A magnetic insulator, LaY₂Fe₅O₁₂, has been shown to generate a spin voltage in response to a temperature gradient, a phenomenon known as the spin Seebeck effect (SSE). This occurs without the need for conduction electrons, as the insulator converts heat into spin voltage through magnetization dynamics. The spin voltage is then converted into an electric voltage by the inverse spin Hall effect (ISHE) in Pt films. The study reveals that thermally activated interface spin exchange between LaY₂Fe₅O₁₂ and Pt is essential for this process. The results extend the range of materials for thermoelectric applications and provide insights into the physics of the SSE. The SSE is analogous to the Seebeck effect but involves spin angular momentum rather than charge. In metallic magnets, the SSE has been observed, but its microscopic mechanism remains unclear. The current study demonstrates that the SSE can occur in a magnetic insulator, indicating that thermally induced spin voltage is associated with magnetization dynamics. This mechanism allows for a new understanding of the SSE in general. The experiment involved a LaY₂Fe₅O₁₂ film with Pt wires attached, where a temperature gradient was applied along the x-direction. The ISHE in Pt converted the spin current generated in the LaY₂Fe₅O₁₂ layer into an electric field. The electric voltage was measured between the Pt wires, showing a dependence on the temperature gradient and magnetic field. The sign of the voltage reversed with the direction of the magnetic field, indicating the role of magnetization reversal. The study also showed that the spin voltage depends on the spatial distribution of the temperature gradient. The voltage varied with the position of the Pt wires relative to the center of the LaY₂Fe₅O₁₂ layer, consistent with the behavior of the SSE in metallic films. The results suggest that the SSE in insulators is driven by an unconventional non-equilibrium state between magnetic moments and electrons. The SSE in magnetic insulators has potential applications in thermo-spin generators and thermoelectric generators. The efficiency of thermoelectric generation can be improved by reducing thermal conductivity and electric resistivity. The SSE in insulators allows for a new principle of thermoelectric devices, with the figure of merit being small but potentially enhanced by reducing phonon contributions and improving interface spin-exchange efficiency. The study provides a new understanding of the SSE in magnetic insulators, demonstrating that the effect can occur without conduction electrons and is driven by spin waves or magnons. This finding has implications for the physics of metallic magnets, suggesting that magnons play a key role in the SSE of both insulators and metals. The results highlight the importance of interface spin exchange and the potential for new thermoelectric technologies.A magnetic insulator, LaY₂Fe₅O₁₂, has been shown to generate a spin voltage in response to a temperature gradient, a phenomenon known as the spin Seebeck effect (SSE). This occurs without the need for conduction electrons, as the insulator converts heat into spin voltage through magnetization dynamics. The spin voltage is then converted into an electric voltage by the inverse spin Hall effect (ISHE) in Pt films. The study reveals that thermally activated interface spin exchange between LaY₂Fe₅O₁₂ and Pt is essential for this process. The results extend the range of materials for thermoelectric applications and provide insights into the physics of the SSE. The SSE is analogous to the Seebeck effect but involves spin angular momentum rather than charge. In metallic magnets, the SSE has been observed, but its microscopic mechanism remains unclear. The current study demonstrates that the SSE can occur in a magnetic insulator, indicating that thermally induced spin voltage is associated with magnetization dynamics. This mechanism allows for a new understanding of the SSE in general. The experiment involved a LaY₂Fe₅O₁₂ film with Pt wires attached, where a temperature gradient was applied along the x-direction. The ISHE in Pt converted the spin current generated in the LaY₂Fe₅O₁₂ layer into an electric field. The electric voltage was measured between the Pt wires, showing a dependence on the temperature gradient and magnetic field. The sign of the voltage reversed with the direction of the magnetic field, indicating the role of magnetization reversal. The study also showed that the spin voltage depends on the spatial distribution of the temperature gradient. The voltage varied with the position of the Pt wires relative to the center of the LaY₂Fe₅O₁₂ layer, consistent with the behavior of the SSE in metallic films. The results suggest that the SSE in insulators is driven by an unconventional non-equilibrium state between magnetic moments and electrons. The SSE in magnetic insulators has potential applications in thermo-spin generators and thermoelectric generators. The efficiency of thermoelectric generation can be improved by reducing thermal conductivity and electric resistivity. The SSE in insulators allows for a new principle of thermoelectric devices, with the figure of merit being small but potentially enhanced by reducing phonon contributions and improving interface spin-exchange efficiency. The study provides a new understanding of the SSE in magnetic insulators, demonstrating that the effect can occur without conduction electrons and is driven by spin waves or magnons. This finding has implications for the physics of metallic magnets, suggesting that magnons play a key role in the SSE of both insulators and metals. The results highlight the importance of interface spin exchange and the potential for new thermoelectric technologies.