[en] Ferromagnetic or antiferromagnetic spin ordering is governed by the exchange interaction, the strongest force in magnetism(1-4). Understanding spin dynamics in magnetic materials is an issue of crucial importance for progress in information processing and recording technology. Usually the dynamics are studied by observing the collective response of exchange-coupled spins, that is, spin resonances, after an external perturbation by a pulse of magnetic field, current or light. The periods of the corresponding resonances range from one nanosecond for ferromagnets down to one picosecond for antiferromagnets. However virtually nothing is known about the behaviour of spins in a magnetic material after being excited on a timescale faster than that corresponding to the exchange interaction (10-100 fs), that is, in a non-adiabatic way. Here we use the element-specific technique X-ray magnetic circular dichroism to study spin reversal in GdFeCo that is optically excited on a timescale pertinent to the characteristic time of the exchange interaction between Gd and Fe spins. We unexpectedly find that the ultrafast spin reversal in this material, where spins are coupled antiferromagnetically, occurs by way of a transient ferromagnetic-like state. Following the optical excitation, the net magnetizations of the Gd and Fe sublattices rapidly collapse, switch their direction and rebuild their net magnetic moments at substantially different timescales; the net magnetic moment of the Gd sublattice is found to reverse within 1.5 picoseconds, which is substantially slower than the Fe reversal time of 300 femtoseconds. Consequently, a transient state characterized by a temporary parallel alignment of the net Gd and Fe moments emerges, despite their ground-state antiferromagnetic coupling. These surprising observations, supported by atomistic simulations, provide a concept for the possibility of manipulating magnetic order on the timescale of the exchange interaction.
Disciplines :
Physics
Author, co-author :
Radu, I.; Radu, I (Reprint Author), Radboud Univ Nijmegen, Inst Mol Mat, Heyendaalseweg 135, NL-6525 AJ Nijmegen, Netherlands. Radu, I.
Vahaplar, K.; Vahaplar, K.
Stamm, C.; Kirilyuk, A.
Kachel, T.; Rasing, Th.
Pontius, N.; Kimel, A. V., Radboud Univ Nijmegen, Inst Mol Mat, NL-6525 AJ Nijmegen, Netherlands. Radu, I.
Duerr, H. A.; Stamm, C.
Ostler, Thomas ; Université de Liège > Département de physique > Physique des matériaux et nanostructures
Barker, J.; Pontius, N.
Evans, R. F. L.; Duerr, H. A., BESSY II, Helmholtz Zentrum Berlin Mat Energie, D-12489 Berlin, Germany. Duerr, H. A., SLAC Natl Accelerator Lab, Menlo Pk, CA 94025 USA. Ostler, T. A.
Chantrell, R. W.; Barker, J.
Tsukamoto, A.; Evans, R. F. L.
Itoh, A.; Chantrell, R. W., Univ York, Dept Phys, York YO10 5DD, N Yorkshire, England. Tsukamoto, A.
Kirilyuk, A.; Itoh, A., Nihon Univ, Coll Sci Technol, Chiba, Japan. Tsukamoto, A., Japan Sci Technol Agcy, PRESTO, Kawaguchi, Saitama, Japan.
Stöhr, J. & Siegmann, H. C. Magnetism: From Fundamentals to Nanoscale Dynamics (Springer, 2006).
Weiss, P. L'hypothèse du champ moléculaire et la propriété ferromagnétique. J. Phys. (Paris) 6, 661-689 (1907).
Neel, L. Influence de fluctuations du champ moleculaire sur les proprieties magnetiques des corps. Ann. Phys. (Paris) 17, 5-9 (1932).
Neel, L. Magnetism and local molecular field. Science 174, 985-992 (1971).
Beaurepaire, E., Merle, J.-C., Daunois, A. & Bigot, J.-Y. Ultrafast spin dynamics in ferromagnetic nickel. Phys. Rev. Lett. 76, 4250-4253 (1996). (Pubitemid 126644988)
Kirilyuk, A., Kimel, A. V. & Rasing, Th. Ultrafast optical manipulation of magnetic order. Rev. Mod. Phys. 82, 2731-2784 (2010).
Gurevich, A. G. & Melkov, G. A. Magnetization Oscillations and Waves (CRC, 1996).
Aeschlimann, M., Vaterlaus, A., Lutz, M., Stampanoni, M. & Maier, F. Ultrafast thermomagnetic writing processes in rare-earth transition-metal thin films. J.Appl. Phys. 67, 4438-4440 (1990).
Stanciu, C. D. et al. Subpicosecond magnetization reversal across ferrimagnetic compensation points. Phys. Rev. Lett. 99, 217204 (2007).
Stanciu, C. D. et al. Ultrafast spin dynamics across compensation points in ferrimagnetic GdFeCo: the role of angular momentumcompensation. Phys. Rev. B 73, 220402(R) (2006).
Stamm, C. et al. Femtosecond modification of electron localization and transfer of angular momentum in nickel. Nature Mater. 6, 740-743 (2007). (Pubitemid 47511275)
Khan, S. et al. Femtosecond undulator radiation from sliced electron bunches. Phys. Rev. Lett. 97, 074801 (2006).
Kazantseva, N. et al. Linear and elliptical magnetization reversal close to the Curie temperature. Europhys. Lett. 86, 27006 (2009).
Koopmans, B. et al. Explaining the paradoxical diversity of ultrafast laser-induced demagnetization. Nature Mater. 9, 259-265 (2010).
Bartelt, A. F. et al. Element-specific spin and orbitalmomentumdynamics of Fe/Gd multilayers. Appl. Phys. Lett. 90, 162503 (2007).
Wietstruk, M. et al. Hot electron driven enhancement of spin-lattice coupling in 4f ferromagnets observed by femtosecond X-ray magnetic circular dichroism. Preprint at Æhttp://arxiv.org/abs/1010.1374æ (2010).
Kazantseva, N. et al. Slow recovery of the magnetization after a sub-picosecond heat pulse. Europhys. Lett. 81, 27004 (2008).
Anisimov, S. I., Kapeliovich, B. L. & Perelman, T. L. Electron emission from metal surfaces exposed to ultrashort laser pulses. Sov. Phys. JETP 39, 375-377 (1974)
