Tunable spin splitting in graphene on a magnetic oxide insulator

In crystals, the spin degeneracy is lifted when time-reversal symmetry is broken, leading to many interesting spintronic, topological phenomena and applications. Owing to the short range of the magnetic-exchange interaction, the spin splitting can be induced in graphene by magnetic proximity effect. For spin-polarized graphene, the frequency of quantum oscillations is determined not only by the Fermi energy, but also by the spin splitting energy. In this work, we demonstrate a strong spin splitting of graphene on a magnetic insulator, Tm₃Fe₅O₁₂ (TmIG), which can be tuned over a broad range. From the gate voltage tuning of quantum oscillations, we extract that the spin splitting energy can be as large as 138 meV at 2 K. Based on the direction and strength of cooling fields, the spin splitting energy can be further tuned between 107 meV and 200 meV, shifting the quantum oscillations and quantum Hall plateau. DFT calculations reveal that an electron doping to graphene is induced by TmIG and the proximity-induced spin splitting energy can be up to 150 meV, consistent with that obtained from transport measurements. Finally, the spin polarization of graphene π orbitals is probed directly using the x-ray magnetic circular dichroism (XMCD) at the C K absorption edges.