Ballistic electron-driven magnetization dynamics induced by femtosecond laser excitation
Final Report Abstract
Further development of spintronics requires miniaturization and reduction of characteristic timescales of spin dynamics combining the nanometer spatial and terahertz frequency ranges. These demands shift the focus of interest towards the fundamental open question of the interaction of femtosecond spin current (SC) pulses with a ferromagnet (FM). The spatio-temporal properties of the spin transfer torque (STT) exerted by ultrashort SC pulses on the FM open the time domain for studying STT fingerprint on spatially non-uniform magnetization dynamics. According to these, the project aimed at fundamental understanding of elementary processes in spin-dependent electron-electron scattering of hot, essentially non-equilibrium, charge carriers in metallic multilayer structures and at the investigation of spin transfer torque effects generated by femtosecond spin current pulses on the magnetization of itinerant ferromagnets. The proposed research program set a new pathway towards a microscopic understanding of spin transport, non-equilibrium spin dynamics, and towards the development of new concepts for metal-based elements in ultrafast spintronics. This dynamics was investigated in highly excited states in epitaxial metallic multilayers composed of FM and non-magnetic (NM) layers. Here, the excited state evolution can be divided into three subsequent stages developing in respective layers of the FM1/NM/FM2 structure: (I) the hot carrier (HC) generation by femtosecond laser pulses in the FM1 layer and injection across the FM1/NM interface, (II) the HC transport through the NM layer forming the SC pulse, and (III) the HC injection across the NM/FM2 interface into the FM2 layer, which excites spin dynamics in the FM2 layer. Our goal was to reveal elementary processes acting at each of these stages, to investigate the responsible interactions, and to develop a microscopic description. Using the sensitivity of magneto-induced second harmonic generation to SC, we have developed technique for SC monitoring. With 20 fs resolution, we have demonstrated the generation of 250 fs-long SC pulses in Fe/Au/Fe/MgO(001) pseudo-spin valve structures. Their temporal profile indicates (i) nearly-ballistic hot electron transport in the Au spacer (which is also confirmed by systematic studies in Au/Fe/MgO(001) bi-layers) and (ii) that the pulse duration is primarily determined by the thermalization time of laser-excited HC in the Fe emitter. The origin of efficient SC generation is the strongly spin-dependent transmittance of Fe/Au interface calculated for these HC together with the spin-dependency of electron transport in Fe: predominantly majority electrons are transported from the bulk of FM emitter towards the FM/NM interface and then emitted into NM spacer. The physics of this phenomenon is similar to that behind the spin-dependent Seebeck effect. However, the non-thermal electron distribution following the femtosecond laser excitation leads to a gaint enhancement of this effect, first of all due to a dramatic difference in the interface transmittance for the majority electrons at elevated energies with respect to the minority electrons and both types of holes. Therefore, we term this phenomenon occurring in an essentially non-equilibrium state of solids as the non-thermal spindependent Seebeck effect. It dominates the generation of ultrashort SC pulses but on a longer (about 0.5 ps) timescale transforms into the normal, thermal spin-dependent Seebeck effect determined by the temperature gradient across the interface and leading to a negligibly small SC in comparison to the non-thermal case. The spin-dependency of the interface transmittance discussed above has another important consequence. The analysis of SC transmission/reflection at the Au/Fe interface between the spacer and collector shows that hot electron spins (i.e. the spins of electrons traversing the Au spacer at essentially elevated energies with respect to the Fermi energy) orthogonal to the Fe magnetization rotate gaining huge parallel (antiparallel) projection in transmitted (reflected) SC. This is similar to the spatial spin separation known from the Stern-Gerlach experiment and holds high potential for future ultrafast spintronic devices giving a pathway to the development of ultrathin spin polarizers and rotators working in both transmission and reflection. Furthermore, this interaction of SC with the Fe collector is accompanied by a STT-induced perturbation of the magnetization localized at the Au/Fe interface, which excites the spatially inhomogeneous high-frequency spin dynamics in the FM. Time-resolved magneto-optical Kerr effect (MOKE) studies reveal the excitation of several standing spin wave modes in the Fe film with their frequencies of up to 0.6 THz. Together with the measured amplitudes of MOKE signals, this indicates the STT spatial confinement to below 2 nm, the initial tilt of the magnetization near the Au/Fe interface above 1 deg., and the density of magnetic moment transferred across the Au/Fe interface per pulse about 7 µB/nm2, which opens intriguing perspectives for this technique including the expansion of coherent magnetization dynamics in FMs into the terahertz domain. Although our conclusions are based on exact ab initio calculations of the interface HC transmittance in the case of Fe/Au(001) interface, its strong spin-dependency has a simplified explanation. The majority electrons are the only HC excited to the sp band of Fe while the other HC are in the d band. Owing to much better band matching betwen the sp bands of Fe and Au than that between d band of Fe and sp band of Au, the interface transmittance for the majority electrons is much higher. Therefore, one can expect similar HC transmittance spin-dependency for other FM/NM interfaces if the considered NM has a gap in d band in the vicinity of Fermi level, like it is in Au. Thus, the phenomena discussed above promise to be rather general and not restricted to the structures with Fe/Au(001) interfaces, which facilitates their applicability for the spintronics and makes the achievements of this project quite significant for the development of ultrafast spin dynamics.
Publications
- Ultrafast magnetization dynamics of Gd(0001): Bulk versus surface, Phys. Status Solidi B 248, 2323 (2011)
M. Sultan, A. Melnikov, and U. Bovensiepen
- Ultrafast transport of laser-excited spin-polarized carriers in Au/Fe/MgO(001), Phys. Rev. Lett. 107, 076601 (2011)
A. Melnikov, I. Razdolski, T. Wehling, E. Papaioannou, V. Roddatis, P. Fumagalli, O. Aktsipetrov, A. Lichtenstein, and U. Bovensiepen
- Electron- and phonon-mediated ultrafast magnetization dynamics of Gd(0001), Phys. Rev. B 85, 184407 (2012)
M. Sultan, U. Atxitia, A. Melnikov, O. Chubykalo-Fesenko, and U. Bovensiepen
(See online at https://doi.org/10.1103/PhysRevB.85.184407) - The role of spin-lattice coupling in the ultrafast demagnetization of Gd1−xTbx alloys, Phys. Rev. B. 89, 214423 (2014)
A. Eschenlohr, M. Sultan, A. Melnikov, N. Bergeard, J. Wieczorek, T. Kachel, C. Stamm, and U. Bovensiepen
(See online at https://doi.org/10.1103/PhysRevB.89.214423) - Second harmonic generation spectroscopy in the Reststrahl band of SiC using an infrared free-electron laser, Appl. Phys. Lett. 107, 081101 (2015)
A. Paarmann, I. Razdolski, A. Melnikov, S. Gewinner, W. Schöllkopf, and M. Wolf
(See online at https://doi.org/10.1063/1.4929358) - Separation of ultrafast spin currents and spin-flip scattering in Co/Cu(001) driven by femtosecond laser excitation employing the complex magneto-optical Kerr effect, Phys. Rev. B 92, 174410 (2015)
J. Wieczorek, A. Eschenlohr, B. Weidtmann, M. Rösner, N. Bergeard, A. Tarasevitch, T. O. Wehling, and U. Bovensiepen
(See online at https://doi.org/10.1103/PhysRevB.92.174410) - Ultrafast laser-excited spin transport in Au/Fe/MgO(001): Relevance of the Fe layer thickness, in Ultrafast Magnetism I, J.-Y. Bigot et al. (eds.), Vol. 159 of Springer Proceedings in Physics, pp. 241-243 (2015)
A. Alekhin, D. Bürstel, A. Melnikov, D. Diesing, and U. Bovensiepen
(See online at https://doi.org/10.1007/978-3-319-07743-7_75) - Ultrafast non-local spin dynamics in metallic bi-layers by linear and non-linear magneto-optics, in Ultrafast Magnetism I, J.-Y. Bigot et al. (eds.), Vol. 159 of Springer Proceedings in Physics, pp. 34-36 (2015)
A. Melnikov, A. Alekhin, D. Bürstel, D. Diesing, T.O. Wehling, I. Rungger, M. Stamenova, S. Sanvito, and U. Bovensiepen
(See online at https://doi.org/10.1007/978-3-319-07743-7_12) - Generation of femtosecond spin current pulses via non-thermal spin-dependent Seebeck effect and their interaction with ferromagnets in spin valves
A. Alekhin, I. Razdolski, N. Ilin, J.P. Meyburg, D. Diesing, V. Roddatis, I. Rungger, M. Stamenova, S. Sanvito, U. Bovensiepen, and A. Melnikov
- Towards the nonlinear acousto-magneto-plasmonics, Journal of Optics 18, 093002 (2016)
V.V. Temnov, I. Razdolski, T. Pezeril, D. Makarov, D. Seletskiy, A. Melnikov, and K.A. Nelson
(See online at https://doi.org/10.1088/2040-8978/18/9/093002) - Analysis of the time-resolved magneto-optical Kerr effect for ultrafast magnetization dynamics in ferromagnetic thin films, Journal of Physics: Condensed Matter 29, 174002 (2017)
I. Razdolski, A. Alekhin, U Martens, D. Bürstel, D. Diesing, M. Münzenberg, U. Bovensiepen, and A. Melnikov
(See online at https://doi.org/10.1088/1361-648X/aa63c6) - Nanoscale confinement of ultrafast spin transfer torque exciting non-uniform spin dynamics by femtosecond spin current pulses, Nature Communications 8, 15007 (2017)
I. Razdolski, A. Alekhin, N. Ilin, J.P. Meyburg, V. Roddatis, D. Diesing, U. Bovensiepen, and A. Melnikov
(See online at https://doi.org/10.1038/ncomms15007)
