Keywords: user research (31) BESSY II (269) materials research (67) spintronics (93) quantum materials (28)

Science Highlight    10.05.2019

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.
Copyright: 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.






You might also be interested in
  • <p>Tomography of a lithium electrode in its initial condition.</p>SCIENCE HIGHLIGHT      06.05.2019

    3D tomographic imagery reveals how lithium batteries age

    Lithium batteries lose amp-hour capacity over time. Microstructures can form on the electrodes with each new charge cycle, which further reduces battery capacity. Now an HZB team together with battery researchers from Forschungszentrum Jülich, the University of Munster, and partners in China have documented the degradation process of lithium electrodes in detail for the first time. They achieved this with the aid of a 3D tomography process using synchrotron radiation at BESSY II (HZB) as well at the Helmholtz-Zentrum Geesthacht (HZG). Their results have been published open access in the scientific journal "Materials Today". [...]

  • <p>SnSe is a highly layered orthorhombic structure. SnSe undergoes a phase transition of second order at 500&deg;C with an increase of the crystal symmetry from space group Pnma (left) to Cmcm (right).</p>SCIENCE HIGHLIGHT      24.04.2019

    High-efficiency thermoelectric materials: new insights into tin selenide

    Tin selenide might considerably exceed the efficiency of current record holding thermoelectric materials made of bismuth telluride. However, it was thought its efficiency became enormous only at temperatures above 500 degrees Celsius. Now measurements at the BESSY II and PETRA III synchrotron sources show that tin selenide can also be utilised as a thermoelectric material at room temperature – so long as high pressure is applied. [...]

  • <p>The enzyme MHETase is a huge and complex molecule. MHET-molecules from PET plastic dock at the active site inside the MHETase and are broken down into their basic building blocks.</p>SCIENCE HIGHLIGHT      12.04.2019

    "Molecular scissors" for plastic waste

    A research team from the University of Greifswald and Helmholtz-Zentrum-Berlin (HZB) has solved the molecular structure of the important enzyme MHETase at BESSY II. MHETase was discovered in bacteria and together with a second enzyme - PETase - is able to break down the widely used plastic PET into its basic building blocks. This 3D structure already allowed the researchers to produce a MHETase variant with optimized activity in order to use it, together with PETase, for a sustainable recycling of PET. The results have been published in the research journal Nature Communications. [...]