Probing magnetic materials with extreme ultraviolet radiation provides a detailed microscopic picture of how magnetic systems interact with light – the fastest way to manipulate magnetic material. A team of researchers led by the Max Born Institute has now provided the experimental and theoretical bases to interpret such spectroscopic signals. The results were published in Physical examination letters.
Studying the interaction between light and matter is one of the most powerful ways to help physicists understand the microscopic world. In magnetic materials, a wealth of information can be retrieved by optical spectroscopy where the energy of individual light particles – photons – favors electrons in the inner shell to higher energies. Indeed, such an approach makes it possible to obtain the magnetic properties separately for the different types of atoms in the magnetic material and allows scientists to understand the role and the interaction of the different constituents. This experimental technique, called X-ray Magnetic Circular Dichroism Spectroscopy (XMCD), was developed in the late 1980s and usually requires a large-scale installation – a synchrotron radiation source or an X-ray laser.
To study how magnetization responds to ultrashort laser pulses – the fastest way to deterministically control magnetic materials – smaller-scale laboratory sources have become available in recent years, delivering ultrashort pulses in the extreme ultraviolet spectral range. (XUV). XUV photons, being less energetic, excite less strongly bound electrons in the material, posing new challenges for the interpretation of the resulting spectra in terms of underlying magnetization in the material.
A team of researchers from the Max Born Institute in Berlin, along with researchers from the Max-Planck-Institute for Microstructure Physics in Halle and the University of Uppsala in Sweden, have now provided a detailed analysis of the magneto-optical response of XUV photons. They combined experiments with ab initio calculations, which only take the types of atoms and their arrangement in the material as input information. For the prototype magnetic elements iron, cobalt and nickel, they were able to measure in detail the response of these materials to XUV radiation. Scientists find that the observed signals are not simply proportional to the magnetic moment at the level of the respective element, and that this deviation is reproduced in theory when the so-called local field effects are taken into account. Sangeeta Sharma, who provided the theoretical description, explains: âLocal field effects can be understood as a transient rearrangement of the electronic charge in the material, caused by the electric field of the XUV radiation used for the investigation. The response of the system to this disturbance must be taken into account when interpreting the spectra. “
This new perspective now makes it possible to quantitatively disentangle the signals of different elements in the same material. âAs most functional magnetic materials are made up of multiple elements, this understanding is crucial for studying such materials, especially when we are interested in the more complex dynamic response when manipulating them with laser pulses,â says Felix Willems, the first author to study it. “By combining experience and theory, we are now ready to study how dynamic microscopic processes can be used to achieve a desired effect, such as switching the magnetization on a very short time scale. This is of a both fundamental and applied interest. “
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