  An effective Hamiltonian, describing quantum tunneling in ferrimagnetic
nanoparticles which includes interactions between the electronic spins of
nanoparticle and microscopic environmental spins (like nuclear spins or
paramagnetic impurities), is obtained. Two limiting cases, describing tunneling
in antiferromagnetic and ferromagnetic regimes are considered, and criterion
for the transition between the two regimes is found. The validity of analytic
results is verified by the exact diagonalization method.
