Spin-Thermoelectric Generation Associated with Magnetization Dynamics in the Insulator-Based Generators Fabricated from Liquid Phase Epitaxial Yttrium Iron Garnet, Bi-Substituted YIG and Bi- and Al-Substituted YIG Films

An insulator-based spin-thermoelectric (STE) generator is composed of a thin paramagnetic metal (PM: Pt) layer for generating the STE voltage V_STE via the inverse spin Hall effect and a ferrimagnetic insulator (FMI) film or slab used for producing spin-wave spin currents due to a temperature gradient $\nabla T$. Yttrium iron garnet (YIG) is a key material used for the FMI of STE generators. We examined in detail the ferromagnetic resonance (FMR) properties of YIG (Y₃Fe₅O₁₂), Bi-substituted YIG (Y_3-xBi_xFe₅O₁₂) and Bi- and Al-substituted YIG (Y_3-xBi_xFe_5-yAl_yO₁₂) films grown by liquid phase epitaxy (LPE). Bi-substituted YIG films exhibited the large FMR damping and high STE voltage when the films were incorporated in the STE generator. The LPE-grown Y_2.35Bi_0.65Fe₅O₁₂ film exhibited the FMR linewidth ΔH of 30 mT and the V_STE of 70 μV for the ∇T of $90\times {10}^{-3}°C/\mathrm{\mu m}$. This paper comprehensively presents the origin of the STE generation in the insulator-based generators on the basis of the results of FMR and STE measurements. To clarify the origin of STE generation in the generators fabricated from single crystal YIG (YIG, Bi-substituted YIG and Bi- and Al-substituted YIG) films grown by LPE, three principal factors are explained: (a) thermal energy transfer from the phonon system to the spin system, which strengthens the heat excitation of spin precession, through the spin–orbit coupling enhanced by the growth-induced magnetic anisotropy of LPE-grown YIG films, (b) in the YIG films incorporated in the STE generators, the generation of spin-wave spin currents owing its origin to the ferromagnetic spin exchange interaction acting between neighboring spins due to $\nabla T$, and (c) how the spin currents pumped into the Pt layer from the YIG film at the Pt/YIG bilayer interface are affected by the magnetization processes of single crystal YIG films. It is concluded that the STE generation of insulator-based STE generators can be explained from a viewpoint of the spin dynamics under the effect of $\nabla T$ in the Pt/YIG bilayer structure. © 2024 Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.