  We consider an effect of a strong magnetic field on the ground state and
macroscopic coherent tunneling in small antiferromagnetic particles with
uniaxial and biaxial single-ion anisotropy. We find several tunneling regimes
that depend on the direction of the magnetic field with respect to the
anisotropy axes. For the case of a purely uniaxial symmetry and the field
directed along the easy axis, an exact instanton solution with two different
scales in imaginary time is constructed. For a rhombic anisotropy the effect of
the field strongly depends on its orientation: with the field increasing, the
tunneling rate increases or decreases for the field parallel to the easy or
medium axis, respectively. The analytical results are complemented by numerical
simulations.
