Effect of the magnetic environment on the tunneling of a giant spin

We study the tunneling of a giant spin coupled to one or several environmental spins 1/2 and to an external magnetic field by means of perturbation theory and numerical simulation. The coupling of an integer spin to an environmental spin or to a magnetic field suppresses the tunneling effect for vanishingly small values of the coupling constant. The tunneling effect can be restored if the giant spin is also coupled to several environmental spins or to a magnetic field. In contrast, the coupling of a half integer spin to one environmental spin induces tunneling.