ULTRAFAST DYNAMICS MEASUREMENTS SURFACES, INTERFACES AND NANOSYSTEMS The demagnetization measurements were performed at the Institut SYNCHROTRON SOLEIL HIGHLIGHTS 2020 Jean Lamour in Nancy, by using a Time-Resolved Magneto-Optical Kerr Effect (TR-MOKE), with a pump-probe geometry. A 35 fs width laser pulse pumps the quenching of the magnetization at time t . The0 change in magnetization is probed afterward at time t by looking at the CASSIOPEE BEAMLINE change in helicity Δθ of a second laser pulse after its passing through the magnetic film (magneto-optical Kerr effect). By adjusting the delay t-tk 0 Associated publication between the pump and probe pulses, one can measure the dynamics Engineering CoMnAlSi Heusler of the magnetization at a sub-picosecond timescale. Results of ultrafast 2 x 1−x Compounds as a Model System to demagnetization dynamics are presented in figure 2-a, together with Correlate Spin Polarization, Intrinsic fits obtained by using the three-temperature model which describes Gilbert Damping and Ultrafast the energy distribution among electrons, phonons, and spins after laser Demagnetization. excitation. The ultra-fast demagnetization time τ decreases from 380 fsM C. Guillemard, W. Zhang, for CoMnSi to 165 fs for CoMnAl. The evolution of 1⁄τ as a function of2 2 M G. Malinowski, C. De Melo, J. Gorchon, 1 – P is presented in figure 2-b, where a clear linear variation is observed S. Petit-Watelot, J. Ghanbaja, in these compounds. As the magnetic damping α is inversely proportional S. Mangin, P. Le Fèvre, F. Bertran & to P in our case, this means that 1⁄τ is proportional to α too.M S. Andrieu. FIGURE 2 Adv. Mater., 32 1908357 (2020). References [1] E. Beaurepaire etal. , Phys. Rev. Lett. 76, 4250 (1996). [2] C. D. Stanciu et al., Phys. Rev. Lett. 99, 047601 (2007). [3] C. Guillemard, PhD thesis, Université de Lorraine (2019). [4] A. Mann et al., Phys. Rev. X 2, 041008 (2012). [5] B. Koopmans et al., Phys. Rev. Lett. 95, 267207 (2005). Corresponding authors Charles Guillemard ALBA Synchrotron, DISCUSSION Carrer de la Llum 2-26, 08290, To the best of our knowledge, the figure 1-b is the first robust demonstration Cerdanyola-del-Vallès, of the link between the magnetic damping and the spin polarization, Barcelona, Spain confirming the theoretical justification of ultra-low damping in half-metal cguillemard@cells.es magnets as a consequence of the reduction of spin flip due to the spin Stéphane Andrieu gap. The dependency between α and τ observed here was a clearM Institut Jean Lamour, opportunity to test the different theoretical explanations proposed in 2, allée André Guinier, the literature. Several groups [4] proposed that the demagnetization 54011 Nancy cedex, France process is linked to the population of minority and majority spin states stephane.andrieu@univ-lorraine.fr at E, leading to a dependence of the spin-scattering rate proportionalF to 1 – P [4]. As this spin scattering rate is linked to the inverse of the Captions demagnetization time, the τ /home/webapps/asp_fr/data/asp/publications/synchrotron-soleil/synchrotron-soleil-2020/soleil-highlights-2020-hd-ss-tc (1 – P)-1M law was proposed. This law FIGURE 1: a) High resolution transmission electron is verified in our samples series and confirms that the spin gap is at microscopy micrograph of Co2MnAl05Si05 showing high the origin of the increase of τ . Finally, an inverse relationship between structural quality with L2 ordered and B2 disordered1 M regions. b)Spin polarization at EF obtained at Cassiopée τ and α is established in these compounds, which agrees well withM beamline and magnetic Gilbert damping α as a function models [4,5]. In summary, this work provides a new milestone toward of the substitution range x in Co2MnAlSix1xcompounds. the understanding of the magnetization dynamics at the sub-picosecond FIGURE 2: a) Ultrafast demagnetization curves measured timescale, and contributes to pave the way for future efficient devices. by TR-MOKE with the corresponding fit for each composition. b) Evidence of 1/τM linear variation with 1 – P. 21