X-rays Expose Magnetism

X-rays have been used to see the internal workings of antiferromagnets for the first time, in work described as a breakthrough in understanding the materials.

Unlike conventional magnets, antiferromagnets (such as the metal chromium) exhibit "secret" magnetism, undetectable at a macroscopic level. Their magnetism is confined to very small regions in which atoms behave as tiny magnets, and they spontaneously align themselves opposite to adjacent atoms, leaving the material magnetically neutral. The internal order of antiferromagnets is on the same scale as the wavelength of x-rays (below 10 nms).

By observing changes in a coherent X-ray speckle pattern, such as the one shown above, researchers are able for the first time to investigate nanoscale dynamics of antiferromagnetic domain walls and to observe a crossover from classical to quantum behavior. (Photo: O. Shpyrko, Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Ill., 60439, USA)

In the latest research, scientists at the London Centre for Nanotechnology, the University of Chicago and the Center for Nanoscale Materials at Argonne National Laboratory used x-ray photon correlation spectroscopy to produce "speckle" patterns, holograms that provide a unique "fingerprint" of a particular magnetic domain configuration.

Eric D. Isaacs, director of the Argonne National Laboratory Center for Nanoscale Materials, said: "Since the discovery of x-rays over 100 years ago, it has been the dream of scientists and engineers to use them to make holographic images of moving objects, such as magnetic domains, at the nanoscale. This has only become possible in the last few years with the availability of sources of coherent x-rays, such as the Advanced Photon Source, and the future looks even brighter with the development of fully coherent x-ray sources called free-electron lasers over the next few years."

The internal order of antiferromagnets is on the same scale as the wavelength of x-rays (below 10 nms). The latest research used x-ray photon correlation spectroscopy to produce 'speckle' patterns; holograms which provide a unique 'fingerprint' of a particular magnetic domain configuration.

Gabriel Aeppli, director of the London Centre for Nanotechnology,a joint venture between University College London and Imperial College London, "This breakthrough takes our understanding of the internal dynamics of antiferromagnets to where we were 90 years ago with ferromagnets. Once you can see something, it makes it that much easier to start engineering it."

The magnetic characteristics of ferromagnets have been studied by scientists since Greek antiquity, enabling them to build up a detailed picture of the regions - or "magnetic domains" - into which they are divided. However, antiferromagnets remained a mystery because their internal structure was too fine to be measured.

Aeppli said, "People have been familiar with ferromagnets for hundreds of years, and they have countless everyday uses: everything from driving electrical motors to storing information on hard-disk drives. We haven't been able to make the same strides with antiferromagnets, because we weren't able to look inside them and see how they were ordered."

In addition to producing the first antiferromagnet holograms, the research also showed that their magnetic domains shift over time, even at the lowest of temperatures. The most likely explanation for this can be found in quantum mechanics, and the experiments open the door to the future exploitation of antiferromagnets in emerging technologies such as quantum computing.

"The key finding of our research provides information on the stability of domain walls in antiferromagnets," said Oleg Shpyrko, lead author of the publication and a researcher at the Center for Nanoscale Materials. "Understanding this is the first step toward engineering antiferromagnets into useful nanoscale devices that exploit it."

Work at the London Centre for Nanotechnology was funded by a Royal Society Wolfson Research Merit Award and the Basic Technologies program of Research Councils UK. Work at the Center for Nanoscale Materials and the Advanced Photon Source was supported by the DOE Office of Science, Office of Basic Energy Sciences. The work at the University of Chicago was supported by the National Science Foundation.

For more information, visit: ucl.ac.uk