  We have fabricated n-layer graphene field effect transistors on epitaxial
ferroelectric Pb(Zr_0.2Ti_0.8)O_3 (PZT) thin films. At low gate voltages, PZT
behaves as a high-k dielectric with k up to 100. An unusual resistance
hysteresis occurs in gate sweeps at high voltages, with its direction opposite
to that expected from the polarization switching of PZT. The relaxation of the
metastable state is thermally activated, with an activation barrier of 50-110
meV and a time constant of 6 hours at 300 K. We attribute its origin to the
slow dissociation/recombination dynamics of water molecules adsorbed at the
graphene-PZT interface. This robust hysteresis can potentially be used to
construct graphene-ferroelectric hybrid memory devices.
