Engineering Proximity Exchange by Twisting: Reversal of Ferromagnetic and Emergence of Antiferromagnetic Dirac Bands in Graphene/Cr$_2$Ge$_2$Te$_6$

Abstract: We investigate the twist-angle and gate dependence of the proximity exchange coupling in twisted graphene on monolayer Cr$2$Ge$_2$Te$_6$ from first principles. The proximitized Dirac band dispersions of graphene are fitted to a model Hamiltonian, yielding effective sublattice-resolved proximity-induced exchange parameters ($\lambda{\textrm{ex}}^\textrm{A}$ and $\lambda_{\textrm{ex}}^\textrm{B}$) for a series of twist angles between 0$^{\circ}$ and 30$^{\circ}$. For aligned layers (0$^{\circ}$ twist angle), the exchange coupling of graphene is the same on both sublattices, $\lambda_{\textrm{ex}}^\textrm{A} \approx \lambda_{\textrm{ex}}^\textrm{B} \approx 4$ meV, while the coupling is reversed at 30$^{\circ}$ (with $\lambda_{\textrm{ex}}^\textrm{A} \approx \lambda_{\textrm{ex}}^\textrm{B} \approx -4$ meV). Remarkably, at 19.1$^{\circ}$ the induced exchange coupling becomes antiferromagnetic: $\lambda_{\textrm{ex}}^\textrm{A} < 0, \lambda_{\textrm{ex}}^\textrm{B} > 0$. Further tuning is provided by a transverse electric field and the interlayer distance. The predicted proximity magnetization reversal and emergence of an antiferromagnetic Dirac dispersion make twisted graphene/Cr$_2$Ge$_2$Te$_6$ bilayers a versatile platform for realizing topological phases and for spintronics applications.