The ground-state tunnel splitting for a biaxial spin molecule in a magnetic field applied in the YZ plane with the model Hamiltonian H=k1Sz2+k2Sy2−gμBHzSz−gμBHySy has been investigated within a continuous spin approach containing the Wess–Zumino–Berry term. The tunneling without Kramers' degeneracy can be completely frozen out at Hy=0 and a certain Hz while the tunnel splitting increases monotonically with Hy at Hz=0. For Hy≠0 and Hz≠0, the tunneling can be partly frozen out. The degree of the non-Kramers freezing is sensitive to the Hy and anisotropy. The stronger the Hy and anisotropy are, the less the freezing is. The field-direction effect is in agreement with that observed in a recent experiment of Fe8 molecular nanomagnets.
Physics Letters A: General, Atomic and Solid State Physics,
Vol. 265, no. 3 (Jan 2000), pp. 217-220