Abstract
Superparamagnetic behavior of gold nanoparticles which are less than 3 nm in diameter and stabilized by polymers has been reported, although the bulk gold metal has non-magnetic (diamagnetic) properties. However, this observation is somewhat ambiguous because conventional magnetometry may include magnetization from trivial origins such as magnetic impurities. Therefore, it is essential to selectively measure the magnetization of gold atoms to prove that their magnetism is intrinsic. We have performed the element specific magnetization measurement for Au nanoparticles with a mean diameter of 1.9 nm using X-ray magnetic circular dichroism (XMCD). Magnetization of gold atoms estimated by XMCD shows a good agreement with the results obtained by conventional magnetometory and consist of a superparamagnetic part and a Pauli-paramagnetic part. The mixture of these component is reasonably explained by the picture that the surface atoms are ferromagnetic and the core atoms are Pauli-paramagnetic.
