Normally gold is only weakly magnetic. This weak magnetism is known as diamagnetism. Researchers from the High Field Magnet Laboratory (HFML) in Nijmegen and Leiden University have now discovered that the diamagnetism of gold nanoparticles is much greater than expected. Making use of a new magneto-optical technique, the researchers discovered that the diamagnetism of gold nanoparticles is fourteen times larger than that of normal gold.
Gold nanoparticles have many special properties. They are extremely small and size-tunable. Their surface can be chemically manipulated so that specific molecules can bind to the nanoparticles. And finally the particles respond to light in a very striking manner. This combination of properties makes gold nanoparticles useful for applications within physics (plasmonics), biology (electron microscopy) and medicine (drug delivery and detection and treatment of diseases).
The magnetic characteristics of gold nanoparticles are, however, less well known. Consequently little use is made of their magnetic properties at present, despite the many possibilities these offer. Gold nanoparticles can be ferromagnetic (like in a permanent magnet), paramagnetic or diamagnetic. The magnetism of the particles depends on exactly how they are prepared.
Since time immemorial metal nanoparticles have been used due to their striking colours, for example for decorations on vases. The fascinating colours are caused by surface plasmons: an unusual type of light wave propagating at the surface of the metal. Utilizing these plasmons, the researchers have developed a new magneto-optical method in which gold nanorods in an aqueous solution align along a strong magnetic field. Using this method, the researchers managed to determine the magnetic properties of nanorods for a very small particle concentration. This revealed that the diamagnetic effect for gold nanorods is fourteen times stronger than expected.
The researchers explain that striking behaviour by the specific trajectories of the free electrons within a gold nanorod. These pathways are like small electric currents each of which generates a magnetic field. Theory had already predicted this new type of orbital magnetism, but up until now it had not yet been observed in metal nanoparticles.