This outstanding term describes a basic result of the interaction between light and matter: In a ferromagnetic material, there is an imbalance of electrons with a certain angular momentum, the spin. If one shines circularly polarized light, which also has a specified angular momentum, through a ferromagnet, a clear difference in transmission for a parallel or anti-parallel alignment of the 2 angular momenta is observable– a so-called dichroism.
This circular dichroism of magnetic origin is especially pronounced in the soft-x-ray area (200 to 2000 eV energy of the light particles, representing a wavelength of just 6 to 0.6 nm), when thinking about the element-specific absorption edges of transition metals, such as iron, nickel, or cobalt, in addition to unusual earths, such as dysprosium or gadolinium. These aspects are particularly important for the technical application of magnetic effects.
The XMCD impact enables specifically determining the magnetic minute of the respective aspects, even in buried layers in a product and without damaging the sample system. If the circularly polarized soft-x-ray radiation can be found in very short femto- to picosecond (ps) pulses, even ultrafast magnetization processes can be kept track of on the appropriate time scale. Previously, access to the needed x-ray radiation has just been possible at scientific massive centers, such as synchrotron-radiation sources or free-electron lasers (FELs), and has thus been strongly restricted.
At the 2 absorption maxima, see insets, significant dichroism for the two various instructions of saturation magnetization of the sample is observable. Credit: Max Born Institute.
A group of researchers around junior research group leader Daniel Schick at limit Born Institute (MBI) in Berlin has now been successful for the very first time in understanding XMCD experiments at the absorption L edges of iron at a photon energy of around 700 eV in a laser lab.
A laser-driven plasma source was used to generate the necessary soft x-ray light, by focusing extremely short (2 ps) and extreme (200 mJ per pulse) optical laser pulses onto a cylinder of tungsten. The produced plasma therefore emits a great deal of light constantly in the appropriate spectral series of 200-2000 eV at a pulse period of smaller than 10 ps. Nevertheless, due to the stochastic generation procedure in the plasma, a really essential requirement to observe XMCD is not satisfied– the polarization of the soft-x-ray light is not circular, as required, however entirely random, comparable to that of a light bulb.
Therefore, the scientists utilized a trick: the x-ray light initially passes through a magnetic polarization filter in which the same XMCD effect as explained above is active. Due to the polarization-dependent dichroic transmission, an imbalance of light particles with anti-parallel vs. parallel angular momentum relative to the magnetization of the filter can be produced. After travelling through the polarization filter, the partially circularly or elliptically polarized soft-x-ray light can be used for the actual XMCD experiment on a magnetic sample.
Magnetic asymmetry behind the polarizer and the taken a look at sample at the Fe L absorption edges. Credit: Max Born Institute.
The work, released in the scientific journal OPTICA, demonstrates that laser-based x-ray sources are overtaking massive centers. “Our concept for creating circularly polarized soft x-rays is not just really flexible but likewise partially superior to standard techniques in XMCD spectroscopy due to the broadband nature of our light source,” states the very first author of the research study and PhD student at the MBI, Martin Borchert. In particular, the already demonstrated pulse period of the produced x-ray pulses of just a couple of picoseconds opens brand-new possibilities to observe and ultimately comprehend even really quick magnetization processes, e.g., when triggered by ultrashort light flashes.
Reference: “X-ray magnetic circular dichroism spectroscopy at the Fe L edges with a picosecond laser-driven plasma source” by Martin Borchert, Dieter Engel, Clemens von Korff Schmising, Bastian Pfau, Stefan Eisebitt and Daniel Schick, 4 April 2023, Optica.DOI: 10.1364/ OPTICA.480221.
By Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI).
May 26, 2023.
Artists impression of the XMCD experiment. The soft-x-ray light from a plasma source is first circularly polarized by the transmission through a magnetic film.
Researchers from the Max Born Institute in Berlin have effectively carried out X-ray Magnetic Circular Dichroism (XMCD) experiments in a laser lab for the first time.
Opening the tricks of magnetic products requires the right lighting. Magnetic x-ray circular dichroism makes it possible to decode magnetic order in nanostructures and to designate it to different layers or chemical aspects. Researchers at limit Born Institute in Berlin have been successful in executing this unique measurement technique in the soft-x-ray range in a laser laboratory. With this development, many highly relevant questions can now be investigated outside of clinical massive centers for the very first time.
Magnetic nanostructures have long been part of our everyday life, e.g., in the type of compact and quick data storage devices or highly delicate sensing units. A major contribution to the understanding of a lot of the pertinent magnetic effects and functionalities is made by an unique measurement technique: X-ray Magnetic Circular Dichroism (XMCD).
The soft-x-ray light from a plasma source is first circularly polarized by the transmission through a magnetic movie. Magnetic x-ray circular dichroism makes it possible to decipher magnetic order in nanostructures and to assign it to chemical elements or different layers. The XMCD impact allows for exactly figuring out the magnetic moment of the respective elements, even in buried layers in a product and without harming the sample system. The scientists utilized a technique: the x-ray light initially passes through a magnetic polarization filter in which the same XMCD result as described above is active. After passing through the polarization filter, the partially circularly or elliptically polarized soft-x-ray light can be employed for the actual XMCD experiment on a magnetic sample.
