  An in situ measurement of spin transport in a graphene nonlocal spin valve is
used to quantify the spin current absorbed by a small (250 nm $\times$ 750 nm)
metallic island. The experiment allows for successive depositions of either Fe
or Cu without breaking vacuum, so that the thickness of the island is the only
parameter that is varied. Furthermore, by measuring the effect of the island
using separate contacts for injection and detection, we isolate the effect of
spin absorption from any change in the spin injection and detection mechanisms.
As inferred from the thickness dependence, the effective spin current $j_e =
\frac{2e}{\hbar} j_s$ absorbed by Fe is as large as $10^8$ A/m$^2$. The maximum
value of $j_e$ is limited by the resistance-area product of the graphene/Fe
interface, which is as small as 3 $\Omega\mu$m$^2$. The spin current absorbed
by the same thickness of Cu is smaller than for Fe, as expected given the
longer spin diffusion length and larger spin resistance of Cu compared to Fe.
These results allow for a quantitative assessment of the prospects for
achieving spin transfer torque switching of a nanomagnet using a graphene-based
nonlocal spin valve.
