Origin of the fast magnetization tunneling in the single-molecule magnet [Ni(hmp)(t-BuEtOH)Cl]4

We present high-frequency angle-dependent EPR data for crystals of [NixZn1−x(hmp)(t-BuEtOH)Cl]4 (x=1 and 0.02). The x=1 complex behaves as a single-molecule magnet at low temperatures, displaying hysteresis and exceptionally fast magnetization tunneling. We show that this behavior is related to a fourth-order transverse crystal-field interaction, which produces a significant tunnel splitting (∼10MHz) of the ground state of this S=4 system. The magnitude of the fourth-order anisotropy, and the dominant axial term (D), can be related to the single-ion interactions (Di and Ei) at the individual NiII sites, as determined for the x=0.02 crystals.