Macroscopic quantum coherence and tunneling of an antiferromagnetic nanoparticle in a magnetic field at an arbitrarily directed angle

Based on the two-sublattice model, we investigate the quantum tunneling beh aviors of the Neel vector in single-domain antiferromagnetic nanoparticles placed in an external-magnetic held at an arbitrarily directed angle in the ZX plane. We consider the magnetocrystalline anisotropy With biaxial, tetr agonal, and hexagonal crystal symmetry, respectively. By applying the stand ard instanton technique in the spin-coherent-state path-integral representa tion, we calculate the tunneling level splittings,the tunneling rates, and the crossover temperatures in the low barrier limit for three angle ranges of the external applied magnetic field (theta(H) = pi/2, pi much less than theta(H) much less than pi, and theta(H) = pi) Our results show that the tu nneling level splittings, the tunneling rates, and the crossover temperatur es depend on the orientation of the external applied magnetic field distinc tly, which provides a possible experimental test for macroscopic quantum tu nneling and coherence of the Neel vector in nanometer-scale single-domain a ntiferromagnets. [S0163-1829(99)01730-X].