Exchange Interactions at the Origin of Slow Relaxation of the Magnetization in {TbCu_3} and {DyCu_3} Single-Molecule Magnets

摘要

New {TbCu_3} and {DyCu_3} single-molecule magnets (SMMs) containing a low-symmetry Ln~(III) center (shape measurements relative to a trigonal dodecahedron and biaugmented trigonal prism are 2.2?2.3) surrounded by three Cu~(II) metalloligands are reported. SMM behavior is confirmed by frequency-dependent out-of-phase ac susceptibility signals and single-crystal temperature and sweep rate dependent hysteresis loops. The ferromagnetic exchange interactions between the central Ln~(III) ion and the three Cu~(II) ions could be accurately measured by inelastic neutron scattering (INS) spectroscopy and modeled effectively. The excitations observed by INS correspond to flipping of Cu~(II) spins and appear at energies similar to the thermodynamic barrier for relaxation of the magnetization, ～15?20 K, and are thus at the origin of the SMM behavior. The magnetic quantum number M_(tot) of the cluster ground state of {DyCu_3} is an integer, whereas it is a half-integer for {TbCu_3}, which explains their vastly different quantum tunneling of the magnetization behavior despite similar energy barriers.