Time-resolved magnetic helicoidal dichroism (MHD) was successfully used to assess the ultra-fast dynamics of magnetization. This first attempt to use an MHD-based technique was demonstrated on a magnetic vortex structure hosted by a permalloy pacman micropillar (diameter~15nm, thickness~80nm). Micromagnetic simulations allowed to link the features identified in the MHD signal to a process of regeneration of the internal structure of the vortex following laser induced demagnetization.
Schematic of the experimental setup, the pacman NiFe pillar in which a magnetic vortex is stabilized after applying an in-plane magnetic H field. Some associated experimental MHD maps are shown.
While the spin angular momentum of light is widely used in dichroic studies (e.g. X-ray magnetic circular dichroism, magneto-optical Kerr effect), the orbital angular momentum (OAM) degree of freedom has been far less explored. Recently, it was demonstrated that OAM can enhance ptychographic imaging resolution. Besides, by controlling the sign of the incoming OAM, it was also showed that it can reveal the orientation of static magnetic textures, such as the winding direction of a magnetic vortex. Here, we have exploited this technique to track the ultrafast dynamics (~ps) of a magnetic vortex following thermal demagnetization.
NiFe pillars were microfabricated at the PTA platform with appropiate size and shape to precisely control the stabilisation of a vortex state at remanence. Light-magnetisation interaction experiments were carried out at Elettra-Sincrotrone (Trieste). An ultra-short infrared laser pulse was used to heat the pillar and partially demagnetise the permalloy layer from its upper surface (see figure). Using a pump-probe method, the dynamics of magnetisation was probed by time-resolved MHD in resonant scattering in the extreme ultraviolet. MHD describes the change in the optical response of a magnetic sample due to a sign reversal in the orbital angular momentum of light. It has been shown to be sensitive to the spin texture of the sample, especially in vortex confguration. In addition to the well-known laser-induced processes of ultra-rapid demagnetisation and re-magnetisation, analysis of the MHD signal provided direct evidence of important transient reorganisations of the spin texture. With the help of micromagnetic modeling, several scenarios of demagnetization were investigated to get insights on the dynamics of the spin texture of the permalloy nanostructure. In particular, we found that an ultrafast laser pulse of sufficient intensity can induce a transient magnetic texture characterized by a vortex surface structure of winding direction opposite to that of the bulk.
A key advantage of time-resolved MHD is its ability to operate without the need for scanning, enabling rapid measurements of magnetization dynamics in magnetic microstructures. These results hold great promise not only for monitoring complex magnetic spin textures but also for uncovering new forms of optical spin-orbit interactions mediated by magnetization.
Team: Theory/Simulations, MRAM
Collaboration: Sorbonne Université, INSP, CEA-LIDYL, Elettra-Sincrotrone Trieste
Funding: HELIMAG (ANR-21-CE30-0037)
Further reading: Magnetic vortex dynamics probed by time-resolved magnetic helicoidal dichroism, M. Fanciulli, M. Pancaldi, A. E. Stanciu, M. Guer, E. Pedersoli, D. De Angelis, P. Rebernik Ribic, D. Bresteau, M. Luttmann, P. Carrara, A. Ravindran, B. Rosner, C. David, C. Spezzani, M. Manfredda, R. Sousa, L. Vila, I. L. Prejbeanu, L. D. Buda-Prejbeanu, B. Dieny, G. De Ninno, F. Capotondi, T. Ruchon and M. Sacchi, Phys. Rev. Lett. 134, 156701 (2025), Open access: hal-05127585v1
Contact: Liliana Buda-Prejbeanu, Bernard Dieny