Contact-Induced Spin Relaxation in Graphene Nonlocal Spin Valves

We report on a systematic study of contact-induced spin relaxation in gated graphene nonlocal spin valves. We demonstrate the enhancement of the nonlocal magnetoresistance (Delta R-NL) as the Co/ AlOx/ graphene interface resistance increases relative to the graphene spin resistance. We measure Hanle precession at many gate voltages on 14 separate spin-valve devices fabricated from graphene grown by chemical vapor deposition (CVD). These measurements are compared by normalizing Delta R-NL to the ideal limit of large contact resistance, and the result is shown to be consistent with isotropic contact-induced spin relaxation caused by spin current flowing from the graphene into the Co contacts. After accounting for this source of spin relaxation, we extract spin lifetimes of up to 600 ps in CVD graphene with a gate-voltage dependence which can be described by a combination of both Elliott-Yafet and D'yakonov-Perel' spin-relaxation mechanisms.