  The ground state tunnel splitting for the biaxial spin model in the magnetic
field, H = -D S_{x}^2 + E S_{z}^2 - g \mu_B S_z H_z, has been investigated
using an instanton approach. We find a new type of spin instanton and a new
quantum interference phenomenon associated with it: at a certain field, H_2 =
2SE^{1/2}(D+E)^{1/2}/(g \mu_B), the dependence of the tunneling splitting on
the field switches from oscillations to a monotonic growth. The predictions of
the theory can be tested in Fe_8 molecular nanomagnets.
