Tailoring magnetic insulator proximity effects in graphene: first-principles calculations

Due to its very long spin diffusion lengths up to room temperature, injecting spins and inducing magnetism in graphene has attracted a lot of research interest in the field of spintronic. Recently, a lot of effort is focused in the production of spin-polarized currents and magnetoresistance signals by growing graphene on magnetic substrates. However, the conductivity mismatch is an important factor that influences the spin injection from magnetic metallic substrates into graphene restricting the design of novel types of spin switches. Therefore, the use of magnetic insulators (MIs) has attracted much interest as an alternative route to induce magnetism in graphene via the exchange-proximity interaction.

Using first-principles calculations we investigated proximity effects induced in graphene by magnetic insulators. Four different MIs have been considered: two ferromagnetic europium chalcogenides (EuO, EuS) and two ferrimagnetic insulators yttrium iron garnet Y3Fe5O12 (YIG) and cobalt ferrite CoFe2O4 (CFO). In all cases, we find that the exchange-splitting induced in graphene varies in the range of tens to hundreds meV . While Dirac cone is negatively doped for graphene on europium chalcogenides and YIG, it is found to be positively doped for graphene on CFO substrate (c. f. Figure 1). In order to bring the Dirac cone closer to the charge neutrality point, we propose to deposit on the top side of the negatively doped structure a material which can positively dope graphene, such as CFO. In such a heterostructure the exchange-coupling parameter induced by proximity effect is expected to be doubled. Moreover, we explored the variation of the extracted magnetic exchange parameters as a function of europium chalcogenides thicknesses. This analysis shows that the extracted parameters are robust to thickness variation and one monolayer of magnetic insulator can induce a large magnetic proximity effect on graphene. These findings pave the way towards possible engineering of graphene spin-gating by proximity effect especially in view of recent progress in experiments

Figure 1. (Top) Side view and top view of the calculated crystalline structures for graphene on top of (a) EuO film (b) e2O4() EuS (d) Y3Fe5O12. (Bottom) Band structures of graphene on (a) EuO, (b) CoFe2O4, (c) EuS and (d) Y3Fe5O12. Blue (green) and red (black) represent spin up and spin down bands of graphene (magnetic insulator), respectively.