Low-energy magnetic excitations of the Mn-12-acetate spin cluster observedby neutron scattering

We studied Mn-12-acetate by inelastic neutron scattering and diffraction. W e separated the energy levels corresponding to the splitting of the lowest S multiplet (S = 10 ground state). The irregular spacing of the transition energies unambiguously shows the presence of high-order terms in the spin H amiltonian [D = -0.457(2) cm(-1), B-4(0) = -2.33(4) x 10(-5) cm(-1)]. The r elative intensity of the lowest energy peaks is very sensitive to the small transverse term that is responsible for quantum tunneling, providing the f irst determination of this term in zero magnetic field [B-4(4) = +/-3.0(5) x 10(-5) cm(-1)].