Spin Seebeck effect and related phenomena in functional magnetic oxides

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