  For longitudinal spin Seebeck effect (LSSE) devices, a multilayer structure
comprising ferromagnetic and nonmagnetic layers is expected to improve their
thermoelectric power. In this study, we developed the fabrication method for
alternately stacked yttrium-iron-garnet (YIG)/Pt multilayer films on a
gadolinium gallium garnet (GGG) (110) substrate, GGG/[YIG(49 nm)/Pt(4 nm)]$_n$
($n =$ 1 - 5) based on room-temperature sputtering and $ex$-$situ$
post-annealing method and we evaluated their structural and LSSE properties.
The fabricated [YIG/Pt]$_n$ samples show flat YIG/Pt interfaces and almost
identical saturation magnetization $M_{\rm s}$, although they contain
polycrystalline YIG layers on Pt layers as well as single-crystalline YIG
layers on GGG. In the samples, we observed clear LSSE signals and found that
the LSSE thermoelectric power factor (PF) increases monotonically with
increasing $n$; the PF of the [YIG/Pt]$_5$ sample is enhanced by a factor of
$\sim 28$ compared to that of [YIG/Pt]$_1$. This work may provide a guideline
for developing future multilayerbased LSSE devices.
