The paper reports the observation of the spin Seebeck effect (SSE) in a magnetic insulator, LaY₂Fe₅O₁₂, which converts heat flow into spin voltage. This effect is demonstrated by attaching Pt films to the LaY₂Fe₅O₁₂ layer, where the inverse spin Hall effect (ISHE) converts the spin voltage into electric voltage. The experimental results require the introduction of thermally activated interface spin exchange between LaY₂Fe₅O₁₂ and Pt. The findings extend the potential materials for thermoelectric applications and provide insights into the physics of the SSE. The SSE in magnetic insulators offers new opportunities for designing thermo-spin generators and thermoelectric generators, potentially improving efficiency by reducing heat conduction and Joule dissipation. The mechanism involves the dynamical spin coupling between LaY₂Fe₅O₁₂ and Pt, distinct from static spin-injection models used in metallic systems.The paper reports the observation of the spin Seebeck effect (SSE) in a magnetic insulator, LaY₂Fe₅O₁₂, which converts heat flow into spin voltage. This effect is demonstrated by attaching Pt films to the LaY₂Fe₅O₁₂ layer, where the inverse spin Hall effect (ISHE) converts the spin voltage into electric voltage. The experimental results require the introduction of thermally activated interface spin exchange between LaY₂Fe₅O₁₂ and Pt. The findings extend the potential materials for thermoelectric applications and provide insights into the physics of the SSE. The SSE in magnetic insulators offers new opportunities for designing thermo-spin generators and thermoelectric generators, potentially improving efficiency by reducing heat conduction and Joule dissipation. The mechanism involves the dynamical spin coupling between LaY₂Fe₅O₁₂ and Pt, distinct from static spin-injection models used in metallic systems.