Recent investigations demonstrate unexpected and unusual magnetic behavior in a wide range of nanoscale materials-metal nanoparticles, metal oxide nanoparticles, nanocrystalline films-which are otherwise diamagnetic (i.e. nonmagnetic) in their bulk phase. Most prominent among these are ligand stabilized ferromagnetic noble metal nanoparticles such as Au, Ag, Cu and Pt. A number of gold nanoparticles stabilized by different ligands have been investigated for their magnetic behavior and it is now well documented that the magnetism is chemically-induced and strongly sizedependent. These magnetic properties are intrinsically related to their electronic structure, which is influenced both by the size of the nanoparticle and the nature of the ligand decorating it. While strongly binding dodecanethiol-capped Au nanoparticles of 1.4 nm in diameter exhibit ferromagnetism, a weakly binding tetraoctylammonium capped ～ 1.5 nm Au nanoparticle is diamagnetic. We have also recently demonstrated ferromagnetism in peptide-capped gold nanoshells (～ 0.5 nm thick) and the possibility to modulate their magnetic behavior by step-wise functionalization of stabilizing ligands. It is generally observed that ferromagnetism at room temperature in gold nanoparticles prevails in thiol-stabilized Au nanoparticles. The crucial electronic event involved on the onset of magnetization in capped gold nanoparticles is a spin symmetry breaking associated with the 5 d and 6 s electrons of the Au atoms involved in the chemical bond with the ligands, which in turn modifies the relative spin densities at the Fermi energy thus creating a non-zero magnetic moment and a corresponding magnetization. Miyake and coworkers have recently reported diameter dependence (size effect) on ferromagnetism of dodecanethiol-capped gold nanoparticles. Table S1 (Supporting Information) summarizes the previously reported literature on size-dependent magnetic properties observed in ligand-stabilized gold nanoparticles. A simple correlation between size and magnetic behavior in gold nanoparticles from the previously published results can be drawn. In several instances, thiol-capped gold nanoparticles of size around 2 to 3 nm have predominantly exhibited ferromagnetic behavior.
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