Laser-driven Spin Dynamics in Ferrimagnets: How does the Angular Momentum flow?
Experiments at the femtoslicing facility of BESSY II revealed the ultrafast angular momentum flow from Gd and Fe spins to the lattice via orbital moment during demagnetization of GdFe alloy. © R. Abrudan/HZB
When exposed to intense laser pulses, the magnetization of a material can be manipulated very fast. Fundamentally, magnetization is connected to the angular momentum of the electrons in the material. A team of researchers led by scientists from the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) has now been able to follow the flow of angular momentum during ultrafast optical demagnetization in a ferrimagnetic iron-gadolinium alloy at the femtoslicing facility of BESSY II. Their results are helpful to understand the fundamental processes and their speed limits. The study is published in Physical Review Letters.
Illumination with an ultrashort laser pulse is a means to demagnetize a material very fast - for the prototypical ferromagnets iron, cobalt and nickel, for example, the magnetization is extinguished within about one picosecond (10-12 s) after the laser pulse has hit the material. This has led to the question, through which channels the angular momentum associated with the magnetization is transferred to other reservoirs during the short time available. Researchers from MBI in Berlin together with scientists from Helmholtz Zentrum Berlin and Nihon University, Japan, have now been able to follow this flow of angular momentum in detail for an iron-gadolinium alloy. In this ferrimagnetic material, adjacent iron (Fe) and gadolinium (Gd) atoms have magnetization with opposite direction.
The researchers have used ultrashort x-ray pulses at the femtoslicing facility of BESSY II to monitor the absorption of circularly polarized x-rays by the Fe and Gd atoms as a function of time after previous laser excitation. This approach is unique in that it allows tracking the magnetic moment during the ultrafast demagnetization at both types of atoms individually. Even more, it is possible to distinguish angular momentum stored in the orbital motion vs. in the spin of the electrons when the respective absorption spectra are analyzed.
W With this detailed “x-ray vision”, the scientists found that during demagnetization process of GdFe alloy the angular momentum flows from Gd and Fe spins to the orbital moments and eventually to the lattice. This means that the surrounding lattice acts as 100 % sink of angular momentum for the demagnetizing Fe and Gd spins on a sub-picosecond timescale.
Given that short laser pulses can also be used to permanently switch magnetization and thus write bits for magnetic data recording, the insight in the dynamics of these fundamental mechanisms is of relevance to develop new approaches to write data to mass data storage media much faster than possible today.
- Energy of charge carrier pairs in cuprate compoundsHigh-temperature superconductivity is still not fully understood. Now, an international research team at BESSY II has measured the energy of charge carrier pairs in undoped La₂CuO₄. Their findings revealed that the interaction energies within the potentially superconducting copper oxide layers are significantly lower than those in the insulating lanthanum oxide layers. These results contribute to a better understanding of high-temperature superconductivity and could also be relevant for research into other functional materials.
- Electrocatalysis with dual functionality – an overviewHybrid electrocatalysts can produce green hydrogen, for example, and valuable organic compounds simultaneously. This promises economically viable applications. However, the complex catalytic reactions involved in producing organic compounds are not yet fully understood. Modern X-ray methods at synchrotron sources such as BESSY II, enable catalyst materials and the reactions occurring on their surfaces to be analysed in real time, in situ and under real operating conditions. This provides insights that can be used for targeted optimisation. A team has now published an overview of the current state of knowledge in Nature Reviews Chemistry.
- BESSY II: Phosphorus chains – a 1D material with 1D electronic propertiesFor the first time, a team at BESSY II has succeeded in demonstrating the one-dimensional electronic properties in phosphorus. The samples consisted of short chains of phosphorus atoms that self-organise at specific angles on a silver substrate. Through sophisticated analysis, the team was able to disentangle the contributions of these differently aligned chains. This revealed that the electronic properties of each chain are indeed one-dimensional. Calculations predict an exciting phase transition to be expected as soon as these chains are more closely packed. While material consisting of individual chains with longer distances is semiconducting, a very dense chain structure would be metallic.