Unraveling the Contributions to Spin–Lattice Relaxation in Kramers Single-Molecule Magnets

The study of how spin interacts with lattice vibrations
and relaxes
to equilibrium provides unique insights into its chemical environment
and the relation between electronic structure and molecular composition.
Despite its importance for several disciplines, ranging from magnetic
resonance to quantum technologies, a convincing interpretation of
spin dynamics in crystals of magnetic molecules is still lacking due
to the challenging experimental determination of the correct spin
relaxation mechanism. We apply ab initio spin dynamics
to a series of 12 coordination complexes of Co2+ and Dy3+ ions selected among ∼240 compounds that largely cover
the literature on single-molecule magnets and well represent different
regimes of spin relaxation. Simulations reveal that the Orbach spin
relaxation rate of known compounds mostly depends on the ions’
zero-field splitting and little on the details of molecular vibrations.
Raman relaxation is instead found to be also significantly affected
by the features of low-energy phonons. These results provide a complete
understanding of the factors limiting spin lifetime in single-molecule
magnets and revisit years of experimental investigations by making
it possible to transparently distinguish Orbach and Raman relaxation
mechanisms.