Background: X-ray Magnetic Circular Dichroism

X-rays are widely used in medicine to image parts of our body, such as bones and internal organs. This imaging technique takes advantage of the property that every material absorbs X-ray radiation in a different way, allowing us to the detect the contrast of different tissues in the body. Due to its very high sensitivity, the absorption of X-ray is also used in material science to detect the composition and electronic properties of a large variety of materials, such as molecular compounds, semiconductors, metals and alloys, providing important information to develop novel materials.
Figure: X-ray absorption (top) and magnetic circular dichroism (XMCD, bottom) of gadolinium atoms on magnesium oxide. The spectra were obtained at the ALBA synchrotron in Spain in a magnetic field of 6 Tesla and at a temperature of 6 K.
X-ray absorption can also be used as a very sensitive magnetic sensor to detect the magnetic properties of ensembles of nanostructures, down to the size of isolated molecules and atoms. To this purpose, one needs to use very bright and controllable sources that can fine tune the polarization and wavelength of the X-ray radiation.

These sources are called synchrotrons. A synchrotron works as a large electron accelerator, with the packets of electrons travelling in circular orbits very close to the speed of light. By bending their trajectories with powerful magnets, part of the energy of the electrons is converted into X-ray radiation that can be used to sense the properties of materials. Differently from STM, this technique probes many atoms at once, which allows performing the measurement much faster.

The figure shows the typical signal that one obtains from a magnetic material, in this case many individual Gd atoms anchored on a magnesium oxide film. The main information is the so-called X-ray magnetic circular dichroism, which is the difference of absorption between X-ray radiation with opposite circular polarization. Analyzing this signal one can determine the quantum levels of atoms and molecules and determine their potential as qubits.

There are many synchrotron sources in the world, and only a few of them are adapted to this kind of experiment. In QNS, we perform experiments at the Pohang Light Source, as well as at other international synchrotrons such as the Swiss Light Source in Switzerland, ALBA in Spain, ESRF and SOLEIL in France.