Electrical tuning of the magnetic properties of 2D magnets: the case of ${\rm Cr}_2{\rm Ge}_2{\rm Te}_6$

Motivated by growing interest in atomically-thin van der Waals magnetic materials, we present an {\it ab initio} theoretical study of the dependence of their magnetic properties on the electron/hole density $ρ$ induced via the electrical field effect. By focusing on the case of monolayer ${\rm Cr}_2{\rm Ge}_2{\rm Te}_6$ (a prototypical 2D Ising ferromagnet) and employing a hybrid functional, we first study the dependence of the gap and effective mass on the carrier concentration $ρ$. We then investigate the robustness of magnetism by studying the dependencies of the exchange couplings and magneto-crystalline anisotropy energy (MAE) on $ρ$. In agreement with experimental results, we find that magnetism displays a bipolar electrically-tunable character, which is, however, much more robust for hole ($ρ>0$) rather than electron ($ρ<0$) doping. Indeed, the MAE vanishes for an electron density $ρ\approx - 7.5 \times 10^{13}~{\rm e} \times {\rm cm}^{-2}$, signalling the failure of a localized description based on a Heisenberg-type anisotropic spin Hamiltonian. This is in agreement with the rapid increase of the coupling between fourth-neighbor atoms with increasing electron density.