Quantum transitions of the Neel vector in small antiferromagnets

The tunneling behaviors of the Neel vector in a small single-domain antiferromagnetic particle out of metastable easy directions or between degenerate easy directions are studied based on the two sublattices model and the instanton method. The previously known Wentzel-Kramers-Brillouin (WKB) exponents for the tunneling rates are supplemented by calculations of the prefactors, for one example of macroscopic quantum tunneling and two examples of macroscopic quantum coherence. The formalism for evaluating both the WKB exponent and the fluctuation determinant is developed in a spin-coherent-state path integral. The topological quenching or quantum interference effect is also discussed for each case. [S0163-1829(97)05238-7].