Synchrotron radiation in materials science
Every new technology (from the AVE to mobile phone batteries) requires the development of materials with specific properties. Materials science has a direct impact on our quality of life, strong and biocompatible implants, lighter and stronger engines, intercontinental high-speed data communication, massive information storage in minimal volumes, improved performance of clean energy sources, huge high-definition displays, and the almost infinite range of plastics we use every day.
The ability to create new materials and structures is inescapably linked to advances in the understanding of these materials and the physical and chemical phenomena at play within them, at various relevant length scales and in many cases involving diverse energy ranges. It is therefore essential to have multiple complementary characterisation techniques, the more versatile the better. Synchrotron radiation is an irreplaceable tool in materials science because it offers a wide range of techniques, ideal and even essential in the process of developing a technological material.
Synchrotron Radiation (SR) is Electromagnetic Radiation (light) generated by charged particles that have been accelerated to almost the speed of light and are forced to follow a curved trajectory and therefore emit such radiation. The charged particles, usually electrons, are kept for hours spinning in a storage ring, which is a particle accelerator dedicated to the production of synchrotron light. SR is produced at points in the ring where a magnetic field bends the path of the electrons. A ring therefore has dozens of beamlines where different experiments are performed simultaneously. The specificity of each line is determined both by the source (depending on how the magnetic field responsible for the emission is generated) and by the optics that the light beam follows (focusing, colour, polarisation, energy resolution, etc.).
In general, a team of scientists needing to use synchrotron radiation agrees to perform an experiment after a selection process of proposals evaluated by an international committee and receives a sufficient number of experiment days to carry out the proposed study, at the optimal beamline to perform it. For the experimental team accessing the SR, the storage ring that produces it is almost non-existent: only the light beam incident on its sample is witness to the existence of the ring. The experimental tooling of each line is independent and the type of experiment that each team performs (absorption, scattering, diffraction, microscopy…) depends almost entirely on it.
Fig. 1. Esquema de un sincrotrón de tercera generación.
Spain is a founding partner of the European Synchrotron Radiation Source, the ESRF in Grenoble. Since its opening in 1995, the Spanish have been entitled to use 4% of the total beam time of the ESRF, the same percentage of its annual budget that we pay for. In addition, Spain maintains two beamlines of its own: BM16 and BM25 are the Spanish beamlines at the ESRF. BM16 is dedicated to the structure of macromolecules and to experiments on light scattering by biological (muscle fibres, natural polymers) and plastic samples. BM25 (Spline) is a line dedicated to various experiments on the structure of solids and surfaces. Since 2004, Spain has launched an ambitious project that will offer the first light to its users towards the end of this decade: ALBA, the Spanish synchrotron is being built in Cerdanyola del Vallés, next to the campus of the Autonomous University of Barcelona. The storage ring will hold electrons at 3 GeV, and will have the capacity for more than twenty lines. Several INMA groups have actively participated in the definition and design of several of the 7 beamlines being built at ALBA.
RS has certain properties that make it an unparalleled tool in science:
it is an immensely bright light source, much brighter than any other laboratory light source
it is a white light source: the incident beam is composed of all colours (i.e. energies) from infrared through visible and X-rays to gamma rays;
RS is very suitable for the study of the structure of solids, proteins, polymers, muscle fibres, by means of diffraction experiments.
RS is a source of light with energies suitable for exciting electronic transitions in materials, which makes it possible to study their electronic structure;
RS allows the beam polarisation to be selected: circular, horizontal or vertical, on demand.
The electrons in the ring do not form a continuous flow, but travel in packets. RS is therefore stroboscopic: pulses of a few picoseconds in duration are repeated every few nanoseconds. This allows time-dependent experiments at very high frequencies.
These combined properties make RS a very versatile tool.
Figure 2. Image of the Spanish ALBA synchrotron project, under construction, en Cerdanyola del Vallès, Barcelona.
At INMA there are several groups that regularly use synchrotron radiation in the study of materials of interest to them. Thus, iron oxides are studied, both to understand some of their basic properties (such as the abrupt variation of the conductivity of magnetite (Fe3O4) on cooling, in the so-called Verwey Transition, which seems not to be as simple as previously thought) and to improve them and use them in the form of nanoparticles in novel biomedical and pharmaceutical technologies that seem straight out of the science fiction of a few decades ago. Other INMA groups use the fact that synchrotron radiation sweeps the energy spectrum typical of the photoelectric effect, which makes it extremely sensitive magnetometry: not only is it possible to study the magnetisation of nanosized samples, but it can also quantitatively measure the magnetisation of non-magnetic elements that are in contact with magnetic elements. These techniques have allowed an INMA team to demonstrate how to improve the magnetic anisotropy of cobalt spheres of only a few thousand atoms by coating them with a layer a few atoms thick of gold, silver or platinum. Magnetic anisotropy is the affinity of a magnet to direct its magnetisation in a particular direction. The greater this anisotropy, i.e. the more the magnet prefers a given direction, the more stable it is as an information store against any disturbance. This property is fundamental and any method to increase it is of paramount importance, because in the case of a magnetic recording element (a bit on a hard disk, for example), the higher the anisotropy, the more certain we are that the stored information will not be erased, and the more interested companies could be in developing a device based on such materials.
Left: Magnetic susceptibility of the 3 nm diameter cobalt ball system covered by a gold layer (green dots) and uncovered (blue dots). The temperature increase of the susceptibility maximum is evidence of an increase of the magnetic anisotropy. Right: Synchrotron radiation magnetic absorption experiment, showing the magnetisation induced in the gold coating the cobalt particles..
RS will be one of the most widely used tools in materials science in the coming decades, given its great versatility, and it is to be hoped that Spanish scientists will make the most of the opportunities that the present moment offers us.
Instituto de Nanociencia y Materiales de Aragón (INMA), CSIC-Universidad de Zaragoza