Precise survey of magnetism with light

The probing of magnetic materials with extreme ultraviolet radiation provides a detailed microscopic picture of how magnetic systems interact with light – the fastest way to manipulate a magnetic material. A team of researchers led by the Max Born Institute provided the experimental and theoretical bases for interpreting such spectroscopic signals. The results were published in Letters of physical examination.

The study of the interaction between light and matter is one of the most powerful ways to help physicists understand the microscopic world. In magnetic materials, optical spectroscopy can recover a wealth of information in which the energy of individual light particles – the photons – promotes the electrons of the inner shell at higher energies. Indeed, such an approach makes it possible to obtain the magnetic properties separately for the different types of atoms of the magnetic material and allows the scientists to understand the role and the interaction of the various constituents. This experimental technique, called X-ray magnetic circular dichroism spectroscopy (XMCD), was developed in the late 1980s and generally requires large-scale installation – a synchrotron radiation source or an X-ray laser.

To study how magnetization responds to ultra-short pulses – the fastest way to deterministically control magnetic materials – smaller-scale laboratory sources have become available in recent years, delivering ultra-short pulses in the range. extreme ultraviolet spectral (XUV). XUV photons, less energetic, excite less strongly bound electrons in the material, which poses 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 and researchers from the Max Planck Institute for Microstructural Physics in Halle and the University of Uppsala in Sweden provided an analysis of the magneto-optical response of XUV photons. They combined experiments with ab initio calculations, which only take as input information the types of atoms and their arrangement in the material. For the prototypical magnetic elements iron, cobalt and nickel, they were able to measure in detail the response of these materials to XUV radiation. Scientists discovered that the observed signals were not simply proportional to the magnetic moment at the respective element level, and that this deviation is theoretically reproduced when local field effects are taken into account. Sangeeta Sharma, who provided the theoretical description, explains: "Local 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. system response to this disturbance must be taken into account when interpreting the spectra ".

This new understanding now makes it possible to disentangle quantitatively the signals of different elements of the same material. "Since most functional magnetic materials are made up of several elements, this understanding is crucial for studying them, especially when we are interested in the more complex dynamic response when manipulating them with laser pulses," explains Felix Willems, lead author. of the book. study. "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 magnetization over a very short period of time." This is of fundamental and applied interest. "