  The control of a ferromagnet's magnetization via only electric currents
requires the efficient generation of current-driven spin-torques. In magnetic
structures based on topological insulators (TIs) current-induced spin-orbit
torques can be generated. Here we show that the addition of graphene, or
bilayer graphene, to a TI-based magnetic structure greatly enhances the
current-induced spin density accumulation and significantly reduces the amount
of power dissipated. We find that this enhancement can be as high as a factor
of 100, giving rise to a giant Edelstein effect. Such a large enhancement is
due to the high mobility of graphene (bilayer graphene) and to the fact that
the graphene (bilayer graphene) sheet very effectively screens charge
impurities, the dominant source of disorder in topological insulators. Our
results show that the integration of graphene in spintronics devices can
greatly enhance their performance and functionalities.
