  Spin transport properties of graphene non-local spin valve devices are
typically determined from Hanle spin precession measurements by using a
simplified solution of the one-dimensional Bloch-Torrey equation which assumes
infinitely long transport channels and uniform spin transport parameter. We
investigate the effects of a finite graphene size and explore the influence of
spatially-varying transport parameters on the measured Hanle curves by finite
element simulations. We assume enhanced spin dephasing in the contact-covered
graphene areas with additional Fermi level pinning and explore the influence of
non-magnetic reference electrodes which are not properly decoupled from
graphene. In experiments, it is typically observed that the spin lifetime
increases with increasing charge carrier density. None of our simulations can
reproduce this trend indicating that this dependency originates from spin
transport through graphene areas which are not covered by contacts. We find
that the extracted spin lifetime might be overestimated in flakes which are
shorter than the spin diffusion length. Moreover, contact-induced spin
dephasing leads to an overall reduction of the extracted spin lifetime.
Additionally, we show that non-magnetic reference electrodes may also influence
the measured spin lifetime even though they are not part of the transport area
under investigation.
