Magnetotransport properties of vertical spin valves of graphene/cobalt junctions

Entani, Shiro; Naramoto, Hiroshi*; Sakai, Seiji

In this work, magnetotransport properties of graphene spin valves were investigated for the vertical Co/graphene/Co devices with a current-perpendicular-to-plane configuration. The devices were fabricated by inserting layer-controlled graphene, which were grown by chemical vapor deposition, between two ferromagnetic electrodes of Co thin films. Linear current-voltage characteristics were observed for all the devices with different layer numbers of graphene, indicating that Ohmic transport in graphene is dominating rather than tunneling transport. A positive MR was commonly observed for a series of the Co/graphene/Co devices at room temperature. The MR ratios were 0.1-0.2% and 0.7-0.8% for the single-layer graphene and tri-layer graphene devices, respectively. Our previous Raman analyses demonstrated that the chemical interactions between grapehene and Co at the graphene/Co interfaces are different depending on the layer number of graphene, and those at the single-layer graphene/Co interface are much stronger than those at the multi-layer graphene/Co interfaces. It is reasonably speculated that the spin injection and ejection efficiencies are reduced in the single-layer graphene device involved with the strong chemical interactions which could modify the electronic states of graphene and Co around the Fermi level, significantly different from the pristine states ideal for the spin filtering.