Macroscopic quantum coherence in small antiferromagnetic particles

The Neel vector of a small antiferromagnetic particle can resonate between degenerate directions. The total moment of the particle will generally be non-zero, and this resonance cannot then occur without transverse anisotropy. Previous work is extended to include such anisotropy via a two-spin model representing the two sublattice moments. The WKB exponent in the tunnel splitting is found exactly when the sublattice moments are equal, and by a variational method that holds over the entire parameter space when the moments are unequal.