Researchers show giant diamagnetism of gold nanorods

Like most materials bulk gold is diamagnetic, exhibiting only a weak response to an external magnetic field. Researchers from the HFML Nijmegen, in collaboration with scientists of Leiden University, have now found drastically enhanced diamagnetism in gold nanoparticles. Using a new, very sensitive, magneto-optical technique the researchers were able to determine the magnetic response of gold nanorods suspended in water, to find a diamagnetic signal that is 14 times larger than that of bulk gold. This effect can be attributed to the behaviour of the free electrons within the small nanoparticles and represents an alternative source of magnetism. The result is published in Physical Review Letters on 17 September 2013.

Scanning Electron Microscope picture of gold nanorodsGold nanoparticles have many special properties: they are small and size-tunable; they have a chemically inert surface acting as a functional molecular binding site; and they have a strong optical response which can be used to trace or heat attached (organic) compounds. These combined properties make them suitable for many applications within research areas such as plasmonics, biological electron microscopy, drug delivery in bio-medical applications and disease detection and treatment. The magnetic properties of gold nanoparticles however, are much less understood and utilised. They can be ferromagnetic – like in permanent magnets – but also paramagnetic and diamagnetic, depending on their preparation method.

Magneto-optical measurements
Already for ages gold nanoparticles are most notably known for their vivid colours, which are originating from their surface plasmon resonance, a collective oscillation of the confined conduction electrons in response to light. Using this optical response the researchers were able to sensitively measure the orientation of nanorods in a water solution as a function of an applied magnetic field. They demonstrate that this novel magneto-optical method is capable of determining the magnetic properties of nanorods at concentrations that are orders of magnitude lower than those required for conventional SQUID magnetometry.

Orbital magnetism
By tuning the size and shape of the nanorods, fundamental theories about the magnetic properties in metallic nanostructures could be studied. The magnetic moment of the rods was found to increase with decreasing rod size and increasing aspect ratio. This behaviour can be explained by the specific trajectories the free electrons have to follow within the nanorods, which were theoretically predicted to give a novel type of orbital magnetism that was never observed in metallic nanorods. This result is expected to stimulate future research on the extraordinary magnetic properties of nanoscale metals and the orbital magnetic susceptibility of confined electron systems in different quantum transport regimes.

´Giant magnetic susceptibility of gold nanorods detected by magnetic alignment´ P.G. van Rhee, P. Zijlstra, T.G.A. Verhagen, J. Aarts, M.I. Katsnelson, J.C. Maan, M. Orrit, and P.C.M. Christianen. Physical Review Letters, 17 September 2013

Left: The magnetically induced optical absorbance difference (ΔA) for different wavelengths. Right: The sign (ΔA) as function of wavelength at 30 T shows that gold nanorods align with its main axis along the magnetic field. From the ratio of the magnetically induced absorbance difference (ΔA), compared to the average 0 T absorbance A, the ratio of aligned nanorods is determined. From this alignment ratio the magnetic moment of a single rod can be accurately determined.