Exchange Interactions at the Origin of Slow Relaxation of the Magnetization in {TbCu₃} and {DyCu₃} Single-Molecule Magnets

New
{TbCu3} and {DyCu3} single-molecule magnets
(SMMs) containing a low-symmetry LnIII center (shape measurements
relative to a trigonal dodecahedron and biaugmented trigonal prism
are 2.2–2.3) surrounded by three CuII 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 LnIII ion and the three CuII ions could be accurately measured by inelastic neutron scattering
(INS) spectroscopy and modeled effectively. The excitations observed
by INS correspond to flipping of CuII 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 Mtot of the cluster ground state of {DyCu3}
is an integer, whereas it is a half-integer for {TbCu3},
which explains their vastly different quantum tunneling of the magnetization
behavior despite similar energy barriers.