Scientists Observe Smallest Atomic Displacements Ever

UCL scientists are part of an international team which has developed a novel X-ray technique for imaging atomic displacements in materials with unprecedented accuracy.

The team has applied the technique to determine how a recently discovered class of exotic materials – multiferroics – can be simultaneously both magnetically and electrically ordered. Multiferroics are candidate materials for new classes of electronic devices.

The discovery, a major breakthrough in understanding multiferroics, is published in Science (2 September 2011) and the authors include scientists from the European Synchrotron Radiation Facility (ESRF) in Grenoble (France), the University of Oxford and the London Centre for Nanotechnology at UCL.

Femtoscale atomic displacements in crystals play a big role in the electronic properties of exotic multiferroics materials, generating a mutual interaction between magnetic and ferroelectric macroscopic properties, of potential interest in the domain of advanced magnetic material for electronics. These materials combine both the crystallographic and magnetic symmetries to produce oriented displacements of atomic positions, as pictorially represented in the art image, leading to a control of electric polarization by a magnetic field and viceversa. Image credit: Ghiandoni/Paolasini

Magnets are polarised with a north and a south pole, a characteristic which is understood to arise from the alignment of magnet moments carried by atoms in magnetic materials. Certain other materials, known as ferroelectrics, exhibit a similar effect for electrical polarisation. The exotic “multiferroic” materials combine both an ordered arrangement of atomic magnetic moments and ferroelectric polarisation, with a strong coupling between these two usually separate phenomena.

The coupling produces unusual characteristics as a magnetic field can electrically polarise the material, and an electric field magnetise it.

In 2003, Kimura and coworkers in Japan sparked interest in multiferroics by discovering the gigantic coupling between magnetism and ferroelectricity within them. The research opened up the possibility of harnessing such exotic properties for novel electronic devices. Proposals include a new type of electronic memory, in which an electric field writes data into the memory and a magnetic detector is used to read it. This process is faster, and uses less energy than today’s hard disk drives.

This photo shows the experimental set-up at ESRF beamline ID20, with the cryostat of a large superconducting magnet in the upper part. The experiment took place in the gray disk-shaped chamber between the two blue split-coils of the magnet. The polarized X-ray beam is led to the experiment in the long steel tube from the right. The detector arm to the left can rotate about the cryostat and may be tilted up and down. It detects scattered X-rays, and in the circular chamber at its end is an analyzer crystal to determine also the polarization state of the scattered X-ray beam. Image courtesy of Luigi Paolasini

Until now, the origin of the electric polarisation in multiferroics has remained elusive. The team’s work unambiguously shows that the electric polarisation in the multiferroic studied proceeds from the relative displacement of charges of different signs, rather than the transfer of charge from one atom to another.

As the displacement involves a high number of electrons, even small distances can lead to significant electrical polarisation. However, the actual distance of the displacement still came as a surprise: about 20 femtometres, or about 1/100,000th of the distance between the atoms in the material. Measuring such small displacements had previously been considered to be impossible.

This photo shows the sample holder, made of copper, which was used in the experiment. The small gray crystal of TbMnO3 that was studied, is in the center, between two electrodes to apply an electric field. Image courtesy of Luigi Paolasini

“I think that everyone involved was surprised, if not staggered, by the result that we can now image the position of atoms with such accuracy. The work is testament, amongst other things, to the fantastic facilities available in Grenoble to the UK science community,” says Prof. Des McMorrow, Deputy Director of the London Centre for Nanotechnology, leader of the UCL part of the project.

Dr. Helen Walker, from the ESRF and lead author on the paper, developed with her colleagues a smart new experimental technique which exploits the interference between two competing processes: charge and magnetic scattering of a powerful, polarized X-ray beam. They studied a single crystal of TbMnO3 which shows a strong multiferroic coupling at temperatures below 30K (-243 degrees centigrade), and were able to measure the displacements of specific atoms within it with an accuracy approaching one femtometre (10-15m). The atoms themselves are spaced apart 100,000 times this distance.

The European Synchrotron Radiation Facility is located in Grenoble, France. The experiments were performed using beams of X-rays provided by one of the world's most brilliant light sources.Image credit: ESRF

The new interference scattering technique has set a world record for accuracy in absolute measurements of atomic displacements. Most significantly, identification of the origin of ferroelectricty in a multiferroic material is a major step forward in the understanding of multiferroics which may in the long term lead to the design of multiferroics for practical applications.

“By revealing the driving mechanism behind multiferroics, which offer so many potential applications, it underlines how experiments designed to understand the fundamental physics of materials can have an impact on the wider world,” concludes Dr. Helen Walker.

*Source: University College London (Images via Eurekalert and UCL)

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