a new spin on microscopy


In 2003, two physicists in Vienna were playing around with equations involving electrons. The resulting formula suggested that something which was once thought impossible might just be possible after all.

In short, it seemed that it might be feasible to use a Transmission Electron Microscope (TEM), of the kind found in labs around the world, to study the magnetic properties of materials in minute detail. At the time, these studies were carried out in synchrotrons, which are large, expensive and rather rare.

To see if their theory would work in practice, the Austrian scientists joined forces with a multidisciplinary team from the Czech Republic, Germany and Italy to work on the CHIRALTEM (Chiral Dichroism in the Transmission Electron Microscope) project. At the time, they were the only team in the world investigating this idea.

The researchers proved that TEMs can indeed reveal information on the magnetic properties of materials. Furthermore, the level of detail surpasses that of other techniques such as x-ray magnetic circular dichroism (XMCD). The discovery could prove useful for many applications, including quantum computers, which store data using the spin of electrons.


Probing matter with electrons

Since the 1930s, researchers have used TEMs to study materials at minute scales. They are used to study the interior of cells and the structure of crystals, for example.

TEMs work in a similar way to ordinary light microscopes, simply replacing the light with a beam of electrons, which is focused with magnetic lenses. As electrons have a much lower wavelength than visible light, TEMs offer much greater magnification and higher resolution than light microscopes.

One drawback of TEMs is their inability to examine the magnetic characteristics of certain atoms in a sample. In theory, a TEM could be used to do this if it could produce a polarised beam of electrons, in which all the electrons are spinning in the same direction. However, as the beam of electrons produced by TEMs is unpolarised, physicists simply assumed that they could not be used to study the magnetic properties of a specimen.

A tricky technique

Until now, the only available method for studying a material’s magnetic properties was ‘X-ray magnetic circular dichroism’ (XMCD). XMCD exploits a phenomenon called circular polarisation. This occurs when electromagnetic waves, such as visible light, appear to spin around the beam of light’s direction of travel, like an arrow spinning in flight.

In XMCD, a polarised beam of x-rays is directed at a sample. The way the x-rays are absorbed by the material is affected by the direction of the magnetic field, an effect called dichroism. XMCD is not without drawbacks – the resolution is not as high as that achieved with a TEM, and one widely used technique – x-ray induced photoemission electron microscopy (XPEEM) – is only able to probe the surface of a material. The main problem with XMCD is the fact that it requires a synchrotron to generate the x-ray beam. Synchrotrons are extremely expensive and relatively rare.

Taking a gamble

The equation the scientists had worked out on paper appeared to challenge the idea that TEMs could not study the magnetic properties of a sample. But would it work in practice? The aim of CHIRALTEM was to find out, and if it worked, compare the new technique to XCMD. No one had tried anything like this before, and there was no guarantee of success.

The project partners investigated magnetic materials in a TEM, to see if the so called dichroic effects could be detected. To achieve this, the scientists had to determine the best way of preparing the samples and controlling the magnetic field within the TEM.

The team soon proved that it was indeed possible to detect magnetic effects using the ordinary beam of electrons found in a normal TEM. The CHIRALTEM researchers called their technique ‘electron energyloss magnetic chiral dichroism’ (EMCD). Their results were published in the prestigious scientific journal Nature in May 2006.

EMCD offers many advantages over other techniques. The high resolution offered by the TEM means it can discern magnetic structures 10 times smaller than those identified by existing x-ray techniques. With EMCD, TEMs can now study the crystallography, morphology, chemistry and magnetic properties of a sample in one go.

A wealth of possibilities

The successful outcome of the project opens up a wealth of possibilities, and EMCD could become a new method for analysing and characterising the magnetic properties of materials at the nanometre scale. It could also be used to improve our understanding of magnetic phenomena in certain chemical elements and is ideal for the study of thin magnetic films.

In biology, EMCD could be a valuable tool for those studying the large numbers of life forms which are able to detect the Earth’s magnetic field and use it to navigate. Many creatures, including birds, bacteria and reptiles, have a built-in compass. Pigeons have magnetic material in their beaks, for example. Studying how living creatures use the Earth’s magnetic field could teach us how animals navigate during migration, for example.

In industry, EMCD could be valuable for investigating the materials to be used in miniaturised recording devices, or in computers that store data on the spins of electrons.

The CHIRALTEM project partners are now working to promote contact between electron microscopists and synchrotron users, as these groups have had little to do with each other in the past and a closer working relationship will be of huge benefit to future study in this field.

By proving that something once thought impossible is in fact possible, CHIRALTEM has highlighted the pioneering nature of European research and opened up a new and exciting field of research whose full potential is just beginning to be realised.