Ultrafast Spin Seebeck Measurements on Rare-Earth Iron Garnets*

Understanding the thermal generation of spin currents in magnetic materials is an important goal for the field of spin caloritronics. Among magnetic materials, rare earth iron garnets (REIG) display intriguing magnetic transport properties as result of strong antiferromagnetic exchange interactions and low magnetic damping. We report on ultrafast longitudinal spin Seebeck effect (LSSE) experiments on thin film REIG / heavy metal (HM) heterostructures (RE: Y, Tm, Eu, Tb; HM: Au). We use time-resolved magneto optic Kerr effect measurements to directly observe the transfer of magnetization from the REIG into the HM on femto-second timescales, allowing us to selectively probe the interfacial SSE. We observe a factor of 4 difference in the magnitude of the LSSE among the different REIG samples. Our results provide insight regarding the different contributions to the spin current from the different REIGs and the relevance of the interface between the REIG and the HM layers.