Séminaire Optique/Spectro

Vendredi 19 Septembre 2014 à 11h00.

Investigation of ultrafast angular momenta transfer in multisublattice magnets by time-resolved X-ray Magnetic Circular Dichroism

''Ultrafast magnetization dynamics is one of the hottest topics in modern magnetism for both fundamental science and technological applications. Although it is well known that ultrashort infrared (IR) laser pulses allow the manipulation of the spins in magnetic films down to the femtosecond time scale [1], the different fundamental processes taking place during the first hundreds of femtoseconds are still debated [2]. One of the key issues is related to the determination of the transfer channels for the angular momentum from J(t) to other external reservoirs. Nowadays time-resolved X-ray magnetic circular dichroism (TR-XMCD) with adapted temporal resolution (130 fs) is achievable using the f-slicing source at the Helmholtz-Zentrum Berlin. It provides element selectivity and electronic sensitivity to distinguish the 3d magnetic momenta in transition metals [3] and the 4f magnetic moments in rare-earth metals [4]. TR-XMCD has also provided access to the determination of the ultrafast dynamics of the spins S(t) and of the orbital L(t) moments [5]. The recent achievements using X-ray based pump-probe techniques are at the basis of numerous applications in the field of ultrafast magnetization dynamics. Important questions can now be addressed as for instance the conservation of the total angular momentum after the laser excitation. In my talk, after a brief historical of ultrafast magnetization dynamics, I will discuss the impact of TR-XMCD on our actual understanding of the different mechanisms involved in this process. Then, I will dedicate the last part of my talk to our recent results obtained on the f-slicing beamline at BESSY II where we have used IR laser pulses to trig ultrafast magnetization dynamics in ferrimagnetic Co74Tb26 alloys [6]. References: [1] Beaurepaire et al. Phys. Rev. Lett., 76, 4250 (1996). [2]: Kirilyuk et al. Rev. Mod. Phys., 82, 2731 (2010). [3]: Stamm et al., Nature Mater. 6, 740 (2007). [4]: Wietstruk et al., Phys. Rev. Lett. 106, 127401 (2011). [5]: Boeglin et al. Nature, 465, 458 (2010). [6]: Bergeard et al. Nature Comm, 5, 3466 (2014).''