Coercivity Mechanisms of Single-Molecule Magnets

Abstract: Magnetic hysteresis has become a crucial aspect for characterizing single-molecule magnets, but the comprehension of the coercivity mechanism is still a challenge. By using analytical derivation and quantum dynamical simulations, we reveal fundamental rules that govern magnetic relaxation of single molecule magnets under the influence of external magnetic fields, which in turn dictates the hysteresis behavior. Specifically, we find that energy level crossing induced by magnetic fields can drastically increase the relaxation rate and set a coercivity limit. The activation of optical-phonon-mediated quantum tunneling accelerates the relaxation and largely determines the coercivity. Intra-molecular exchange interaction in multi-ion compounds may enhance the coercivity by suppressing key relaxation processes. Unpaired bonding electrons in mixed-valence complexes bear a pre-spin-flip process, which may facilitate magnetization reversal. Underlying these properties are magnetic relaxation processes modulated by the interplay of magnetic fields, phonon spectrum and spin state configuration, which also proposes a fresh perspective for the nearly centurial coercive paradox.