  The degradation mechanisms of multilayer graphene spin valves are
investigated. The spin injection signals in graphene spin valves have been
reported to be linearly dependent on the drain bias voltage, which indicates
that the spin polarization of injected spins in graphene is robust against the
bias voltage. We present that the disappearance of this robustness is due to
two different degradation mechanisms of the spin valves. Our findings indicate
that the disappearance of the robustness is due to degradation rather than an
intrinsic characteristic of graphene. Thus, the robustness can be greatly
enhanced if degradation can be prevented.
