Transverse magnetic anisotropy in Mn12 acetate: Direct determination by inelastic neutron scattering

A high-resolution inelastic neutron scattering (INS) study of fully deuterated Mn₁₂ acetate provides accurate spin-Hamiltonian parameters for this prototype single-molecule magnet. The Mn₁₂ clusters deviate from axial symmetry, a nonzero rhombic term in the model Hamiltonian leading to excellent agreement with observed positions and intensities of the INS peaks. The following parameter set provides the best agreement with the experimental data: D=-0.0570(1)meV , B₄⁰=-2.78(7)×10^-6meV , B₄⁴=-3.2(6)×10^-6meV , and ∣E∣=6.8(15)×10^-4meV . Crystal dislocations are not the likely cause of the symmetry lowering. Rather, this study lends strong support to a recently proposed model, which is based on the presence of several molecular isomers with distinct spin-Hamiltonian parameters.