6. August 2015

Magnets made of non-magnetic metals

Media Releases Materials Research Matter and Material Research Using Muons

For the first time, an international research team headed by the University of Leeds has demonstrated how to generate magnetism in metals that aren’t naturally magnetic. The discovery could help develop novel magnets for a wide range of technical applications, such as power generators or hard drives. Crucial measurements to understand this phenomenon were carried out at the Paul Scherrer Institute (PSI) – the only place where magnetic processes inside materials can be studied in sufficient detail. The results have now been published in the journal Nature.

Magnets are used in many technological applications: power generators, data storage on hard drives or medical imaging devices. Permanent magnets can only be produced from the three ferromagnetic elements iron, cobalt and nickel. In order to adapt the magnets to the needs of individual applications, small amounts of other, sometimes rare or harmful elements are often added to these elements.

Buckyballs make copper magnetic

In a research project headed by the University of Leeds, researchers have now demonstrated how magnetism can be generated in metals that aren’t naturally magnetic. As Fatma Al Ma’Mari from the School of Physics & Astronomy at the University of Leeds stresses: This opens new paths to devices that use abundant and hazardless elements, such as carbon and copper.

For their experiments, the researchers applied a layer of carbon-60 molecules – also dubbed buckyballs on account of their shape – to a thin strip of copper. The movement of the electrons through the interface between the two layers alters the magnetic properties of the combined material to such an extent that it becomes ferromagnetic, which means that it can be permanently magnetised.

Experiments only possible at PSI

Experiments conducted with muons at the Paul Scherrer Institute (PSI) in Villigen, Switzerland, revealed that the interface between the two materials is actually responsible for the magnetic behaviour. Muons are unstable elementary particles that enable the magnetism at different points inside materials to be studied specifically. It isn’t easy to study the magnetic properties of a buried interface. Slow muons, which can be placed near the interface with high precision, are just the ticket. PSI is currently the only place where slow muons can be used for this kind of study, stresses Thomas Prokscha, head of the Low-Energy Muons Group at PSI.

In the experiment, the muons are fired into the material studied. As they behave like tiny compass needles themselves, they react to the magnetic field where they are located in the material. After a short space of time, the muons break down into other particles. If you observe the trajectory of these particles, you can work out the behaviour of the muons in the material and thus the magnetic processes inside it, explains Hubertus Luetkens, who supervised the experiment with Prokscha on behalf of PSI.

Still too weak

As Oscar Céspedes, head of the research project at the University of Leeds, explains: We and other researchers had noticed that creating a molecular interface changed how magnets behave. For us, the next step was to test if molecules could also be used to bring magnetic ordering into non-magnetic metals.

The researchers stress that while they have demonstrated the fundamental principle, they still need to work on making the magnets stronger. Currently, you wouldn’t be able to stick one of these magnets to your fridge. But we are confident that applying the technique to the right combination of elements will yield a new form of designer magnets for current and future technologies, says Céspedes.

Text: Text based on a press release published by the University of Leeds

Background

The condition that determines whether a material is ferromagnetic is referred to as the Stoner Criterion, which was formulated by E.C. Stoner, a theoretical physicist at the University of Leeds. It explains why iron is ferromagnetic while manganese is not, even though they are situated right next to each other in the periodic table.

The criterion is based on two key factors: the Density of States (DOS) and the exchange interaction. The DOS reflects the number of states that the electrons are able to occupy in orbitals around the atomic nucleus. The exchange interaction refers to the interaction between electrons within an atom, which is determined by the orientation of the electron spin – a quantum property responsible for magnetism in materials. The spin can only have two possible orientations: up or down. According to the Stoner Criterion, if the DOS is multiplied by the exchange interaction and the result is higher than one, the material is ferromagnetic. The researchers have now demonstrated that the DOS and the exchange interaction can be changed in a non-magnetic material if the electrons are partly able to flow into a layer of a suitable second material – in this particular case, the carbon-60 molecule. Thanks to the movement of electrons between the metal and the molecules, the Stoner Criterion can be overcome.

About PSI
The Paul Scherrer Institute PSI develops, builds and operates large, complex research facilities and makes them available to the national and international research community. The institute's own key research priorities are in the fields of matter and materials, energy and environment and human health. PSI is committed to the training of future generations. Therefore about one quarter of our staff are post-docs, post-graduates or apprentices. Altogether PSI employs 1900 people, thus being the largest research institute in Switzerland. The annual budget amounts to approximately CHF 380 million.
(Last updated on April 2015)
Additional information
Research with Muons at the PSI
Low-Energy Muons Group at the PSI
Condensed Matter Physics Group at the University of Leeds
Contact
Dr. Thomas Prokscha, head of the Low-Energy Muons Group,
Paul Scherrer Institut, Villigen PSI, Switzerland
Telephone: +41 56 310 4275; E-Mail: thomas.prokscha@psi.ch

Dr. Oscar Céspedes, Condensed Matter Group, School of Physics and Astronomy,
University of Leeds, UK
Telephone: +44 113 3431885; E-Mail: O.Cespedes@leeds.ac.uk
Original Publication
Beating the stoner criterion using molecular interfaces
Fatma Al Ma’Mari, Timothy Moorsom, Gilberto Teobaldi, William Deacon, Thomas Prokscha, Hubertus Luetkens, Steve Lee, George E. Sterbinsky, Dario A. Arena, Donald A. MacLaren, Machiel Flokstra, Mannan Ali, May Wheeler, Gavin Burnell, Bryan J. Hickey and Oscar Céspedes,
Nature 06 August 2015, Vol. 524, pp. 69–73
DOI: 10.1038/nature14621