Tracking Ultrafast Vortex Magnetism with Twisted XUV Light
Micro- and nanostructures of magnetic materials underpin many key modern technologies, ranging from sensors to high-density data storage. Controlling and understanding their magnetisation on ultrafast time scales is essential for developing devices such as all-optical memories, spin-based oscillators, or THz emitters. Spin dynamics is most often probed through the interaction between magnetic moments and polarised light beams. Magnetic circular dichroism (MCD), defined as the difference in the response to the interaction with a right- or left-handed circularly polarized beam, is among the most widely used techniques for characterising the magnetic state of a material.
A promising new approach exploits phase-structured light, namely, laser pulses carrying orbital angular momentum (OAM). Unlike conventional polarisation (spin angular momentum), OAM endows the wavefront of light a helical structure, sometimes referred to as “twisted light” or "vortex beam". When interacting with magnetic structures, these beams create new opportunities for probing chiral spin textures. To harness this property, researchers have recently proposed a method called magnetic helicoidal dichroism (MHD), which exploits the fact that magnetic materials respond differently to OAM beams depending on the handedness of their magnetization. Following an initial theoretical prediction and a static proof-of-concept, the question remained whether MHD can reveal magnetisation dynamics in real time.
An international collaboration led by teams from CEA–LIDYL (France), CY Cergy Paris University, Sorbonne University (France), and Elettra–Sincrotrone Trieste has now provided the answer. Using the DiProI beamline at the FERMI free-electron laser, they have carried out the first time-resolved MHD experiments, capturing on the picosecond scale how a magnetic vortex responds to ultrashort laser excitations at the sub-picosecond time scale. In the experiment, micrometre-sized permalloy dots hosting a central magnetic vortex were first excited by a femtosecond infrared (IR) laser pulse, see Figure 1. The ensuing magnetisation dynamics was then probed by extreme ultraviolet (XUV) pulses carrying OAM. By comparing the scattering signal recorded for opposite vortex magnetisations, the team retrieved the MHD signal as a function of time delay.
Figure 1: Sketch of the experiment: a femtosecond IR laser pulse excites a magnetic vortex in a permalloy dot. At a given delay time, an XUV pulse carrying orbital angular momentum (twisted light) probes the system. By comparing the signals recorded for opposite vortex directions, the time-dependent magnetic helicoidal dichroism (MHD) response is retrieved.
The results extend far beyond the well-known phenomenon of ultrafast demagnetisation and recovery. The analysis reveals that the laser pulse induces a transient reorganisation of the spin texture, including a striking temporary reversal of the vortex curling direction at the surface, see Figure 2. Micromagnetic simulations confirm that this effect arises from a depth-dependent recovery of magnetization, with the surface layers responding differently from the bulk
Figure 2. Example of MHD signals: before laser excitation (left), the static vortex structure is clearly observed. About 13 ps after excitation (right), the MHD signal reveals a transient reversal of the vortex curling direction, highlighted by an inversion of the colour pattern.
This pioneering work demonstrates the power of time-resolved MHD as a tool for visualising ultrafast spin texture dynamics in magnetic microstructures. Beyond revealing fundamental insights, the technique opens new avenues for all-optical preparation of complex metastable states in thin films and nanostructures, with potential applications in future information technologies. These results mark an important step towards optically controlled magnetic information processing.
This research was conducted by the following research team:M. Fanciulli1,2, M. Pancaldi3,4, A.E. Stanciu5, M. Guer1, E. Pedersoli3, D. De Angelis3, P. Rebernik Ribič3, D. Bresteau1, M. Luttmann1, P. Carrara6, A. Ravindran7, B. Rösner8, C. David8, C. Spezzani3, M. Manfredda3, R. Sousa5, L. Vila5, I.-L. Prejbeanu5, L. Buda-Prejbeanu5, B. Dieny5, G. De Ninno3,7, F. Capotondi3, T. Ruchon1, M. Sacchi6,9
1 Lidyl - AttoLab, CEA Saclay, France
2 LPMS - Cergy Paris Université, France
3 Elettra - Sincrotrone Trieste, Italy
4 Università di Ca’ Foscari, Venezia, Italy
5 Spintec, CEA Grenoble, France
6 CNRS – INSP, Sorbonne Université, Paris, France
7 LQO, University of Nova Gorica, Slovenia
8 Paul Scherrer Institut, Villigen, Switzerland
9 Synchrotron SOLEIL, Université Paris-Saclay, France
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Reference
M. Fanciulli et al., “Magnetic Vortex Dynamics Probed by Time-Resolved Magnetic Helicoidal Dichroism”, Phys. Rev. Lett. 134, 156701 (2025); DOI: 10.1103/PhysRevLett.134.156701.
