In recent years, Spin Seebeck effect (SSE) emerges as one of the efficient and easiest ways to generate pure spin current for spintronics devices. In this dissertation, we have systematically studied the SSE and related phenomena like spin Hall magneto-resistance (SMR), anomalous Nernst effect (ANE) in functional magnetic oxides for both fundamental understanding of their origins and practical ways to apply into technological devices. The research has been performed on three different systems of topical interest: (i) Y3Fe5O12 (YIG)/Pt and YIG/C60/Pt, (ii) CoFe2O4 (CFO)/Pt and CFO/C60/Pt, and (iii) Nd0.6Sr0.4MnO3 (NSMO). In case of the YIG/Pt structure, we have achieved a new consensus regarding the temperature dependence of the longitudinal SSE (LSSE). For the first time, we have demonstrated the temperature dependence of LSSE in association with the magnetocrystalline anisotropy (HK) and surface perpendicular magnetic anisotropy field (HKS) of YIG in the same YIG/Pt system. We show that on lowering temperature, the sharp drop in LSSE signal (VLSSE) and the sudden increases in HK and HKS at ~175 K are associated with the spin reorientation due to single ion anisotropy of Fe2+ ions. The VLSSE peak at ~75 K is attributed to the HKS and MS (saturation magnetization) whose peaks also occur at the same temperature. The effects of surface and bulk magnetic anisotropies are corroborated with those of thermally excited magnon number and magnon propagation length to satisfactorily explain the temperature dependence of LSSE in the Pt/YIG system. As a new way to reduce conductivity mismatch, promote spin transport, and tune the spin mixing conductance (G) at the YIG/Pt interface, we have deposited an organic semiconductor (OSC), C60, between ferrimagnetic material (FM) and Pt. Transverse susceptibility study on YIG/C60/Pt has shown that the deposition of C60 has reduced HKS at the surface of YIG significantly, due to the hybridization between the dz2 orbital in Fe and C atoms, leading to the overall increase in spin moments and G and consequently the LSSE. Upon lowering temperature from 300 K, we have observed an exponential increase in LSSE at low temperature (a ~800% increment at 150 K) in this system, which is attributed to the exponential increase in the spin diffusion length of C60 at low temperature. On the other hand, similar experiments on CoFe2O4 (CFO)/C60/Pt show a reduction in the LSSE signal at room temperature, due to the hybridization between the dz2 orbital in Co and C atoms that results in the increased magnetic anisotropy. Upon decreasing the temperature below 150 K, we have interestingly observed that LSSE signal from CFO/C60/Pt exceeds that of CFO/Pt and increases remarkably with temperature. This finding confirms the important role played by the spin diffusion length of C60 in enhancing the LSSE. A systematic study of SMR, SSE, and HKS on the YIG/Pt system using the same YIG single crystal has revealed a low-temperature peak at the same temperature (~75 K) for all the phenomena. Given the distinct origins of the SSE and SMR, our observation points to the difference in spin states between the bulk and surface of YIG as the main reason for such a low-temperature peak, and suggests that the ‘magnon phonon drag’ theory developed to explain the temperature-dependent SSE behavior should be adjusted to include this important effect. SSE and ANE studies on NSMO films have revealed the dominance of ANE over SSE in this class of perovskite-structured materials. The substrate-dependent study of the films shows that compressive strain developed due to the large lattice mismatch from LAO gives rise to the enhanced ANE signal. On the same substrate, ANE signal strength increases as the thickness increases. A sign change in ANE has been observed at a particular temperature, which explains that the Anomalous Hall effect (AHE) and ANE in these systems arise due to intrinsic scattering mechanisms. Overall, we have performed the SSE and related studies on the three important classes of functional magnetic oxide materials. We demonstrate the important role of magnetic anisotropy in manipulating the SSE in these systems. With this knowledge, we have been able to design the novel YIG/C60/Pt and CFO/C60/Pt heterostructures that exhibit the giant SSEs. The organic semiconductor C60 has been explored for the first time as a means of controlling pure spin current in inorganic magnetic oxide/metal heterostructures, paying the way for future spintronic materials and devices.

The main objective of this thesis was to bring the first experimental evidence about the spin-Hall magnetoresistance (SMR) in the Terahertz (THz) spectral range. The time-domain THz spectroscopy (TDTS) was chosen as the main method, and we presented a new experimental scheme, which allows us to observe SMR or magnetoresistive effects with similar symmetry at a wide range of THz frequencies very efficiently. We focused on the study of SMR in the bilayers consisting of either a prototypical ferrimagnetic isolator or of heavy metal (FI/HM) and metallic stacks of ferromagnetic CoFeB and heavy metal Pt layer (FM/HM). While SMR shows a rapid decrease already at the lowest THz frequencies (< 1 THz) in the FI/HM structures, the SMR in FM/HM bilayers persists above 30 THz. These observations are then explained by a different mechanism of SMR. The second part of the thesis is devoted to the construction of the new TDTS setup and an easy-to-use model of the THz setup using the Gaussian description of THz radiation. The simulated results have been compared to corresponding experimental measurements using a spintronic THz emitter (STE). The last part of the thesis describes the emission of intensive THz pulses from large-area STE.

Magnetoresistive effects are powerful tools for studying the intricate structure of solid state electronic systems, and have many applications in our current information technology. In particular, the electronic system reflects the crystal symmetry and the orbital structure of the atoms of a given solid, and thus is crucial to understanding magnetism, superconductivity and many other effects which are of key interest to current solid state research. Consequently, studies of the electrical transport properties of solid state matter allow to evaluate this imprint and in turn draw conclusions about the interactions within a material. In this thesis, we will exploit the capabilities of magnetotransport measurements to infer the properties of a multitude of magnetic systems. In turn, this allows us to push the understanding of transport phenomena in magnetic materials. The first part of this work is focused on the magnetoresistance observed in spin Hall active metals in contact with a magnetic insulator. In such bilayers, the interfacial spin accumulation caused by the spin Hall effect in the metal can interact with the magnetic insulator, giving rise to interesting magnetoresistive effects. In the framework of this thesis, bilayers with several magnetic insulators are studied, including antiferomagnets, ferrimagnets and paramagnets (disordered magnets). For the disordered magnetic insulators, we find that the established spin Hall magnetoresistance framework does not allow to consistently describe the observed transport response. Consequently, we propose an alternative explanation of the magnetoresistance in such heterostructures, using the Hanle magnetoresistance and assuming an interface which has a finite electrical conductivity. This alternative model can serve to generalize the theory of the spin Hall magnetoresistance, providing addition information on the microscopic picture for the loss of the transverse spin component. Additionally, by partly removing the magnetic insulator and studying the ensuing changes, we verify that magnons are crucial for the observation of a non-local magnetoresistance in bilayers of a magnetic insulator and a metal. Finally, the local and non-local spin Seebeck effect (i.e. the electric field generated by a thermally driven pure spin current) is investigated in bilayers of Cr2O3 and Pt where the occurrence of a spin superfluid ground state was reported. In our sample, however, the transport response is consistent with the antiferromagnetic spin Seebeck effect mediated by the small magnetic field induced magnetization also reported for other antiferromagnet/metal heterostructures. As such, we cannot verify the presence of a spin superfluid ground state in the system. In the second part of this thesis, the topological properties of the electronic system and the related changes of the magnetoelectric and magnetothermal transport response are investigated. To that end, we first demonstrate a novel measurement technique, the alternating thermal gradient technique, allowing to separate the relevant thermovoltages from spurious other voltages generated within the measurement setup. We employ this novel technique for measuring the topological Nernst effect in Mn 1.8 PtSn and show the possibility to combine the magnetoelectric and magnetothermal transport response to evaluate the presence of topological transport signatures without requiring magnetization measurements. Additionally, we show that the anomalous Nernst effect in the non-collinear antiferromagnet Mn3Sn is connected to the antiferromagnetic domain structure: Using spatially resolved measurements of the anomalous Nernst effect, direct access to the antiferromagnetic domain structure is demonstrated. Additionally, a thermally assisted domain writing scheme is implemented, allowing the preparation of Mn3Sn into a defined antiferromagnetic domain state.