  We present a study of the transport properties of thermally generated spin
currents in an insulating ferrimagnetic-antiferromagnetic-ferrimagnetic
trilayer over a wide range of temperature. Spin currents generated by the spin
Seebeck effect (SSE) in a yttrium iron garnet (YIG) YIG/NiO/YIG trilayer on a
gadolinium gallium garnet (GGG) substrate were detected using the inverse spin
Hall effect in Pt. By studying samples with different NiO thicknesses, the NiO
spin diffusion length was determined to be 4.2 nm at room temperature.
Interestingly, below 30 K, the inverse spin Hall signals are associated with
the GGG substrate. The field dependence of the signal follows a Brillouin
function for a S=7/2 spin ($\mathrm{Gd^{3+}}$) at low temperature. Sharp
changes in the SSE signal at low fields are due to switching of the YIG
magnetization. A broad peak in the SSE response was observed around 100 K,
which we associate with an increase in the spin-diffusion length in YIG. These
observations are important in understanding the generation and transport
properties of spin currents through magnetic insulators and the role of a
paramagnetic substrate in spin current generation.
