Spin injection in graphene using ferromagnetic indium-cobalt van der Waals contacts

Graphene based spintronic devices require efficient spin injection and collection. Oxide dielectric tunnel barriers obtained by direct deposition or oxidizing thin metal films are typically used to facilitate spin injection. However, the yield of working devices with such tunnel barriers is low (typically 5-10%) as direct growth of oxides on dangling bond free 2D surfaces is challenging. Here we report a robust and simple method for spin injection in graphene lateral spin valves (LSVs) using ferromagnetic (FM) van der Waals (vdW) contacts of indium and cobalt (In/Co) that does not require deposition of dielectric tunnel barriers. We obtain magnetoresistance (MR) values of 1.5 % + 0.5% (spin signal ~ 50 ohms) - comparable to state-of-the-art graphene LSVs with oxide tunnel barriers – with a working device yield of >70% with FM In/Co vdW contacts. In contrast, LSVs with contacts containing only Co are highly inefficient and in the best cases show an MR ~ 0.2% (spin signal ≤ 3 ohms, only 10% devices work). The contact resistance of FM In/Co vdW contacts is 2 – 5 kohms, making them compatible for integration with complementary metal oxide semiconductor devices for electrical switching. Our results promise facile and high yield fabrication of spintronic devices using direct deposition of In/Co vdW contacts.