High-frequency electron paramagnetic resonance investigations of tetranuclear nickel-based single-molecule magnets

We report preliminary high-frequency electron paramagnetic resonance (EPR) investigations for several tetranuclear nickel complexes which exhibit single-molecule magnetism, including low-temperature (below ∼1 K) hysteresis loops and resonant magnetic quantum tunneling. The combination of a cavity perturbation technique and a split-coil magnet facilitates high-sensitivity, multifrequency (40 to 200+ GHz), angle dependent single-crystal EPR measurements. The data confirm the expected S=4 ground states, and a negative magnetocrystalline anisotropy for each member in the series. An unusual splitting of the easy-axis EPR peaks is observed, which may be interpreted in terms of distinct Ni4 species within the crystals. Overall, however, the trends associated with the splitting, as well as the EPR linewidths and shapes, suggest that intermolecular exchange interactions are important. Indeed, differences between the EPR spectra obtained for different complexes correlate nicely with the expected strength of exchange interactions, as determined both from intermolecular contact distances and from independent hysteresis measurements.