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Magnetic gold is evidence of relativity, study finds

A curious property of nanoscale gold explains why the metal doesn’t tarnish – and needs Einstein’s theory to explain it. Joel F. Hooper reports.

Gold: attractive, but not magnetic.
Gold: attractive, but not magnetic.

Scientists have spent years debating a strange property of gold: on the macro scale it is both physically and magnetically inert, but on the nano-scale, it is magnetic.

A new paper by Robson Fernandes de Farias of the Federal University of Rio Grande in Brazil hopes to shed light on this by examining the smallest possible gold cluster, particles of just two atoms. The explanation may trace its way back to Einstein’s theory of relativity.

For as long as humans have inhabited the earth, gold has been an object of desire. Gold flakes have been found in Palaeolithic caves dating back to 40,000 BCE, and gold was prized by the pharaohs of ancient Egypt. Lust for gold led the conquistadores to ravage the New World in the sixteenth century, while solutions of gold were consumed in the Middle Ages as a cure for syphilis.

In the modern world, gold is in our jewellery as well as in our electronics, and it is is increasingly finding its way back into medical practice in the form of nanoparticles used in tissue engineering. We might think that gold holds few surprises, but it’s only in recent years that scientists have discovered its magnetism.

Gold (Au) in its bulk form, like the metal in a wedding ring, is not considered a magnetic material. Technically, it is classified as “diamagnetic”, meaning that it can be repelled by a magnetic field, but cannot form a permanent magnet.

This is true on the macro scale, but as we know, when things get small their properties get weird.

In 2004, gold nanoparticles, tiny clusters of gold atoms a few nanometres in size, were shown to have paramagnetic properties, meaning they can attract other magnetic materials, just like miniature bar magnets.

Magnetism is caused by unpaired electrons surrounding the atoms of the material. Due to a quantum mechanical property called “spin”, unpaired electrons induce a magnetic dipole (like the two poles of a bar magnet). However, electrons often like to team up in pairs, and the opposing spin of the two electrons cancels out the magnetic effect.

A single atom of gold has an odd number of electrons, so it will always have one unpaired electron. But in bulk gold, these unpaired electrons can be shared between atoms, allowing them to find a buddy and form a pair. This means that metallic gold has no unpaired electrons, and it does not display classical magnetism.

Fernandes de Farias has tackled this problem by calculating the electronic structure of the smallest possible gold cluster, a two atom Au-Au dimer. His calculations show that in the Au-Au cluster, the two unpaired electrons held by the gold atoms do not form a pair, but are more stable on their own. He proposes that this effect becomes weaker as the gold cluster gets larger, meaning the bigger the gold particle, the weaker its magnetic properties.

Curiously, the properties of unpaired electrons in gold are largely explained by Einstein’s theory of relativity.

Gold is a large atom, so the unpaired electron orbiting furthest from the nucleus is moving very, very fast. Relativistic effects cause these electrons to be drawn closer to the nucleus, making them more stable than would be expected without the effects of relativity.

The stability of this unpaired electron is responsible for the very low reactivity of gold, explaining why it does not tarnish. It also affects the likelihood that this electron will pair with another electron in a gold nanoparticle. Thus, the magnetic properties of nanoscale gold are likely caused by the effects of relativity.

Joel Hooper is a senior research fellow at Monash University, in Melbourne, Australia.
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