GOLD readily forms alloys with the precious metals silver and palladium, but it normally blends with cheap iron about as well as oil mixes with water. That has now changed, with the creation of a gold-iron alloy that is held together by magnetism.
The arrangement of atoms in an alloy changes the chemical properties of its constituent metals and makes it potentially useful to catalyse reactions. This prompted and colleagues at the Denis Diderot University, Paris, and the French National Centre for Scientific Research to explore creating one from gold and iron. But creating a gold-iron alloy is problematic because of the differing sizes of the atoms. Locked in a crystal lattice, each gold atom has an effective radius of 0.29 nanometres, while an iron atom has a radius of just 0.256 nanometres.
The researchers overcame this obstacle by using ruthenium as a “bridge” between the two. With an effective atomic radius of 0.273 nanometres, a bed of ruthenium can guide the growth of a gold-iron lattice.
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Rousset’s team vaporised iron and gold and deposited them on a slab of ruthenium, before heating the slab to 330 °C to allow the atoms to migrate into a single-layered lattice.
The team tested a number of combinations of iron and gold to see which led to the most stable arrangement. Theoretically, stability should be found in a mix containing about 80 per cent iron, as this minimises the mechanical strain caused by the atoms’ different sizes. However, to their surprise they found that the most stable lattice contained approximately one iron atom to every two gold atoms (Physical Review Letters, ).
In this combination there was evidence of long-range order: a repeating pattern of interconnected hexagons of gold, each with an iron atom at its centre.
Rousset suggests that iron’s magnetism is behind this stability. The iron atoms have their strongest magnetic properties when they make up one-third of the alloy, she says.
“What is remarkable is the absolute dominance of the magnetic interaction,” says Gayle Thayer of Sandia National Laboratories in Albuquerque, New Mexico. “The long-range order is also spectacular.”
“The absolute dominance of the magnetic interaction in a gold-iron alloy is remarkable”
If the magnetism can be switched between two orientations when a magnetic field is applied, and maintain that orientation at the temperatures found inside computers, the alloy could function as a high-density computer memory array, says Rousset.
