  The ultimate goal of spintronics is achieving electrically controlled
coherent manipulation of the electron spin at room temperature to enable
devices such as spin field-effect transistors. With conventional materials,
coherent spin precession has been observed in the ballistic regime and at low
temperatures only. However, the strong spin anisotropy and the valley character
of the electronic states in 2D materials provide unique control knobs to
manipulate spin precession. Here, by manipulating the anisotropic spin-orbit
coupling in bilayer graphene by the proximity effect to WSe$_2$, we achieve
coherent spin precession in the absence of an external magnetic field, even in
the diffusive regime. Remarkably, the sign of the precessing spin polarization
can be tuned by a back gate voltage and by a drift current. Our realization of
a spin field-effect transistor at room temperature is a cornerstone for the
implementation of energy-efficient spin-based logic.
