Bias-dependent spin injection into graphene on YIG through bilayer hBN tunnel barriers

We study the spin injection efficiency into single and bilayer graphene on the ferrimagnetic insulator yttriumiron-garnet (YIG) through an exfoliated tunnel barrier of bilayer hexagonal boron nitride (hBN). The contacts of two samples yield a resistance-area product between 5 and 30 k Omega mu m(2). Depending on an applied dc bias current, the magnitude of the nonlocal spin signal can be increased or suppressed below the noise level. The differential spin injection efficiency reaches values from -60% to +25%. The results are confirmed with both spin valve and spin precession measurements. The proximity induced exchange field is found in single layer graphene on YIG to be (85 +/- 30) mT and in bilayer graphene on YIG close to the detection limit. Our results show that the exceptional spin injection properties of bilayer hBN tunnel barriers reported by Gurram et al. [Nat. Commun. 8, 248 (2017)] are not limited to fully encapsulated graphene systems but are also valid in graphene/YIG devices. This further emphasizes the versatility of bilayer hBN as an efficient and reliable tunnel barrier for graphene spintronics.