Exact instanton solution for quantum tunneling in an uncompensated antiferromagnet

An exact instanton solution describing macroscopic quantum tunneling for a small antiferromagnetic particle with uncompensated spin and biaxial quadra tic anisotropy is constructed. The solution is valid for any relation betwe en anisotropy parameters and relative value of uncompensated spin. The obta ined solution is used for calculating the tunneling amplitude taking into a ccount the pre-exponential factor. The amplitude is characterized by a nona nalytic dependence on the ratio of small parameters of the problem, viz., a nisotropy in the basal plane and the value of uncompensated spin. (C) 1999 American Institute of Physics. [S1063-777X(99)00712-4].