  Improved fabrication techniques have enabled the possibility of ballistic
transport and unprecedented spin manipulation in ultraclean graphene devices.
Spin transport in graphene is typically probed in a nonlocal spin valve and is
analyzed using spin diffusion theory, but this theory is not necessarily
applicable when charge transport becomes ballistic or when the spin diffusion
length is exceptionally long. Here, we study these regimes by performing
quantum simulations of graphene nonlocal spin valves. We find that conventional
spin diffusion theory fails to capture the crossover to the ballistic regime as
well as the limit of long spin diffusion length. We show that the latter can be
described by an extension of the current theoretical framework. Finally, by
covering the whole range of spin dynamics, our study opens a new perspective to
predict and scrutinize spin transport in graphene and other two-dimensional
material-based ultraclean devices.
