Two coats are better than one to bring nanomagnetism out of its shell.
Diamond Light Source has been used to determine that at least two chromium atomic layers are required to stabilise important magnetic effects at the interface of iron-chromium nanoparticles. Understanding these new nanoscale effects should help efforts to develop advanced computer memory devices.
“It was a very exciting result for us” says Professor Chris Binns, of the University of Leicester, “and I think we are one of the first groups to achieve this”.
The combination of a ferromagnet (where all the atomic magnetic moments align in the same direction) with an antiferromagnet(where all the magnetic moments face in opposite directions) causes what is known as the exchange bias effect. This extremely useful phenomenon is used in magnetic recording, where it maximises the sensitivity of the read head in hard disk drives.
So far, this has primarily been achieved using devices based on flat atomic layers, but the researchers, from the University of Leicester, Hazara University (Pakistan), CNR (Italy), University of York, Rutherford Appleton Laboratory and Diamond Light Source, were able to study similar effects in nanoscale spherical particles with a controlled core-shell structure. These particles consisted of an incredibly small iron core made up of just 850 atoms (~2.7 nm diameter), which was then coated with layers of chromium either one or two atoms thick. It is only very recently that methods to synthesise such novel nanoparticles with good control over the core size and shell thickness have been developed.
The investigations used the advanced superconducting vector magnet on beamline I06 of Diamond Light Source, where the researchers were able to use X-ray Magnetic Circular Dichroism (XMCD) to probe the iron and chromium layers separately. “The ability to probe the constituent elements separately really was the key for this work” adds Professor Chris Binns. Combined with laboratory based measurements, the team demonstrated that a chromium layer of only one layer exhibited no exchange bias effect at all, but a coating of two atomic layers is necessary to stabilise exchange bias in core-shell nanoparticles.
For more information about using the superconducting magnet on beamline I06 please contact Prof. Sarnjeet Dhesi (
dhesi@diamond.ac.uk)
Exchange Bias in Fe@Cr Core-Shell Nanoparticles
Chris Binns , Muhammad Tauseef Qureshi , Davide Peddis , Steve Baker , Paul Howes , Adrian Boatwright , Stuart Cavill , Sarnjeet Dhesi , Lari , Roland Kröger , Sean Langridge Nano Letters DOI: 10.1021/nl401587t
Science