Slow magnetic dynamics in the Ni-10 family of compounds

We report on a family of molecular crystals containing non-interacting Ni-10 magnetic molecules. These crystals display slow relaxation of the magnetization below a temperature as high as 17 K. This behavior is not associated with an anisotropy energy barrier. The magnetic energy spectrum of a Ni-10 molecule is structured in dense well-separated bands, the lowest of which makes the crystal nearly opaque to phonons of energy below about 1 meV. We interpret the observed non-equilibrium behavior as due to resonant trapping of these low-energy phonons.