The Spin Seebeck Effect (SSE) generates a spin current from a temperature gradient in a magnetic insulator (e.g. YIG). Magnons — quantized spin waves — have a thermal distribution that shifts with temperature: hotter regions have more magnons. The magnon chemical potential gradient drives a spin current Js ∝ −∇T, detected via the inverse spin Hall effect (ISHE) in an attached Pt layer as a voltage signal.