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The wider application of spintronic devices requires the development of several new material platforms that can address the following challenges: 1) efficient conversion between spin and charge currents, 2) storage of magnetic information that can be easily manipulated, and 3) transmission of spin information that can be easily detected. With respect to the first challenge, Bismuthate-based superconductors are systems that are generally thought to offer weak spin-orbit coupling, despite the heavy elements that make up such compounds. Here we use spin-torque ferromagnetic resonance to measure a large spin-orbit torque efficiency driven by spin polarization generated in heterostructures based on BaPb$_{1-x}$Bi$_{x}$O$_3$ in a non-superconducting state. We suggest that the unexpectedly large current-induced torques could stem from an orbital Rashba effect associated with local inversion symmetry breaking in BaPb$_{1-x}$Bi$_x$O$_3$.In response to the second challenge, Bismuth Ferrite has garnered considerable attention as a promising candidate for magnetoelectric spin-orbit coupled logic-in memory. While this model system offers a magnetic texture controllable by electric fields, epitaxial BiFeO$_3$ films have typically been deposited at temperatures higher than allowed for direction integration with silicon-CMOS platforms. Here, we solve this engineering problem by growing La-doped BiFeO$_3$ at temperatures reasonably compatible with silicon-CMOS integration on BaPb$_{1-x}$Bi$_x$O$_3$ electrodes. Despite the large lattice mismatch between the two materials, all layers are well-ordered with a [001] texture, and the La-doped BiFeO$_3$ exhibits desirable ferroelectric properties. These results provide a possible route for realizing epitaxial multiferroics on complex-oxide buffer layers at low temperatures and opens the door for potential silicon-CMOS integration. Furthermore, the incorporation of a material with efficient conversion between spin and charge with a multiferroic will drive innovation and application of new spintronic devices, such as all-oxide multiferroic magnonic memory architectures.In response to the second and third challenges, spin waves in magnetic materials, or magnons, are promising information carriers due to their ultra-low energy dissipation and long coherence length. Antiferromagnets are strong candidate materials for magnetic information storage due in part to their stability to external fields and larger group velocities. Multiferroic antiferromagnets such as BiFeO$_3$ have an additional degree of freedom stemming from magnetoelectric coupling, allowing for control of the magnetic structure, and thus magnons, with an electric field. Unfortunately, spin-wave propagation in BiFeO$_3$ is not well-understood due to the complexity of the magnetic structure. In this work, we discover an anisotropy in spin transport within the spin cycloid magnetic structure of BiFeO$_3$. We also show that through Lanthanum substitution, a single ferroelectric domain can be engineered with a stable, single-variant spin cycloid controllable by electric field. The strong anisotropy discussed is an important development in the understanding of magnon-spin currents in the spin cycloid magnetic texture.Finally, understanding the anisotropies and other characteristics of magnon-spin currents in a multiferroic requires an understanding of the symmetries inherent to the magnetic and polar orders of the multiferroic. In this work, we present a phenomenological model to elucidate the existence of magnon spin currents in generalized multiferroics. This model takes inspiration from the symmetries of multiferroics such as BiFeO$_3$, and is grounded in experimental data obtained from BiFeO$_3$ and its derivatives. By introducing this model, we address the issue of symmetry-allowed, switchable magnon spin transport in multiferroics, thereby establishing a critical framework for comprehending magnon transport within complex magnetic textures.Our responses to the three challenges posed above all point to a magnon-based multiferroic memory founded in epitaxial oxide heterostructures. In researching a path towards this application, we make new discoveries of the physics of spin-charge interconversion, heterostructure growth, and magnon dynamics in complicated magnetic textures. We hope that the following research will prove applicable not only towards the specific goal of multiferroic magnonic memory, but also to the wider field of epitaxial oxide spintronics.

This dissertation details methods of manipulating and dynamic studies of ferroic order in the multiferroic material, BiFeO3, and related material systems. The primary advances made as a result of the work described herein are understanding of intrinsic and extrinsic limits on ferroelectric switching (in particular free- and bound-charge dynamics, the role of, and pathways to manipulate the Landau free energy landscape and lattice dynamics) and implications for switching of magnetic order, mechanisms and factors impacting depolarization in ferroelectric materials, and a novel manifestation of magnetoelectric coupling enabling nonvolatile electric field control of magnon spin transport.

Magnons, i.e. the quanta of spin waves, are considered to be promising information carriers. Unlike electric currents, magnon-based spin currents could be used to transport information coherently over long distance without generating any Joule heat. Magnons in antiferromagnetic materials exhibit resonance frequencies extending up to the THz frequency range, which promises rapid response of magnon-based devices. However, it becomes difficult to control such rapid oscillations of magnetization using circuit-based electronic control. Instead, optical techniques have been investigated for the generation and control of AF magnons – however most techniques have been based upon ultrafast near-IR pump-probe techniques. In this thesis, I investigate the feasibility of designing electromagnetic metastructures with subwavelength effective cavity volumes to realize strong light-matter coupling between suitable antiferromagnetic materials operating at over 1 THz. In these systems, light and material excitations are strongly coupled and mixed into superposition states with hybrid dispersion relation, which are termed as polaritons. Such magnon-polariton systems are of interest as they enable coherent information transferring between distinct physical platforms. While polaritons have been demonstrated between GHz-frequency photons and ferromagnetic magnons, only limited reports have been found on antiferromagnetic magnon-polaritons. Moreover, as a potential application, the feasibility of THz magnon-polariton lasers is studied. In this thesis, I propose an idea which combines metal-metal waveguide with LC circuit- based microcavity, and introduce a design of metastructure with strong evanescent magnetic field, enabling the generation of magnon-polaritons even in a 200 nm thin antiferromagnetic film. Quantum cascade active region with intersubband transitions falling into THz frequency range can be applied into the metastructure, enabling direct amplification of magnon-polaritons. Magnon-polariton lasing becomes possible when this hybrid active metastructure/ antiferromganet is paired with an output coupler, building up a quantum cascaded vertical external cavity surface emitting laser (QC-VECSEL). The opportunity of magnon-polariton quantum cascade laser is also discussed when a thick antiferromagnetic slab is inserted in a standard VECSEL cavity.

The development of ultrafast spectroscopy gives us access to the time dimension to study the dynamics of condensed matter systems. Different from conventional condensed matter experimental techniques, the pump-probe technique allows us not only to study the relaxation of nonequilibrium to equilibrium states, but also the possibility to selectively control the properties of a material using ultrafast light. In my thesis, I will introduce the fundamental of optics in the first chapter. Second chapter consists of the experimental set-ups I developed and built during my Ph.D.: Terahertz time-domain spectroscopy (THz-TDS), broadband THz source from two-color laser-induced gas plasma, intensive mid-infrared pump Kerr rotation probe and data acquisition (DAQ) system. The third chapter is the background of spin-orbit coupled Mott insulator Sr2IrO4,and the last chapter includes my work of ultrafast magnetic control in Sr2IrO4: we study the magnetic dynamics of the Jeff = 1/2 Mott state using strong mid-Infrared 9 µm (below the charge gap), and near-infrared 1.3 µm (above the charge gap) circularly polarized excitations, and monitor the pump induced Kerr signal. For both pump wavelength, the 2D in-plane B2g coherent magnon oscillation of frequency ~ 0.5 THz was observed in the pump-induced Kerr rotation signal. The circularly polarized 9 µm pumps of opposite helicities excite oscillations of opposite phase, while 1.3 µm pumps excite oscillations of same phase. The quadratically scaling of the fluence dependent magnon amplitude for the 9 µm pump indicates a novel photon-two-magnon coupling mechanism for the magnon generation. The directly excitation (9 µm) of the spin spectrum without photodoping electrons permits extremely efficient magnon generation, almost ten times better than the resonant 1.3 µm pump.

The anticipated end of Moore’s law has accelerated the exploration and development of new approaches to more scalable and more functional logic devices that consume less power. Of the promising technologies that have emerged, magnon spintronics is innovative for its use of magnetic excitation called spin waves. There are several appealing properties inherent in spin waves. For instance, the spin wave can exhibit nanometer wavelengths and micrometer travel distances. This advantage brings spin-based devices with scalability, which can be a substitute and continues the trend of Moore’s law for minimal sized devices. Importantly, spin waves provide Joule-heat-free transfer of information, since the spins do not involve the motion of electrons. This property can prevent excessive energy consumption compared to CMOS. Also, because spin wave devices utilize phase in addition to amplitude, their operations are based on vectors instead of scalar variables, that makes spin waves devices more functional compared to the conventional digital circuitry. I have dedicated myself to studying the excitation, propagation, and control of spin waves in magnetic micro- and nanostructures. This dissertation is based on my published works, where I contributed to make spin wave devices based on Y3Fe5O12 (YIG) structures, as well as on more recent results based on Ni80Fe20 (Py) nanostructures. YIG and Py are the best materials for prototyping spin wave devices. YIG is an excellent material for building spin wave-based devices because of its extremely low magnetic damping and its status as an insulator. Then, I give results based on Ni80Fe20 (Py) nanostructures. Py has relatively low spin wave damping among conducting materials and is compatible with CMOS technology. Investigation of spin waves in Py nanostructures is expected to lead to the building of scalable and multi-functional devices with capabilities far beyond the scaled CMOS. In this dissertation, I presented spin wave excitation and propagation in magnetic ferrimagnetic and ferromagnetic materials, as well as innovative structural design and spin wave controls for novel applications. I believe this dissertation will contribute greatly to the community.

This dissertation reports the investigation into temperature dependence of magnon and phonon energies, and the strength of spin – phonon coupling in antiferromagnetic nickel oxide (NiO) - an important material for spintronic devices. A combination of Brillouin–Mandelstam and Raman spectroscopy techniques were used to elucidate the evolution of the phonon and magnon spectral signatures from the Brillouin zone center (GHz frequency range) to the second-order peaks from the Brillouin zone boundary (THz frequency range). The temperature dependent behavior of the magnon and phonon bands in the NiO spectrum indicates the presence of the antiferromagnetic order fluctuation or a persistent antiferromagnetic state, at temperatures substantially above the Neel temperature (TN = 523 K). Tuning the intensity of the excitation laser provided a method for disentangling the magnon signatures from those of the acoustic phonons in scattering spectra of NiO without the need for a magnetic field. The ultraviolet Raman spectroscopy was utilized in order to determine the spin-phonon coupling coefficients. The analysis of the second-order phonon scattering and ultraviolet laser excitation (λ=325 nm) was essential for overcoming the problem of the optical selection rules and dominance of the two-magnon band in the visible Raman spectrum of NiO. The results of this study helped to establish that the spins of Ni atoms interact more strongly with the longitudinal optical (LO) phonons than the transverse optical (TO) phonons and produce an opposite effect on the phonon energies. The experimentally determined characteristics of the spin-phonon coupling were found to be in agreement with the trends calculated by density functional theory. The results of this dissertation research can be used for the interpretation of the inelastic-light scattering spectrum of NiO and other antiferromagnetic materials. They also shed light on the nature of the spin-phonon coupling in antiferromagnetic insulators, which is important for developing spintronic devices.

This thesis describes three experiments in the topics of magnetism and superconductivity. The first experiment demonstrates the ability to have magnetic control of the triplet component amplitude in a Nb/Co/Cu/Co/CoO superconducting spin valve. The experiment is done by measuring the superconducting transition temperature, Tc, in the multilayers as a function of the angle alpha between the magnetic moments of the Co layers. The measurements reveal that Tc(alpha) is a nonmonotonic function, with a minimum near alpha = pi/2. The experimental data were compared with numerical self-consistent solutions of the Bogoliubov--de Gennes equations calculated by our collaborators. This thesis shows that experimental data and theoretical evidence agree in relating Tc(alpha) to enhanced penetration of the triplet component of the condensate into the Co/Cu/Co spin valve in the maximally noncollinear magnetic configuration. The second experiment is a crystallographic characterization of a magnetron sputtered IrMn3 thin film which is a metallic non-collinear antiferromagnet. It has been predicted to have a large anomalous Hall effect, which enables magnetic state readout in antiferromagnetic spintronic devices. The third experiment is a demonstration of unusual magneto-dynamic phenomena in nanoscale ferromagnets due to resonant three-magnon scattering. In this work, some numerical calculations are presented of a theory made by a collaborator.

Spin current transport plays an important role in modern solid state physics. Research efforts on this field not only reveal fundamental principles, but also promote engineering applications. We explore spin current transport in various heterostructures.We first give a brief introduction to the recent advancements in spin current transport, including the observation of spin Hall effect (SHE) in materials with strong spin-orbit coupling, the discovery of spin Seebeck effect (SSE) in both conductive and insulating magnetic materials, and the realization of spin current transport without free charge motion in magnetic insulators. Then we present the results related to spin current transport in ferromagnetic metal (FM)/normal metal (NM) heterostructures, driven by spin pumping or heat flow. In the spin pumping experiment, we observe inverse SHE signals at ferromagnetic resonance states and anomalous Nernst effect (ANE) signals due to a vertical temperature gradient. In the longitudinal SSE experiment, we disentangle SSE, ANE and proximity effect contributions and discover the spin current draining effect, i.e., an adjacent NM changes the spin chemical potential and induces an additional spin current in the FM.The third part provides details about the observation of magnon-mediated current drag effect. We grow high quality Pt/yttrium iron garnet (YIG)/Pt(Ta) trilayer heterostructures and realize electronic signal transmission through YIG, a magnetic insulator, by magnon current. A charge current in the bottom Pt layer induces spin accumulation by SHE. The electronic spins convert to magnons in YIG and convert back to electronic spins in the top Pt(Ta) layer.The study of topological SSE in YIG/topological insulator (TI) heterostructures is presented at last. Magnons driven by a vertical temperature gradient are converted to a charge current in TI surface states. We tune the Fermi level by changing TI composition ratio or applying a top gate and demonstrate the dominant role of the topological surface states in the magnon-charge conversion.