Unlocking the Secret Nanostructures of Magnetic Materials With the Right Illumination

Researchers from the Max Born Institute in Berlin have efficiently carried out X-ray Magnetic Circular Dichroism (XMCD) experiments in a laser laboratory for the primary time.

Unlocking the secrets and techniques of magnetic supplies requires the suitable illumination. Magnetic x-ray round dichroism makes it potential to decode magnetic order in nanostructures and to assign it to completely different layers or chemical components. Researchers on the Max Born Institute in Berlin have succeeded in implementing this distinctive measurement method within the soft-x-ray vary in a laser laboratory. With this improvement, many technologically related questions can now be investigated outdoors of scientific large-scale amenities for the primary time.

Magnetic nanostructures have lengthy been a part of our on a regular basis life, e.g., within the type of quick and compact knowledge storage units or extremely delicate sensors. A serious contribution to the understanding of lots of the related magnetic results and functionalities is made by a particular measurement methodology: X-ray Magnetic Circular Dichroism (XMCD).

This spectacular time period describes a basic impact of the interplay between gentle and matter: In a ferromagnetic materials, there’s an imbalance of electrons with a sure angular momentum, the spin. If one shines circularly polarized gentle, which additionally has an outlined angular momentum, by means of a ferromagnet, a transparent distinction in transmission for a parallel or anti-parallel alignment of the 2 angular momenta is observable — a so-called dichroism.

This round dichroism of magnetic origin is especially pronounced within the soft-x-ray area (200 to 2000 eV vitality of the sunshine particles, equivalent to a wavelength of solely 6 to 0.6 nm), when contemplating the element-specific absorption edges of transition metals, corresponding to iron, nickel, or cobalt, in addition to uncommon earths, corresponding to dysprosium or gadolinium. These components are significantly vital for the technical software of magnetic results.

The XMCD impact permits for exactly figuring out the magnetic second of the respective components, even in buried layers in a fabric and with out damaging the pattern system. If the circularly polarized soft-x-ray radiation is available in very quick femto- to picosecond (ps) pulses, even ultrafast magnetization processes may be monitored on the related time scale. Until now, entry to the required x-ray radiation has solely been potential at scientific large-scale amenities, corresponding to synchrotron-radiation sources or free-electron lasers (FELs), and has thus been strongly restricted.

The averaged transmission by means of the investigated pattern on the Fe L absorption edges (black knowledge factors) may be measured exactly and is properly described by a simulation (black line). At the 2 absorption maxima, see insets, important dichroism for the 2 completely different instructions of saturation magnetization of the pattern is observable. So far, such experiments have solely been potential at large-scale amenities. Credit: Max Born Institute

A crew of researchers round junior analysis group chief Daniel Schick on the Max Born Institute (MBI) in Berlin has now succeeded for the primary time in realizing XMCD experiments on the absorption L edges of iron at a photon energy of around 700 eV in a laser laboratory.

A laser-driven plasma source was used to generate the required soft x-ray light, by focusing very short (2 ps) and intense (200 mJ per pulse) optical laser pulses onto a cylinder of tungsten. The generated plasma thereby emits a lot of light continuously in the relevant spectral range of 200-2000 eV at a pulse duration of smaller than 10 ps. However, due to the stochastic generation process in the plasma, a very important requirement to observe XMCD is not met — the polarization of the soft-x-ray light is not circular, as required, but completely random, similar to that of a light bulb.

Therefore, the researchers used a trick: the x-ray light first passes through a magnetic polarization filter in which the same XMCD effect as described above is active. Due to the polarization-dependent dichroic transmission, an imbalance of light particles with parallel vs. anti-parallel angular momentum relative to the magnetization of the filter can be generated. 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.

Magnetic asymmetry behind the polarizer and the examined sample at the Fe L absorption edges. The two colors correspond to measurements with reversed magnetization of the polarizer — the magnetization direction of the sample is immediately evident from the sign of the dichroism observed (blue vs. red curve). The measurements can be reproduced very accurately by simulations (lines). Credit: Max Born Institute

The work, published in the scientific journal OPTICA, demonstrates that laser-based x-ray sources are catching up with large-scale facilities. “Our concept for generating circularly polarized soft x-rays is not only very flexible but also partly superior to conventional methods in XMCD spectroscopy due to the broadband nature of our light source,” says the first author of the study and PhD student at the MBI, Martin Borchert. In particular, the already demonstrated pulse duration of the generated x-ray pulses of only a few picoseconds opens up new possibilities to observe and ultimately understand even very fast 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