A novel technique for nanomagnetism

Diamond scientists are planning new types of experiment that will enable them to study magnetic structures on the nanometer scale. The study of magnetic properties on this length scales has already had a significant impact on society – for example, the phenomenon of giant magnetoresistance is now used in mp3 players to increase the amount of music they can store.

Diamond scientist Gerrit van der Laan reviews the state of the art in a special issue of Comptes Rendus Physique on synchrotron X-rays and condensed matter, and looks at the future and the exciting new physics that this novel technique can unleash.

In Detail

X-ray diffraction has had a huge impact on our understanding of the crystalline structure of materials, including proteins and macromolecules, and relies on the interaction between the photons and charge density of electrons in the sample. The sensitivity of the technique to magnetism was investigated as far back as the 1970s, but this application was something of a curiosity and remained little used compared to the study of the more versatile technique of magnetic scattering by neutrons.

 

The cover of Comptes Rendus Physique, showing a three-dimensional image of the intensity fluctuations in the magnetic satellite for coherent soft X-ray resonant magnetic scattering at the Co L₃ edge from a Co/Pt magnetic nanostructure (Results by Guillaume Beutier and Gerrit van der Laan).

However, in the mid 1980s access to dedicated synchrotron radiation sources resulted in two significant breakthroughs. First, X-ray absorption experiments showed a huge polarization dependence in the electric-dipole allowed transitions for electrons between the core and the magnetically polarized valence shell. Second, the X-ray absorption followed by photon emission in resonant scattering is an elastic process and can be considered as a virtual transition. The huge sensitivity to magnetism of the X-ray absorption and hence resonant scattering techniques arises from the strong spin-orbit interaction of electrons in the probed core level.

Since these initial studies, access to synchrotrons, which can generate light that is tunable in both energy and polarization, has led to X-ray resonant magnetic scattering becoming a mature technique, which has given insights into the magnetic structure of rare earth, actinide and transition metal compounds. It complements advances in neutron scattering by providing element, chemical-valence and electron-shell specificity, and also separating the spin and orbital contributions to the magnetic moments.

An application of this technique, soft X-ray resonant magnetic scattering (SXRMS), has now been established to examine technologically important structures, such as magnetic thin films, used in data storage devices, where the wavelength of the soft X-rays is a perfect match for the nanolength scale. This application is unique in that it provides information on structural and magnetic properties of layers, interface roughness, induced magnetic order in non-magnetic spacer layers and layer-resolved magnetic moments.

G. van der Laan "Soft X-ray resonant magnetic scattering of magnetic nanostructures", Comptes Rendus Physique 9 (2008) 570 – 584.
http://france.elsevier.com/direct/COMREN/