Partly non-Kramers freezing of tunneling in a spin molecule

The ground-state tunnel splitting for a biaxial spin molecule in a magnetic field applied in the YZ plane with the model Hamiltonian H = k(1)S(z)(2) k(2)S(y)(2) - g mu(B) HzSz - g mu(B) HySy has been investigated within a c ontinuous spin approach containing the Wess-Zumino-Berry term. The tunnelin g without Kramers' degeneracy can be completely frozen out at H-y = 0 and a certain H-z while the tunnel splitting increases monotonically with H-y at H-z = 0. For H-y not equal 0 and H-z not equal 0, the tunneling can be par tly frozen out. The degree of the non-Kramers freezing is sensitive to the H-y and anisotropy, The stronger the H-y and anisotropy are, the less the f reezing is. The field-direction effect is in agreement with that observed i n a recent experiment of Fe-8 molecular nanomagnets. (C) 2000 Elsevier Scie nce B.V. All rights reserved.