X-ray holograms reveal secret magnetism

Today’s edition of Nature journal reveals how collaboration between scientists in the USA and the UK has led to a major breakthrough in the understanding of antiferromagnets. Scientists at the Center for Nanoscale Materials at Argonne National Laboratory, the University of Chicago and the London Centre for Nanotechnology have exploited a technique called “X-ray photon correlation spectroscopy,” to see the internal workings of antiferromagnets, such as the metal chromium, for the very first time.

Unlike conventional magnets, antiferromagnets are materials that exhibit ‘secret’ magnetism that is not easily detectable. Instead, their magnetism is confined to very small regions where atoms behave as tiny magnets by spontaneously aligning themselves oppositely to adjacent atoms thus neutralising the overall magnetism of the material.

Gabriel Aeppli, director of the London Centre for Nanotechnology, said, “People have been familiar with ferromagnets for hundreds of years and they are being used in everything from driving electrical motors to storing the information in hard disk drives. But we haven’t been able to make the same strides forward with antiferromagnets because, until relatively recently, we couldn’t look inside them to see how they were ordered. Once you can see something, it makes it 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 is far too fine to be measured using techniques ultimately relying on visual inspection.

The internal order of antiferromagnets is on the same scale as the wavelength of X-rays — below 10 nanometers — and these have now been used to produce a ‘speckle’ pattern which is actually a hologram, or more loosely speaking, a unique fingerprint of a particular magnetic domain configuration. Eric D. Isaacs, director of the Center for Nanoscale Materials, said, “Since the discovery of X-rays more than 100 years ago, it has been the dream of scientists and engineers to use them to make holographic images of moving objects, like 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 over the next few years of fully coherent X-ray sources called Free Electron Lasers.”

In addition to producing the first holograms of an antiferromagnet, the research revealed that the holograms are actually time-dependent, even down to the lowest temperatures. This implies that the antiferromagnet is never truly at rest, and the responsibility for this most likely lies with quantum mechanics and the uncertainties it imposes not only on conventional particles such as electrons and atoms, but also on objects such as domain walls in magnets. The new experiments thus help to open the prospect of exploiting 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 on the publication and researcher at the Center for Nanoscale Materials. “Understanding this is the first step towards engineering antiferromagnets into useful nanoscale devices that exploit it.”

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. 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 United Kingdom. Work at the University of Chicago was supported by the National Science Foundation.

Other researchers involved in the publication are Paul Zschack, Michael Sprung, Suresh Narayanan and Alec R. Sandy of the Advanced Photon Source and Jonathan Logan, Yejun Feng, Rafael Jaramillo, H.C. Kim and Thomas F. Rosenbaum of the University of Chicago.