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 semiclassical predictions for 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.