QUANTUM TRANSITIONS OF THE NEEL VECTOR IN SMALL ANTIFERROMAGNETS

The tunneling behaviors of the Neel vector in a small single-domain an tiferromagnetic particle out of metastable easy directions or between degenerate easy directions are studied based on the two sublattices mo del and the instanton method. The previously known Wentzel-Kramers-Bri llouin (WKB) exponents for the tunneling rates are supplemented by cal culations of the prefactors, for one example of macroscopic quantum tu nneling and two examples of macroscopic quantum coherence. The formali sm for evaluating both the WKB exponent and the fluctuation determinan t is developed in a spin-coherent-state path integral. The topological quenching or quantum interference effect is also discussed for each c ase. [S0163-1829(97)05238-7].