Dichroism Method Adapted for Lab Use Unlocks Materials’ Secrets

Researchers at the Max Born Institute (MBI) used x-ray magnetic circular dichroism (XMCD), a technique that can be used to measure the spin and charge dynamics in multi-elemental magnetic materials, in a laser laboratory setting and performed experiments at the absorption L edges of iron at a photon energy of around 700 eV.

Until now, use of XMCD in the soft x-ray range has been limited to synchrotron-radiation sources and free-electron lasers. The researchers believe that their approach could be adapted to other transition metal and rare-earth absorption edges and, with a temporal resolution of less than 10 ps, to a wide range of ultrafast magnetization studies.

In addition, the pulse duration of the generated x-ray pulses of only a few picoseconds could make it possible to observe and ultimately understand very fast magnetization processes — for example, those triggered by ultrashort light flashes.

To generate soft x-ray light via a laser-driven plasma source, the researchers focused short (2 ps), intense (200 mJ per pulse) laser pulses onto a cylinder of tungsten. The plasma that was generated emitted large amounts of light continuously, in the relevant spectral range of 200 to 2000 eV, at a pulse duration of smaller than 10 ps.

However, the generation process was arbitrary. As a result, the polarization of the soft x-ray light was not circular, as is required for XMCD, but completely random, like that of a light bulb.

To generate circular polarization in the soft x-ray regime, the researchers combined the laser-driven plasma source with a magnetic thin-film polarizer. The x-ray light passed through a magnetic polarization filter, in which the random XMCD effect was active. Due to the polarization-dependent dichroic transmission, an imbalance of light particles, with parallel versus anti-parallel angular momentum relative to the magnetization of the filter, was generated.

Artist’s depiction of the XMCD experiment. First, the soft-x-ray light from a plasma source is circularly polarized by the transmission through a magnetic film. Subsequently, the magnetization in the actual sample can be determined accurately. Courtesy of Christian Tzschaschel.

After passing through the polarization filter, the partially circularly or elliptically polarized soft x-ray light could be used for an XMCD experiment on a magnetic sample.

The broadband characteristics of this approach to XMCD enabled the researchers to measure XMCD spectra across the entire spin-orbit pair of free electron edges in a single acquisition, with a signal-to-noise ratio comparable to the bending-magnet beamlines at synchrotron-radiation facilities.

Circular dichroism of magnetic origin is particularly pronounced in the soft x-ray region, covering 200 to 2000 eV energy of the light particles, corresponding to a wavelength of only 6 to 0.6 nm, when the element-specific absorption edges of transition metals, such as iron, and rare earths, such as dysprosium, are considered. These elements are important for the technical application of magnetic effects. XMCD allows scientists to precisely determine the magnetic moment of an element without damaging the sample system, even when it is buried within layers of a material. If the circularly polarized soft-x-ray radiation comes in very short femtosecond to picosecond pulses, even ultrafast magnetization processes can be monitored on a relevant timescale.

The work of the MBI team to perform XMCD measurements in a laser laboratory suggests that the capabilities of laser-based x-ray sources are catching up with those of 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,” researcher Martin Borchert said.

Based on the researchers’ success, the number of scientific and technological investigations that take place outside of large-scale facilities could grow. The flexibility in the sample environment, the short iteration cycles, and the availability of a laboratory-based, ultrafast XMCD setup would allow for detailed, systematic studies of time- and element-resolved spin dynamics, the researchers said.

The research was published in Optica (www.doi.org/10.1364/OPTICA.480221).