The research field of ultrafast demagnetization is a very young one, with the first proof of magnetization dynamics on a femtosecond (fs) timescale dating back to 1996 [1]. Since then, fs laser pulses have been used to trigger spin waves [2], measure spin polarization [3], and even to directly switch the direction of magnetizaiton [4]. The all-optical studies of magnetic thin films attracted many researchers intent on understanding the elemental processes governing the dynamics of the fastest possible demagnetization. It became clear very soon that the key issue is the redistribution of deposited energy between electrons, lattice, and spins. However, up to date, we are still lacking a complete microscopic description of the physics at hand.
In this project, we aim to further our understanding of the magnetizaiton dynamics on the sub-picosecond timescale and the transfer of spin angular momentum. It has recently been demonstrated [5] that it is possible to influence the ultrafast demagnetization of a sample by tailoring the spin-dependent transfer of hot electrons. This is a first step in the transition from observing magnetization dynamics towards achieving control over it. In fact, there is an intense discussion going on whether demagnetization can be explained by phonon-mediated spin-flip scattering [6] or by superdiffusive transport of hot electrons [7]. Further investigations on this topic are necessary to clarify this point and find the long-desired microscopic explanation for ultrafast demagnetization. We conduct such studies using a capable fs pump-probe setup to examine a variety of magnetic thin films. Results are evaluated both for possible applications and to enhance our theoretical understanding of energy and spin transfer.
[1] E. Beaurepaire, J.-C. Merle, A. Daunois, and J.-Y. Bigot. Ultrafast Spin Dynamics in Ferromagnetic Nickel. Phys. Rev. Lett., 76(22):4250-, May 1996.[2] M. Djordjevic, M. Lüttich, P. Moschkau, P. Guderian, T. Kampfrath, R. G. Ulbrich, M. Münzenberg, W. Felsch, and J. S. Moodera, Comprehensive view on ultrafast dynamics of ferromagnetic films, Physica Status Solidi C 3 (2006), no. 5, 1347-1358.
[3] G. M. Müller, J. Walowski, M. Djordjevic, G.-X. Miao, A. Gupta, A. V. Ramos, K. Gehrke, V. Moshnyaga, K. Samwer, J. Schmalhorst, A. Thomas, A. Hütten, G. Reiss, J. S. Moodera, and M. Münzenberg. Spin polarization in half-metals probed by femtosecond spin excitation. Nat Mater, 8(1):56-61, January 2009.
[4] C. D. Stanciu, F. Hansteen, A. V. Kimel, A. Kirilyuk, A. Tsukamoto, A. Itoh, and T. Rasing. All-Optical Magnetic Recording with Circularly Polarized Light. Phys. Rev. Lett. 99, 047601 (2007).
[5] G. Malinowski, F. D. Longa, J. H. H. Rietjens, P. V. Paluskar, R. Huijink, H. J. M. Swagten, and B. Koopmans, Nature Physics 4, 855 (2008).
[6] B. Koopmans, G. Malinowski, F. Dalla Longa, D. Steiauf, M. Fähnle, T. Roth, M. Cinchetti, and M. Aeschlimann. Explaining the paradoxical diversity of ultrafast laser-induced demagnetization. Nat Mater, 9:259-265, March 2010.
[7] M. Battiato, K. Carva, and P. M. Oppeneer. Superdiffusive Spin Transport as a Mechanism of Ultrafast Demagnetization. Phys. Rev. Lett. 105, 027203 (2010).