Thermographic measurements of spin-current-induced temperature modulation in metallic bilayers

Temperature modulation due to a pure spin current has been investigated in bilayer metallic films consisting of a paramagnetic metal (PM; Pt, W, or Ta) and a ferromagnetic metal (FM; CoFeB or permalloy). When a charge current is applied to the PM/FM bilayer film, a spin current is generated across the PM/FM interface owing to the spin Hall effect in PM. The spin current was found to exhibit cooling and heating features depending on the sign of the spin Hall angle of PM; the spin-current-induced temperature modulation is estimated by subtracting the contribution of the anomalous Ettingshausen effect in FM monolayer films, and attributed to the conduction-electron-driven spin-dependent Peltier effect and magnon-driven spin Peltier effect in FM. To reveal the origin of the spin-current-induced contribution in the PM/FM films, we compared the experimental results with the phenomenological calculations based on the spin and magnon diffusion models. We found that the spin-current-induced temperature modulation is greater than that expected from the spin-dependent Peltier coefficients reported in earlier studies and its characteristic length is around 10 nm, possibly larger than typical spin-diffusion lengths of conduction electrons in FM. These facts indicate that the signals in the PM/FM films contain the substantial contribution from the magnon-driven spin Peltier effect.