Series of Lanthanide Organometallic Single-Ion Magnets

The synthesis, structures, and magnetic properties of
a series of lanthanide organometallic mixed sandwich molecules, (Cp*)­Ln­(COT),
are investigated, where Cp* is the pentamethylcyclopentadiene anion
and COT is the cyclooctatetraene dianion and Ln represents TbIII, DyIII, HoIII, ErIII,
and TmIII. Among the five complexes, Dy and Ho complexes
are determined to be single-ion magnets in addition to the previously
reported Er complex. Both Dy and Ho complexes show obvious quantum
tunneling magnetization relaxation in the absence of a static field.
The diluted Ho complex behaves two sets of thermally activated relaxation
as we reported previously in Er due to the COT ring static disorder.
A stair-shaped hysteresis for the Er compound can be observed at 1.6
K with Hc = 1 kOe at a sweeping rate over 700 Oe/s. The quantum tunneling
decoherence relaxation rate increases from Er to Ho to Dy, which may
be caused by the relative increase of transverse anisotropy coming
from the larger tilting of the two aromatic rings within the molecule.
The fine electronic structure is analyzed with ligand-field theory employing
the effective Hamiltonian method. The zero-field splitting is determined
to be Ising type, and the energy gap between the ground state and
the first excited one is consistent with the barrier obtained by Arrhenius
analysis.