X-ray holograms reveal secret magnetism

ARGONNE, Ill. (May 3, 2007) — 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.

About the London Centre for Nanotechnology

The London Centre for Nanotechnology is a joint enterprise between University College London and Imperial College London. In bringing together world-class infrastructure and leading nanotechnology research activities, the Centre aims to attain the critical mass to compete with the best facilities abroad. Furthermore by acting as a bridge between the biomedical, physical, chemical and engineering sciences the Centre will cross the'chip-to-cell interface' - an essential step if the UK is to remain internationally competitive in biotechnology. Website: www.london-nano.ucl.ac.uk

About Imperial College London

Consistently rated in the top three UK university institutions, Imperial College London is a world leading science-based university whose reputation for excellence in teaching and research attracts students (11,000) and staff (6,000) of the highest international quality. Innovative research at the College explores the interface between science, medicine, engineering and management and delivers practical solutions that enhance the quality of life and the environment - underpinned by a dynamic enterprise culture. Website: www.imperial.ac.uk

About Argonne National Laboratory

Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation's first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America 's scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy's Office of Science.

About the Center for Nanoscale Materials

The Center for Nanoscale Materials, or CNM, at Argonne is a joint partnership between the U.S. Department of Energy and the State of Illinois, as part of DOE'S Nanoscale Science Research Center program. The CNM serves as a user-based facility, providing the expertise and capabilities for nanoscience and nanotechnology research. The CNM's mission includes supporting basic research and the development of advanced instrumentation that will help generate new scientific insights and create new materials with entirely new properties. The existence of the CNM, with its centralized facilities, controlled environments, technical support, and scientific staff, enables researchers to excel and significantly extend their reach.

About the University of Chicago

Founded by oil magnate John D. Rockefeller, the University of Chicago is a private, nondenominational, coeducational institution of higher learning. Scientists at the University are working at the cutting edge of virtually every field of science, from cosmological astrophysics to molecular genetics and from high-energy particle physics to psychoneuroimmunology. Seventy-nine recipients of the Nobel Prize have been researchers, students or faculty members at the University at some point in their careers. Web site: www.uchicago.edu.

For more information, please contact Steve McGregor (630/252-5580 or media@anl.gov) at Argonne.