  We study the effect of a magnetic insulator (Yttrium Iron Garnet - YIG)
substrate on the spin transport properties of Ni$_{80}$Fe$_{20}$/Al nonlocal
spin valve (NLSV) devices. The NLSV signal on the YIG substrate is about 2 to 3
times lower than that on a non magnetic SiO$_2$ substrate, indicating that a
significant fraction of the spin-current is absorbed at the Al/YIG interface.
By measuring the NLSV signal for varying injector-to-detector distance and
using a three dimensional spin-transport model that takes spin current
absorption at the Al/YIG interface into account we obtain an effective
spin-mixing conductance $G_{\uparrow\downarrow}\simeq 5 - 8\times
10^{13}~\Omega^{-1}$m$^{-2}$. We also observe a small but clear modulation of
the NLSV signal when rotating the YIG magnetization direction with respect to
the fixed spin polarization of the spin accumulation in the Al. Spin relaxation
due to thermal magnons or roughness of the YIG surface may be responsible for
the observed small modulation of the NLSV signal.
