Tracking down antiferromagnets
Magnetic materials are an integral part of our everyday life, e.g. as a needle in a compass or as a data storage in a hard disk. Usually one thinks of ferromagnets, where all magnetic moments point in the same direction. A typical example is the fridge magnet at home. Most magnetic materials, however, form a completely different - namely antiferromagnetic - order, in which the magnetic moments align themselves periodically, but e.g. in opposite directions, and thus no net magnetization can be detected. This is also the reason why this class of materials was discovered only very late in the 1930s by the French Nobel laureate Louis Néel. For a long time, antiferromagnets were considered to be rather academic systems for fundamental research without any application potential. However, this opinion has changed drastically, especially in the last decades, due to the discovery of new material systems and methods for characterization as well as control of the magnetic order. In this context, antiferromagnets exceed ferromagnets with their significantly higher speed, stability and energy efficiency, e.g. in data processing and storage.
One of the most important methods for studying antiferromagnets is resonant magnetic scattering. In this mixture of spectroscopy and scattering experiment, light with a very specific frequency (analogous to the color in the visible range) is required to visualize the magnetism. Also the wavelength of the light, corresponding to the frequency, must be smaller than the antiferromagnetic periodicity in the sample under investigation. These two criteria are met in the so-called soft X-ray region for many antiferromagnetic systems, which in turn can provide direct insights into magnetic order on length scales of a few nanometers. Unfortunately, the required light sources with appropriate brightness have so far only been available at scientific large-scale facilities such as synchrotrons and free-electron lasers, significantly limiting the availability of this powerful experimental method.
Figure 1: Schematic setup of the magnetic scattering experiment. A very intense pulsed laser in the infrared range is focused onto a rotating tungsten cylinder. The generated plasma emits soft X-rays over a very wide spectral range, see source spectrum. Using a reflective zone plate, a selected fraction of this light is collected and sent through a slit onto the sample under investigation. The scattered light from the sample is detected by a camera. The structure of the investigated artificial antiferromagnet is sketched in the top-right corner.
Researchers from the Max Born Institute, Forschungszentrum Jülich, and Helmholtz-Zentrum Berlin have now succeeded for the first time in conducting such an experiment on a laboratory scale. To do so, they utilized and optimized an established technique for generating soft X-rays - a laser-driven plasma source. The thin-disk laser used was developed specifically for this and similar applications at the Max Born Institute. Extremely energetic and very short (2 ps = 0.000 000 002 s) light flashes from the laser are focused onto a metal cylinder made of tungsten. In the focus, conditions similar to those on the surface of the sun prevail for 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 broad spectral range, just like the sun. Since the plasma is driven by very short laser pulses, the light flashes generated are also only slightly longer. With the help of special optics, a reflection zone plate, it is possible to sufficiently collect soft X-rays from this plasma emission and utilize them for magnetic scattering experiments.
To demonstrate their new concept, the researchers examined an artificial antiferromagnet. This was tailored by alternately growing several layers of pure iron and chromium each only about a nanometer thick. The iron layers are pure ferromagnets, but they align themselves exactly antiparallel to each other by coupling across 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 iron layers. Both periodicities can be resolved by resonant scattering, allowing direct insight into the structural and antiferromagnetic order in the sample system, see Fig. 2.
Figure 2: Reflectivity of the artificial antiferromagnet as a function of the normalized angle of incidence, L, of the soft X-ray radiation. The blue and orange circles represent the measured data at different frequencies of the X-ray radiation. The intense reflectivity at L=1 provides direct information about the structural periodicity of the sample and can be measured independently of the light frequency. The increased reflectivity at L=0.5 occurs only for specific, magnetically sensitive light frequencies and is an unambiguous and quantitative measure for determining the strength and periodicity of the antiferromagnetic order in the sample. The blue line shows a very good agreement between experiment and theory.
As mentioned above, the laser-driven plasma source not only provides sufficient soft X-ray radiation for magnetic scattering experiments in the lab, but at the same time its light flashes are also particularly short - namely only a few picoseconds. Accordingly, the measurements as described above can also be performed in a stroboscopic mode, allowing, for example, the investigation of light-induced dynamics on time scales of the pulse duration. The corresponding time-resolved measurements of the artificial antiferromagnet impressively demonstrate the advantages of this work compared to current and future synchrotron sources, which offer a temporal resolution worse by a factor of 10.
Artistic impression of the magnetic scattering experiment. The butterfly-shaped X-ray spectrum is scattered by an artificial antiferromagnet. The elliptical structure of the reflection zone plate appears in the background. Credit: Moritz Eisebitt.