Stability and quenching of plasmon resonance absorption in magnetic gold nanoparticles

The transition from diamagnetic to ferromagnetic behavior is one of the most dramatic quantum size-dependent effects in noble metals. The origin of the ferromagnetic behavior of Au nanoparticles is associated with a spin symmetry breaking at the Fermi level, which leads to a quasi-degeneracy of states whose magnetic properties differ markedly from the diamagnetic behavior encountered in bulk Au. We have performed quantum-chemical density functional theory-based calculations of the electronic, optical, and magnetic properties of Au clusters to clarify several aspects of the behavior of nanogold. In some cases, we found a remarkable stability of the localized magnetic moments of the clusters that support a dual domain model for Au nanoparticles to explain their magnetic properties, that is, a diamagnetic core and localized surface moments. This magnetic transition influences the optical response of the nanoparticles, quenching the intensity of the absorption associated to the plasmon resonance. We found that this striking inverse relationship between the onset of magnetism and the reduction of the absorption intensity is due to a reduction of the oscillator strengths of the transitions involved in the optical response. This reduction is in turn caused by an enhanced participation of d electrons in the magnetic state.