A research team from Helmholtz-Zentrum Berlin and Freie Universität demonstrated that laser pulses can affect 4f electrons and alter their magnetic properties. Experiments at EuXFEL and FLASH made this finding possible, providing a new avenue for storing data using rare earth elements. The journal Science Advances published this study.
The image shows the terbium orbitals between which the excitation takes place and a schematic sketch of the excitation process. Image Credit: Helmholtz-Zentrum Berlin
The electrons in the 4f shell are responsible for the unique characteristics of rare earth magnetic materials. Until now, it was thought to be nearly impossible to control the magnetic characteristics of 4f electrons.
The strongest magnets known are based on rare earths. Their magnetic characteristics are caused by their 4f electrons, which produce a strong magnetic moment that remains constant despite changes in their chemical surroundings.
This implies that the unique magnetic properties of rare earths can be maintained when they are included in a wide range of compounds and alloys.
Previously, it was thought that even when a laser pulse was used to excite the material, the magnetic characteristics of 4f electrons could not be altered. This is feasible as demonstrated by a group from the European X-ray laser XFEL, DESY, HZB, Freie Universität Berlin, and other institutions.
One can momentarily change their spatial configuration by using a laser to excite the 4f electrons. This also alters their magnet. This effect creates new opportunities for the quick and energy-saving manipulation of rare-earth magnetic materials.
Terbium Studied at the X-ray Lasers EuXFEL and FLASH
The group examined samples of terbium, a rare earth element with atomic number 65 and a total of eight electrons in 4f orbitals, and conducted tests at the X-ray lasers EuXFEL and FLASH. After being stimulated by an ultrashort laser pulse, the sample was examined using X-ray spectroscopy.
The study's soft X-ray radiation has a particularly sensitive way of determining a material's electrical structure. The experiment demonstrates that 4f electrons momentarily transition to an orbital with an altered spatial distribution following laser activation.
This results from a previously unrecognized scattering process using 5D electrons. The laser stimulation generates a momentary change in the magnetic characteristics of the 4f electrons due to their redistribution.
Rare Earth Materials as Data Storage Devices
Rare earth minerals can now be used in controlled switching devices, such as quick and energy-efficient data storage systems. Rare earth elements have not yet been included in the magnetic storage medium.
The newest storage mediums are data storage devices known as HAMR (Heat-Assisted Magnetic Recording) devices, in which magnetic structures are heated by a laser pulse such that a magnet can switch them.
Thanks to the considerably stronger rare-earth magnets, an ultrashort laser pulse could now excite the 4f electrons and enable switching. This electronic effect would be significantly faster and more effective than the heating mechanism in HAMR memory.
High-Resolution Spectroscopy with Ultrashort X-ray Pulses at BESSY II
The advent of accelerator-based X-ray sources that produce ultrashort X-ray pulses in recent decades has made this research conceivable. On periods of a few fs, these X-ray sources enable the observation of basic processes in magnetic materials. One-millionth of a billionth of a second is called a femtosecond (10–15 s). A hair's breadth's worth of light travels in 300 fs.
The work was done at FLASH in Hamburg and the European X-ray laser EuXFEL. Additionally, the HZB runs a short-pulse X-ray source that will be enhanced by the end of the year, especially for high-spectroscopic-resolution investigations. Then, BESSY II will likewise provide ideal settings for this kind of study. Berlin is one of the top locations in the world for studying ultrafast magnetic effects.
Journal Reference:
Kühn, T. N., et al. (2024) Optical control of 4 f orbital states in rare-earth metals. Science Advances. doi.org/10.1126/sciadv.adk9522.