Iron doped gold cluster nanomagnets: ab initio determination of barriers for demagnetization.

Magnetic properties of small- and nano-sized iron doped gold clusters are calculated at the level of second order multireference perturbation theory. We first assess the methodology for small Au 6 Fe and Au 7 Fe clusters, which are representative of even and odd electron count systems. We find that larger active spaces are needed for the odd electron count system, Au 7 Fe, which exhibits isotropic magnetization behaviour. On the other hand, the even electron count system, Au 6 Fe, exhibits strong axial magnetic anisotropy. We then apply this methodology to the tetrahedral and truncated pyramidal nano-sized Au 19 Fe (with S = 3/2) and Au 18 Fe (with S = 2) clusters. We find that face substitutions result in the most stable structures, followed by edge and corner substitutions. However, for Au 18 Fe, corner substitution results in strong magnetic anisotropy and a large barrier for demagnetization while face substitution does not. Thus, although corner and face substituted Au 18 Fe have the same spin, only corner substituted Au 18 Fe can act as a single nanoparticle magnet.