  We propose a scheme to manipulate the spin coherence in vertically coupled
GaAs double quantum dots. Up to {\em ten} orders of magnitude variation of the
spin relaxation and {\em two} orders of magnitude variation of the spin
dephasing can be achieved by a small gate voltage applied vertically on the
double dot. Specially, large variation of spin relaxation still exists at 0 K.
In the calculation, the equation-of-motion approach is applied to obtain the
electron decoherence time and all the relevant spin decoherence mechanisms,
such as the spin-orbit coupling together with the electron--bulk-phonon
scattering, the direct spin-phonon coupling due to the phonon-induced strain,
the hyperfine interaction and the second-order process of electron-phonon
scattering combined with the hyperfine interaction, are included. The condition
to obtain the large variations of spin coherence is also addressed.
