Uncovering secret nanostructures of magnetic materials with the right lighting

Author: Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI), May 26, 2023

Artist’s impression of the XMCD experiment. Soft X-ray light from a plasma source is first circularly polarized by passing through a magnetic film. Subsequently, the magnetization of a real sample can be accurately determined. Credit: Christian Zschaschel

Researchers at the Max Born Institute in Berlin have successfully carried out X-ray magnetic circular dichroism (XMCD) experiments for the first time in a laser laboratory.

Revealing the secrets of magnetic materials requires proper lighting. Magnetic X-ray circular dichroism makes it possible to decipher the magnetic order in nanostructures and assign it to different layers or chemical elements. Researchers at the Max Born Institute in Berlin have succeeded in implementing this unique soft X-ray measurement technique in a laser laboratory. Thanks to this development, many technologically important questions can now be explored for the first time outside of large-scale scientific installations.

Magnetic nanostructures have long been part of our daily lives, for example in the form of fast and compact data storage devices or highly sensitive sensors. A special measurement technique contributes greatly to the understanding of many relevant magnetic effects and functions: X-ray magnetic circular dichroism (XMCD).

This impressive term describes a fundamental effect of the interaction of light and matter: in a ferromagnetic material, there is an imbalance of electrons with a certain angular momentum, spin. If you shine circularly polarized light, which also has a certain angular momentum, through a ferromagnet, you can observe a clear difference in transmission for parallel or anti-parallel alignment of two angular momentum – the so-called dichroism.

This circular dichroism of magnetic origin is especially pronounced in the region of soft X-rays (the energy of light particles is from 200 to 2000 eV, which corresponds to a wavelength of only 6 to 0.6 nm), when considering the transition absorption edges characteristic of the element metals such as iron , nickel or cobalt, as well as rare earth elements such as dysprosium or gadolinium. These elements are especially important for the technical application of magnetic effects.

The XMCD effect makes it possible to accurately determine the magnetic moment of the corresponding elements even in hidden material layers and without damaging the sample system. If circularly polarized soft x-rays come in the form of very short femtosecond to picosecond (ps) pulses, then even ultrafast magnetization processes can be tracked on the appropriate time scale. Until now, access to the necessary X-rays has only been possible in large scientific facilities such as synchrotron radiation sources or free electron lasers (FELs) and has therefore been severely limited.

The average transmission through the test sample at the Fe L absorption edges (black dots) can be accurately measured and is well described by simulation (black line). In two absorption maxima, see insets, significant dichroism is observed for two different directions of sample saturation magnetization. So far, such experiments are possible only on large-scale facilities. Credit: Max Born Institute.

A team of researchers led by junior research team leader Daniel Schick of the Max Born Institute (MBI) in Berlin succeeded for the first time in realizing XMCD experiments at the absorption edges of L iron at a photon energy of about 700 eV in a laser. laboratory.

To generate the required soft X-ray radiation, a laser plasma source was used by focusing very short (2 ps) and intense (200 mJ per pulse) optical laser pulses onto a tungsten cylinder. Thus, the generated plasma continuously emits a lot of light in the corresponding spectral range of 200-2000 eV with a pulse duration of less than 10 ps. However, due to the stochastic generation process in the plasma, a very important requirement for XMCD observation is not met – the polarization of soft X-rays is not circular, as required, but completely random, like a light bulb.

So the researchers used a trick: X-ray light first passes through a magnetic polarization filter, in which the same XMCD effect is active as described above. Due to polarization-dependent dichroic transmission, an imbalance of light particles with parallel and anti-parallel angular momentum relative to the filter magnetization can occur. After passing through a polarizing filter, soft X-rays with partial circular or elliptical polarization can be used for the actual XMCD experiment on a magnetic sample.

Magnetic asymmetry behind the polarizer and the sample under study at the absorption edges of Fe L. Two colors correspond to measurements with the reverse magnetization of the polarizer – the direction of the sample magnetization is immediately visible from the observed sign of dichroism (blue and red curves). Measurements can be reproduced very accurately by modeling (lines). Credit: Max Born Institute.

The paper, published in the scientific journal OPTICA, shows that laser X-ray sources are catching up with large-scale installations. “Our concept of generating circularly polarized soft X-rays is not only very flexible, but also partially superior to traditional XMCD spectroscopy methods due to the broadband nature of our light source,” says the first author of the study and an MBI graduate student. , Martin Borchert. In particular, the already demonstrated pulse duration of generated X-ray pulses of only a few picoseconds opens up new possibilities for observing and ultimately understanding even very fast magnetization processes, for example, when initiated by ultrashort light flashes.

Reference: “X-ray spectroscopy of circular magnetic dichroism at the edges of Fe L with a picosecond laser plasma source”, Martin Borchert, Dieter Engel, Clemens von Korf Schmiesing, Bastian Pfau, Stefan Eisebitt and Daniel Schick, April 4, 2023, Optica.
DOI: 10.1364/OPTICS.480221