Vibrational Bottleneck of Magnetic Relaxation in Single-Ion Endohedral Fullerenes

Endohedral-fullerene-based molecular magnets containing
a single
lanthanide ion demonstrate high blocking temperature and long magnetic
relaxation time despite their weak crystal field anisotropy. This
work presents a comprehensive ab initio investigation
of the spin–vibrational couplings responsible for the unique
magnetic properties of endohedral fullerenes with a single magnetic
ion, focusing on the Dy@C81N complex as an example. The
study reveals that only two molecular vibrations, characterized by
translational motion of the ion along the interior surface of the
fullerene cage, drive high-temperature magnetic relaxation. These
low-frequency modes represent a vibrational bottleneck for magnetic
relaxation, similar to the one identified in the single-atom magnets
with magnetic ions placed on metal oxide or graphene surfaces. However,
the translational motions of the ion inside the fullerene cage can
hybridize with the internal rotation of fullerenes in the molecular
crystal, resulting in more vibrations strongly coupled to electron
spin.