Spin-parity effects in the resonant quantum coherence of the Neel vector in antiferromagnets with m-fold rotational symmetries

Based on the two-sublattice model, the quantum interference effects induced by the topological phase term in the Euclidean action are studied in reson ant quantum coherence of the Neel vector between energetically degenerate e asy directions in single-domain antiferromagnetic nanoparticles with m-fold rotational symmetries around the z axis and reflection symmetry in the x-y plane at zero magnetic field, where m = 3, 4, and 6, which corresponds to the trigonal, tetragonal, and hexagonal crystal symmetries, respectively. B y applying the standard instanton technique in the spin-coherent-state path -integral representation, we evaluate both the Wentzel-Kramers-Brillouin ex ponent and the preexponential factors in the instanton's contribution to th e tunneling level splitting. The Euclidean transition amplitudes between en ergetically degenerate easy directions are obtained by making use of the di lute instanton-gas approximation. The effective Hamiltonian approach is app lied to give the final results of the ground-state tunneling level splittin gs for each kind of crystal symmetry. The low-lying tunneling level spectru m and the thermodynamic properties of magnetic tunneling states are found t o depend significantly on the parity of the excess spin of single-domain an tiferromagnet. The topological quenching of the ground-state tunneling leve l splitting for the half-integer excess spins obtained previously for the b iaxial crystal symmetry (i.e., double-well system at zero magnetic field) c an be recovered by a simple diagonalization of the effective Hamiltonian. I t is shown that the effective Hamiltonian approach is equivalent to the dil ute instanton-gas approximation. Possible relevance to experiments is also discussed. [S0163-1829(99)01229-1].