtain the self-consistent solutions of the Bogoliubov-de Gennes (BdG) equations and use these solutions to compute the relevant physical quantities. In F1F2S trilayers, we find that the critical temperature, Tc, can be a non-monotonic function of the angle &alpha between magnetizations in F layers. The minimum Tc(&alpha) often occurs when magnetizations are mutually perpendicular (&alpha=&pi/2). In addition, we demonstrate that the Tc minimum corresponds to the maximum of the penetration of the long-range triplet amplitudes. We compare our theoretical results with experiment and find that they are in excellent agreement. We also study other aspects of proximity effects such as the local density of states, local magnetizations, and thermodynamic functions. In conical-ferromagnet/superconductor bilayers, we obtain the relation between Tc and the thickness dF of the magnetic layer, and find that the Tc(dF) curves include multiple oscillations. Moreover, for a range of dF, the superconductivity is reentrant with temperature T: as one lowers T the system turns superconducting, and when T is further lowered it returns to normal. We demonstrate that the behavior of both m=0 and m=&plusmn1 triplet amplitudes are related to the intrinsic periodicity of conical ferromagnet. Our theoretical fits of Tc(dF) are in good agreement with experimental data. The transport properties, including the tunneling conductance and the spin polarized transport, in F1F2S trilayers are investigated. To fully take into account proximity effects, we adopt a transfer matrix method incorporated with the Blonder-Tinkham-Klapwijk formalism and self-consistent solutions to the BdG equations. We show that our method ensures that conservation laws are properly satisfied. Our results indicate that the behavior of tunneling conductance depends on the misorientation angle between magnetizations, and also exhibits resonance effects. We also investigate the bias dependence of non-equilibrium spin transfer torque and its connection to both spin currents and local magnetizations.

The ferromagnetic semiconductor EuO, which possesses several record properties relevant to devices, has been researched in an attempt to enhance and understand this novel material, i.e., raising its Curie temperature (TC) and developing it as a viable electronic material, by investigating three distinct research directions for EuO: chemical doping via trivalent rare-earth cations, biaxial strain via substrates with varying degrees of lattice mismatch, and preservation via application of a well-chosen capping layer. The use of Lu3+ as a chemical dopant for enhancing the TC of EuO up to 119 K was established. The spin-polarization for the Lu-doped EuO thin film was measured to be 96%, confirming its near-complete spin-polarization. The effect of biaxial strain on TC for EuO thin films that are commensurately strained to the substrate was explored. The use LuAlO3, which imparts an asymmetric tensile strain of +0.4% strain and +1.5% strain along the [110] and [110] EuO directions, respectively, resulted in the reduction of the TC for commensurate EuO / LuAlO3 thin films of varying thicknesses. The strain-dependence was confirmed by comparing the series to an identical series grown on the lattice-matched, i.e., unstrained, substrate yttria-stabilized zirconia (YSZ). Biaxial compressive strain was explored by growing EuO thin films on both singlecrystal diamonds and epitaxial diamond thin films grown on silicon. Diamond is 2.2% smaller than EuO, so epitaxial integration of EuO on diamond was attempted in hopes of achieving symmetric 2.2% compressive strain. Though the EuO was epitaxial and free of extraneous phases and orientations, demonstrating the first instance of a ferromagnet epitaxially integrated with diamond, the EuO was not compressively strained, and no change in TC was observed. Finally, the formation of a protective Eu2O3 capping layer by controlled oxidation of the surface of EuO in vacuum was studied. The Eu2O3 was effective at preventing the degradation of the underlying EuO, as determined by the scanning tunneling electron microscopy and electron energy loss spectroscopy analysis. The analysis, in conjunction with X-ray diffraction experiments, demonstrated that the oxygen diffusion during the capping layer formation is heavily influenced by the presence of defects in the film.

The various thin film systems studied in this dissertation are outlined in Chapter one. Two groups of nitride and oxide thin film semiconductors were studied. The nitride group includes c-InxGa1-xN (x = 12, 19, and 55%), c-(In0.06Ga0.94N/GaN)-SLs, and a-In0.74B0.26N. The oxide group includes a-In0.06Ga0.47Zn0.03O0.45 and various compositions of a-InxGayZnzOβ alloys. All films were grown using reactive magnetron sputtering except c-(In0.06Ga0.94N/GaN)-SLs, which were grown using metal {organic chemical vapor deposition (MOCVD). For c-InxGa1-xaN thin films, two crystallographic orientations are observed, (0002) and (101̅1). The optical bandgap decreases with increasing In at.%. In addition, we characterize the electrical properties using Hall effect measurements,and relate them to the structure and composition. c-(In0.06Ga0.94N/GaN)-SLs are studied by X-ray diffraction. Simulation for the as-deposited SL was achieved in the frame work of fully strained SL structure. It was found that implantation with Eu3+ ions causes partial degradation of the SL structure, which can be highly recovered by annealing at high temperatures. For a-In0.74B0.26N thin films, we mainly characterize the optical properties using spectroscopic ellipsometry (SE). The measured SE spectra are described very well by a two-layer model structure, which consists of a transparent layer on top of an absorbing layer. We believe that oxidizing the upper layer is caused by formed voids due to alloying with boron, that creates paths for oxygen to diffuse into the upper layer and oxidize it. The optical and magnetic properties of a-In0.06Ga0.47Zn0.03O0.45 thin films were studied in depth. The bandgap determined by various methods has an average value of 3.85 eV, and the re ectivity shows that a-In0.06Ga0.47Zn0.03O0.45 could be a good candidate for anti-reflective coating. We prove that some or all of the indium, gallium, and oxygen ions exist in mixed oxidation states, which manifest an induced FM interaction. We then engineer the optical bandgap and optical functions for various compositions of a-InxGayZnzOβ with relatively small In at.%. By changing the composition at.%, we were able to switch the FM interaction on and off, which is caused by switching the oxidations states of ions between mixed and pure states.

The work in this dissertation discusses the interplay between chemical, magnetic and orbital order. Polycrystalline double perovskites with isovalent A-cation substitutions are discussed in detail in Chapters 2 and 3. The crystal structure and physical properties are closely examined to understand the relationship between the two. In the second chapter the structural and magnetic properties of a perovskite system, BixLa2-xMnMO6 with M = Ni, Co for varying x is studied. It is seen that only less than 25% bismuth may be added to the perovskite and that some manganese substitutes onto the M site as Mn3+. Additionally the materials have magnetic and electrical properties, but these are not coupled to each other.It is shown in Chapter 3 that isovalent substitutions can dramatically alter the physical properties, even when the substituted cations are not directly responsible for the behavior. In particular the double perovskite SrMn0.5Ru0.5O3 has an antiferromagnetic C-type arrangement of magnetic spins. The addition of calcium in Sr(1-x)Ca(x)Mn0.5Ru0.5O3 for x ≥ 0.3 disrupts the orbitally-ordered driven system and resulting perovskites are ferromagnetic. The magnetization is much lower than expected for a genuine ferromagnet and neutron powder diffraction is suggestive of a complex magnetic arrangement.The final chapter investigates the possibility of double perovskite materials as thin films for both technological and scientific exploration. Specifically the novel growth of three double perovskites, Sr2CrNbO6, Sr2GaNbO6, and Sr2GaTaO6 are investigated. Pulsed laser deposition on stoichiometric targets is used to grow thin films on single crystal substrates. A number of growth parameters are explored. A detailed X-ray diffraction is carried out to determine the phase purity and lattice constants. Additional techniques are used to further characterize the surface features and metal content.

The study of quantum phase transitions (QPT) provides a new route to find and understand unconventional phases in condensed matter physics. The presently studied alloy, Ni(1-x)Vx, offers an opportunity to investigate a ferromagnetic quantum phase transition, a transition from a ferromagnetic ordered state into a paramagnetic state at T = 0 K, by varying the vanadium concentration, x. Magnetization and transport measurements are used to probe the critical behavior of the phase transition and characterize the onset of “unconventional behavior” such as non-Fermi liquid behavior, which signals a deviation from Fermi liquid theory, a fundamental concept in metals. Towards 11.2 % vanadium, the Curie temperature (Tc) is reduced to zero from its pure nickel value of Tc = 627 K. The critical behavior of the phase transition in samples with the higher nickel content (x < 11%) at a finite Tc essentially follows theories as expected for weak itinerant magnets. The samples with more vanadium (x > 11.2%) do not show a conventional ferromagnetic transition or the typical properties of an ordinary paramagnet. Instead, we see evidence for power laws with unusual exponents in the temperature dependence of the magnetization and the resistivity due to an inhomogeneous magnetic moment distribution. We compare our data findings with recent theories addressing a new critical scenario, quantum phase transitions with disorder. One signature is a Quantum Griffiths’ phase which is observed as power laws with non-universal exponents heading towards a T → 0 instability. At very low temperatures, the quantum Griffiths phase in Ni-V leads to the formation of a frozen cluster glass phase. To our knowledge, our compound is the first to experimentally show all signatures of a quantum Griffiths phase in an extended regime, and therefore provides an ideal model system for a disordered itinerant 3-d Heisenberg system.

Magnetic Resonance Force Microscopy (MRFM) is a novel approach to scanned probe imaging, combining the advantages of Magnetic Resonance Imaging (MRI) with Scanning Probe Microscopy. It has extremely high sensitivity that has demonstrated detection of individual electron spins and small numbers of nuclear spins.Here we describe our MRFM experiments on Ferromagnetic thin film structures. Unlike ESR and NMR, Ferromagnetic Resonance (FMR) is defined not only by local probe field and the sample structures, but also by strong spin-spin dipole and exchange interactions in the sample. Thus, imaging and spatially localized study using FMR requires an entirely new approach.In MRFM, a probe magnet is used to detect the force response from the sample magnetization and it provides local magnetic field gradient that enables mapping of spatial location into resonance field. The probe field influences on the FMR modes in a sample, thus enabling local measurements of properties of ferromagnets. When sufficiently intense, the inhomogeneous probe field defines the region in which FMR modes are stable, thus producing localized modes. This feature enables FMRFM to be important tool for the local study of continuous Ferromagnetic samples and structures.In our experiments, we explore the properties of the FMR signal as the strength of the local probe field evolves from the weak to strong perturbation limit. This underlies the important new capability of Ferromagnetic resonance imaging, a powerful new approach to imaging ferromagnet. The new developed FMR imaging technique enables FMR imaging and localized FMR spectroscopy to combine spectroscopy and lateral information of ferromagnetic resonance images.Our theoretical approach agrees well with spatially localized spectroscopy and imaging results. This approach also allows analysis and reconstruction of FMR modes in a sample.Finally we consider the effect of strong probe fields on FMR modes. In this regime the probe field significantly modifies the FMR modes. In particular we observe the complete local suppression of the FMR mode under the probe. This provides as a new tool for local study of continuous ferromagnetic thin films and microstructures.