The semiclassical approach to macroscopic quantum coherence (MQC) neglects the intrinsic zero point motion of a vector order parameter when symmetry is approximately unbroken in the ground state. An exact treatment of MQC is carried out on the two-dimensional anisotropic Heisenberg antiferromagnet to illustrate this effect. For small barriers, the order parameter tunneling rate is seen to exhibit the energy scale of the massive rotator states which form the quantum counterpart of classical long range Néel order. Based on these results, predictions relevant to mesoscale magnets are made.

• Received 20 September 1994

DOI:https://doi.org/10.1103/PhysRevLett.75.4142