  We study macroscopic quantum coherence in antiferromagnetic molecular magnets
in the presence of magnetic fields. Such fields generate artificial tunnel
barriers with externally tunable strength. We give detailed semi-classical
predictions for the tunnel splitting in various regimes for low and high
magnetic fields. We show that the tunneling dynamics of the Neel vector can be
directly measured via the static magnetization and the specific heat. We also
report on a new quantum phase arising from fluctuations. The analytic results
are complemented by numerical simulations.
