  We experimentally study the transient voltage response of yttrium iron
garnet/platinum bilayer samples subject to periodic heating up to gigahertz
frequencies. We observe an intrinsic cutoff frequency of the induced
thermopower voltage, which characteristically depends on the thickness of the
yttrium iron garnet film. The cutoff frequency reaches values of up to 350 MHz
in a 50 nm thick yttrium iron garnet film, but drops to below 1 MHz for
several-micrometer-thick films. These data corroborate the notion that the
magnon spectrum responsible for the spin current emission in the spin Seebeck
effect can be shaped by tuning the thickness of the ferromagnetic layer.
