Ultrafast Opto-thermal measurements of spin-Seebeck effect

The spin-Seebeck effect (SSE) remains one of the most intriguing and potentially useful spin-thermal phenomena of the last decade. In conjunction with the inverse-spin hall effect, the SSE results in the generation of an electric potential from a thermally-induced spin current. It has been reported in magnetic metals, semiconductors, and insulators and has been measured in a variety of different configurations, most recently in the opto-thermal setup reported by Weiller. In this method a laser pulse generates a temperature gradient that induces a spin current to flow from an insulating ferromagnet into a spin Hall metal, thereby inducing a transverse voltage. The underlying physics of the SSE remain somewhat elusive, although theoretical work explains that the SSE develops due to temperature gradients between the local magnon, phonon and electron populations within the magnetic material. We attempt to clarify the physics by presenting the first SSE induced transient electric current measurements from a custom built cryogenic opto-thermal setup. Single femtosecond laser pulses are used to generate transient thermal gradients allowing insight to the time-dependence of the SSE. Photo-carrier effects on SSE materials acquired by systematically tuning the wavelength of the femtosecond laser above the bandgap of the material will also be reported.