Hyperfine structure of magnetic excitations in a Tb-based single-molecule magnet studied by high-resolution neutron spectroscopy

We report inelastic neutron scattering results on a rare-earth-based single-molecule magnet, the Tb-Cu dinuclear complex. By means of a high-resolution neutron chopper spectrometer, the details of the excitations were clarified. The magnetic excitations are clearly observed at (h) over bar omega=1.7 and 12.3 meV. The transition energy of 1.7 meV corresponds to the energy barrier of magnetic relaxation estimated from an ac magnetic susceptibility measurement. This indicates that the magnetization reversal occurs through quantum tunneling between pairs of degenerated excited states at 1.7 meV, which is called a thermally activated tunneling process. Interestingly, these excitations exhibit further peak splitting in energy and their energies are independent of momentum transfer Q. Our calculation, which is based on a spin Hamiltonian, suggests that (i) the excitation at 1.7 meV originates from the exchange coupling between Tb3+ and Cu2+, (ii) the excitation at 12.3 meV corresponds to the transition between the multiplets J of the Tb3+ moment, and (iii) the fine peak splitting in energy is due to the hyperfine interaction between the nuclear spin and electron spin and orbital magnetic moments.