Transport between metals and magnetic insulators

We derive the Onsager response matrix of fluctuation-mediated spin-collinear transport through a ferromagnetic insulator and normal metal interface driven by a temperature difference, spin accumulation, or magnetic field. We predict magnon-squeezing spin currents, magnetic field-induced cooling (magnon Peltier effect), temperature induced magnetization (thermal magnetic field) as well as universal spin Seebeck/Peltier coefficients.