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      Ultrafast optical demagnetization manipulates nanoscale spin structure in domain walls

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          Abstract

          During ultrafast demagnetization of a magnetically ordered solid, angular momentum has to be transferred between the spins, electrons, and phonons in the system on femto- and picosecond timescales. Although the intrinsic spin-transfer mechanisms are intensely debated, additional extrinsic mechanisms arising due to nanoscale heterogeneity have only recently entered the discussion. Here we use femtosecond X-ray pulses from a free-electron laser to study thin film samples with magnetic domain patterns. We observe an infrared-pump-induced change of the spin structure within the domain walls on the sub-picosecond timescale. This domain-topography-dependent contribution connects the intrinsic demagnetization process in each domain with spin-transport processes across the domain walls, demonstrating the importance of spin-dependent electron transport between differently magnetized regions as an ultrafast demagnetization channel. This pathway exists independent from structural inhomogeneities such as chemical interfaces, and gives rise to an ultrafast spatially varying response to optical pump pulses.

          Abstract

          Ultrafast demagnetization occurs when magnetically ordered solids are exposed to femtosecond light pulses, yet the exact spin-transfer mechanism is still debated. Combining ultrashort X-rays and infrared laser pulses, Pfau et al. show the importance of spin transport between domains in thin magnetic films.

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          Most cited references 4

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          Fast current-induced domain-wall motion controlled by the Rashba effect.

          The propagation of magnetic domain walls induced by spin-polarized currents has launched new concepts for memory and logic devices. A wave of studies focusing on permalloy (NiFe) nanowires has found evidence for high domain-wall velocities (100 m s(-1); refs,), but has also exposed the drawbacks of this phenomenon for applications. Often the domain-wall displacements are not reproducible, their depinning from a thermally stable position is difficult and the domain-wall structural instability (Walker breakdown) limits the maximum velocity. Here, we show that the combined action of spin-transfer and spin-orbit torques offers a comprehensive solution to these problems. In an ultrathin Co nanowire, integrated in a trilayer with structural inversion asymmetry (SIA), the high spin-torque efficiency facilitates the depinning and leads to high mobility, while the SIA-mediated Rashba field controlling the domain-wall chirality stabilizes the Bloch domain-wall structure. Thus, the high-mobility regime is extended to higher current densities, allowing domain-wall velocities up to 400 m s(-1). © 2011 Macmillan Publishers Limited. All rights reserved
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            Creep and flow regimes of magnetic domain-wall motion in ultrathin Pt/Co/Pt films with perpendicular anisotropy.

             V Baltz,  P Jamet,  B Rodmacq (2007)
            We report on magnetic domain-wall velocity measurements in ultrathin Pt/Co(0.5-0.8 nm)/Pt films with perpendicular anisotropy over a large range of applied magnetic fields. The complete velocity-field characteristics are obtained, enabling an examination of the transition between thermally activated creep and viscous flow: motion regimes predicted from general theories for driven elastic interfaces in weakly disordered media. The dissipation limited flow regime is found to be consistent with precessional domain-wall motion, analysis of which yields values for the damping parameter, alpha.
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              Ultrafast transport of laser-excited spin polarized carriers in Au/Fe/MgO(001)

              A time domain approach to probe hot carrier-induced spin dynamics is demonstrated. The experiments are performed in epitaxial Au/Fe/MgO(001), where spin-polarized hot carriers are excited in the Fe layer by 35 fs laser pulses. They propagate to the Au surface where the transient spin polarization is detected by magneto-induced second harmonic generation. Different energies of majority and minority hot carriers excited in the exchange-split Fe band structure lead to their spindependent lifetimes in Au. Accordingly, two spin-polarized current contributions which propagate superdiffusively at different velocities result in a spin current pulse of about 100 fs duration.
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                Author and article information

                Journal
                Nat Commun
                Nat Commun
                Nature Communications
                Nature Pub. Group
                2041-1723
                02 October 2012
                : 3
                : 1100
                Affiliations
                [1 ]TU Berlin, Institut für Optik und Atomare Physik , 10623 Berlin, Germany.
                [2 ]Deutsches Elektronen-Synchrotron DESY , 22607 Hamburg, Germany.
                [3 ]Johannes Gutenberg-Universität Mainz, Institut für Physik , 55099 Mainz, Germany.
                [4 ]SwissFEL, Paul Scherrer Institut , 5232 Villigen PSI, Switzerland.
                [5 ]École Polytechnique Fédérale de Lausanne , 1015 Lausanne, Switzerland.
                [6 ]Université Pierre et Marie Curie, Laboratoire de Chimie Physique – Matière et Rayonnement – CNRS UMR 7614 , 75005 Paris, France.
                [7 ]Universität Hamburg, Institut für Angewandte Physik , 20355 Hamburg, Germany.
                [8 ]Helmholtz-Zentrum Berlin für Materialien und Energie , 14109 Berlin, Germany.
                [9 ]Synchrotron SOLEIL, L'Orme des Merisiers, Saint-Aubin , 91192 Gif-sur-Yvette Cedex, France.
                [10 ]Linac Coherent Light Source, SLAC National Accelerator Laboratory , Menlo Park, California 94025, USA.
                [11 ]Laboratoire d'Optique Appliquée, ENSTA ParisTech – CNRS UMR 7639 – École polytechnique, Chemin de la Hunière , 91761 Palaiseau, France.
                Author notes
                Article
                ncomms2108
                10.1038/ncomms2108
                3493637
                23033076
                Copyright © 2012, Nature Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved.

                This work is licensed under a Creative Commons Attribution-NonCommercial-Share Alike 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/

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