  The bias dependence of spin injection in graphene lateral spin valves is
systematically studied to determine the factors affecting the tunneling spin
injection efficiency. Three types of junctions are investigated, including MgO
and hexagonal boron nitride (hBN) tunnel barriers and direct contacts. A DC
bias current applied to the injector electrode induces a strong nonlinear bias
dependence of the nonlocal spin signal for both MgO and hBN tunnel barriers.
Furthermore, this signal reverses its sign at a negative DC bias for both kinds
of tunnel barriers. The analysis of the bias dependence for injector electrodes
with a wide range of contact resistances suggests that the sign reversal
correlates with bias voltage rather than current. We consider different
mechanisms for nonlinear bias dependence and conclude that the energy-dependent
spin-polarized electronic structure of the ferromagnetic electrodes, rather
than the electrical field-induced spin drift effect or spin filtering effect of
the tunnel barrier, is the most likely explanation of the experimental
observations.
