  X-band electron spin resonance (ESR) spectroscopy has been performed for gold
nanorods (AuNRs) of four different sizes covered with a diamagnetic stabilizing
component, cetyltrimethylammmonium bromide. The ESR spectra show ferromagnetic
features such as hysteresis and resonance field shift, depending on the size of
the AuNRs. In addition, the ferromagnetic transition is indicated by an abrupt
change in the spectra of the two smallest AuNRs studied. A large g-value in the
paramagnetic region suggests that the ferromagnetism in the AuNRs originates
from strong spin-orbit interaction.
