Magnon shot noise in a longitudinal spin Seebeck device*

If an angular momentum current in a magnetic insulator is carried by discrete magnons, each with angular momentum ℏ, there is expected to be a fundamental angular momentum current noise analogous to charge shot noise.  In the spin Seebeck effect (SSE), an angular momentum current is driven by a temperature gradient, and this may be detected with a Pt wire through the inverse spin Hall effect (ISHE).  In a longitudinal SSE structure based on a yttrium iron garnet film/gadolinium gallium garnet substrate, at low temperatures in addition to the spin Seebeck voltage we measure the voltage noise in the ISHE.  We find an additional magnetic field-dependent component of the noise when the field is oriented to produce a non-zero spin Seebeck voltage.  We compare this noise with rough theoretical estimates for a possible magnon shot noise detected via the ISHE.