Magnetic Ordering in Gold Nanoclusters

Several research groups have observed magnetism in monolayer-protected gold cluster samples, but the results were often contradictory, and thus, a clear understanding of this phenomenon is still missing. We used Au₂₅(SCH₂CH₂Ph)₁₈ ⁰, which is a paramagnetic cluster that can be prepared with atomic precision and whose structure is known precisely. Previous magnetometry studies only detected paramagnetism. We used samples representing a range of crystallographic orders and studied their magnetic behaviors using electron paramagnetic resonance (EPR). As a film, Au₂₅(SCH₂CH₂Ph)₁₈ ⁰ exhibits a paramagnetic behavior, but at low temperature, ferromagnetic interactions are detectable. One or few single crystals undergo physical reorientation with the applied field and exhibit ferromagnetism, as detected through hysteresis experiments. A large collection of microcrystals is magnetic even at room temperature and shows distinct paramagnetic, superparamagnetic, and ferromagnetic behaviors. Simulation of the EPR spectra shows that both spin-orbit (SO) coupling and crystal distortion are important to determine the observed magnetic behaviors. Density functional theory calculations carried out on single cluster and periodic models predict the values of SO coupling and crystal-splitting effects in agreement with the EPR-derived quantities. Magnetism in gold nanoclusters is thus demonstrated to be the outcome of a very delicate balance of factors. To obtain reproducible results, the samples must be (i) controlled for composition and thus be monodisperse with atomic precision, (ii) of known charge state, and (iii) well-defined in terms of crystallinity and experimental conditions.