Magnetic circular dichroism and the nature of ferromagnetism in colloidal gold nanoparticles

The spectra of optical absorption and of magnetic circular dichroism (MCD) have been measured in the $350$–$1150$ nm wavelength range for a set of colloidal solutions containing Au nanoparticles with the average size of $6$ nm having thiolate coatings with different degrees of chirality. The form of absorption and MCD spectra suggests the dipole character of interband transitions involving the $5d-6(sp)$ orbitals. The absence (within the experimental error) of the dependence of the MCD spectra on the coating type rules out the hypothesis on the orbital nature of the observed magnetism. We argue that the spin polarization plays the dominant role in the magnetism of gold nanoparticles with a thiolate coating.