Today, however, antiferromagnetics are prime candidates for faster, more energy-efficient data processing and storage. In this way, magnetic scattering in the soft X-ray region – a combination of spectroscopy and scattering experiment – provided direct insight into the magnetic order of antiferromagnetics and thus allowed important contributions to the study. field. However, until now such experiments could only be performed in large-scale scientific facilities, such as synchrotrons and free-electron lasers, which provide sufficient light in the soft X-ray range. At the Max Born Institute, it is now possible for the first time to study an antiferromagnetic sample by magnetic scattering on a laboratory laser-controlled source. The work was published in the journal Optica and was also chosen for the cover image.
Magnetic materials are an integral part of our daily life, for example as a needle in a compass or as data storage on a hard drive. We usually think of ferro-magnets, where all the magnetic moments point in the same direction. A typical example is the home refrigerator magnet. Most magnetic materials, however, form a completely different order – namely antiferromagnetic – in which the magnetic moments periodically align, but for example in opposite directions, and therefore no clear magnetization can be detected. This is also the reason why this class of materials was not discovered until very late in the 1930s by the French Nobel laureate Louis Néel. For a long time, antiferromagnetics were regarded as rather academic systems for basic research without any potential for application. However, this view has changed dramatically, especially in recent decades, due to the discovery of new material systems and methods for characterization as well as control of magnetic order. In this context, antiferromagnetics overtake ferro-magnets with their significantly higher speed, stability and energy efficiency, for example in data processing and storage.
One of the most important methods for studying antiferromagnetics is resonant magnetic scattering. In this mixture of spectroscopy and scattering experiment, light with a very specific frequency (analogous to color in the visible domain) is needed to visualize magnetism. In addition, the wavelength of light, corresponding to the frequency, must be less than the antiferromagnetic periodicity in the sample studied. Both of these criteria are met in the so-called soft X-ray region for many antiferromagnetic systems, which in turn can provide direct information on the magnetic order on scales of a few nanometers in length. Unfortunately, the required light sources with the appropriate brightness have so far only been available in large-scale scientific facilities such as synchrotrons and free electron lasers, severely limiting the availability of this powerful experimental method.
Researchers from the Max Born Institute, Forschungszentrum Jülich and Helmholtz-Zentrum Berlin have for the first time succeeded in carrying out such a laboratory-scale experiment. To do this, they used and optimized an established technique for generating soft x-rays – a laser-driven plasma source. The thin disc laser used was developed specifically for this application and similar applications at the Max Born Institute. The extremely energetic and very short (2 ps = 0.000 000 002 s) flashes of light from the laser are focused on a tungsten metal cylinder. At the focus, conditions similar to those on the surface of the sun prevail during the short duration of the laser pulse and lead to the generation of a plasma. This form of matter, also known as the fourth aggregate state, itself emits light over a very wide spectral range, just like the sun. Since the plasma is driven by very short laser pulses, the generated flashes of light are also only slightly longer. With the help of special optics, reflection zone plate, it is possible to collect enough soft x-rays of this plasma emission and use them for magnetic scattering experiments.
To demonstrate their new concept, the researchers looked at an artificial antiferromagnetic. This was adapted by alternately growing several layers of pure iron and chromium each about one nanometer thick. The iron layers are pure ferromagnetics, but they align exactly antiparallel to each other by coupling through the chromium layers, see Fig. 1. In addition to a structural periodicity due to the alternating layers, this also results in a so-called antiferromagnetic superstructure, which contains exactly two layers of iron. Both periodicities can be resolved by resonant scattering, allowing direct insight into the structural and antiferromagnetic order in the sample system, see Fig. 2.
As mentioned above, the laser driven plasma source not only provides sufficient soft X-ray radiation for laboratory magnetic scattering experiments, but at the same time its flashes of light are also particularly short, i.e. only a few picoseconds. Accordingly, measurements as described above can also be performed in a strobe mode, allowing, for example, the study of light-induced dynamics over time scales of pulse duration. The corresponding time-resolved measurements of the artificial antiferromagnetic impressively demonstrate the advantages of this work over current and future synchrotron sources, which offer a 10-fold worse time resolution.