Unusual attraction

Materials World magazine
3 Oct 2015

Early work on overcoming the Stoner criterion has resulted in a number of non-ferromagnetic metals including copper being granted magnetic properties. Khai Trung Le spoke with Dr Oscar Céspedes on the future of his findings.

A thin layer of buckyballs is all that is required to generate magnetism in metals that are not naturally ferromagnetic, overcoming the Stoner criterion that typically determines whether elements are magnetic. But this simple observation made at the University of Leeds, UK, has the potential to transform the use of materials in numerous industries, should the team overcome a number of obstacles.

The study Beating the Stoner criterion using molecular interfaces, led by Dr Oscar Céspedes, details altering the quantum interfaces of matter to adjust the outcome of the criterion by using a thin layer of C60, commonly referred to as buckyballs. The C60 electrons are trapped within the football–like structure of the atoms, and it is speculated that the electron–free surface takes electrons from other surfaces. This removes some of the electrons on the non-ferromagnetic metal surface, and the research team identified that it is the movement of remaining electrons between the metal and C60 that subverts the Stoner criterion, allowing the metal to develop magnetic properties. ‘This is the first universal method that you can apply to any metal to try and make it magnetic,’ Céspedes said.

The team first observed the change with a 20 atom-thick strip of copper wrapped in a C60 layer six atoms-thick, with an ultra-sensitive magnetometer measuring slight magnetism. This was repeated with another 100 samples, following Céspedes’ initial scepticism about the results – ‘OK, nobody is going to believe this.’ The paper speculates that ‘density functional theory simulations suggest a mechanism based on magnetic hardening of the metal atoms, owing to electron transfer’.

However, the reasoning behind the subversion is still an unknown quality, and there is considerable work to be done before this discovery has commercial application. Céspedes commented, ‘Currently, the magnetic strength is quite weak – you wouldn’t be able to stick one of these magnets on your fridge – and we are limited to using 2–3nm of metal with a large surface-to-volume ratio. The C60 layer does not have any impact on thicker metal samples,’ with the ferromagnetic state existing over several layers of the metal before being quenched by large same thickness.

Despite this, Céspedes was optimistic about both the discovery and its future application, noting in the paper that it may allow for ‘the exploitation of molecular coupling to design magnetic metamaterials using abundant, non-toxic components such as organic semiconductors […] with consequences for the design of devices for electronic, power or computing applications.’

Following the team’s ongoing research, Céspedes noted, ‘We believe all transition metals can have their properties changed, but this will be part of our continued work, along with exploring how to make these effects viable in bulk materials and making the effect stronger. We are also working on creating a device that can implement and control this effect.’

Co-lead author Tim Moorson commented, ‘Having such a small variety of magnetic materials limits our ability to tailor magnetic systems to the needs of applications without using very rare or toxic materials. Building devices with only the three magnetic metals [iron, cobalt and nickel] naturally available to us is rather like trying to build a skyscraper using only wrought iron. Why not add a little carbon and make steel?’

Céspedes believes that the team’s discovery has the most direct application in magnetic recording devices, and hopes that the creation of organic carbon-based semiconductors will enable future devices to become more environmentally friendly. ‘There are other uses we are thinking of, but these start to move into science fiction so, for now, we’re concentrating on the immediate future.’