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      Investigating the role of superdiffusive currents in laser induced demagnetization of ferromagnets with nanoscale magnetic domains

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          Abstract

          Understanding the loss of magnetic order and the microscopic mechanisms involved in laser induced magnetization dynamics is one of the most challenging topics in today's magnetism research. While scattering between spins, phonons, magnons and electrons have been proposed as sources for dissipation of spin angular momentum, ultrafast spin dependent transport of hot electrons has been pointed out as a potential candidate to explain ultrafast demagnetization without resorting to any spin dissipation channel. Here we use time resolved magneto-optical Kerr measurements to extract the influence of spin dependent transport on the demagnetization dynamics taking place in magnetic samples with alternating domains with opposite magnetization directions. We unambiguously show that whatever the sample magnetic configuration, the demagnetization takes place during the same time, demonstrating that hot electrons spin dependent transfer between neighboring domains does not alter the ultrafast magnetization dynamics in our systems with perpendicular anisotropy and 140 nm domain sizes.

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          Most cited references 16

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          Ultrafast Spin Dynamics in Ferromagnetic Nickel

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            Femtosecond modification of electron localization and transfer of angular momentum in nickel.

            The rapidly increasing information density required of modern magnetic data storage devices raises the question of the fundamental limits in bit size and writing speed. At present, the magnetization reversal of a bit can occur as quickly as 200 ps (ref. 1). A fundamental limit has been explored by using intense magnetic-field pulses of 2 ps duration leading to a non-deterministic magnetization reversal. For this process, dissipation of spin angular momentum to other degrees of freedom on an ultrafast timescale is crucial. An even faster regime down to 100 fs or below might be reached by non-thermal control of magnetization with femtosecond laser radiation. Here, we show that an efficient novel channel for angular momentum dissipation to the lattice can be opened by femtosecond laser excitation of a ferromagnet. For the first time, the quenching of spin angular momentum and its transfer to the lattice with a time constant of 120+/-70 fs is determined unambiguously with X-ray magnetic circular dichroism. We report the first femtosecond time-resolved X-ray absorption spectroscopy data over an entire absorption edge, which are consistent with an unexpected increase in valence-electron localization during the first 120+/-50 fs, possibly providing the driving force behind femtosecond spin-lattice relaxation.
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              Probing the timescale of the exchange interaction in a ferromagnetic alloy.

              The underlying physics of all ferromagnetic behavior is the cooperative interaction between individual atomic magnetic moments that results in a macroscopic magnetization. In this work, we use extreme ultraviolet pulses from high-harmonic generation as an element-specific probe of ultrafast, optically driven, demagnetization in a ferromagnetic Fe-Ni alloy (permalloy). We show that for times shorter than the characteristic timescale for exchange coupling, the magnetization of Fe quenches more strongly than that of Ni. Then as the Fe moments start to randomize, the strong ferromagnetic exchange interaction induces further demagnetization in Ni, with a characteristic delay determined by the strength of the exchange interaction. We can further enhance this delay by lowering the exchange energy by diluting the permalloy with Cu. This measurement probes how the fundamental quantum mechanical exchange coupling between Fe and Ni in magnetic materials influences magnetic switching dynamics in ferromagnetic materials relevant to next-generation data storage technologies.
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                Author and article information

                Journal
                Sci Rep
                Sci Rep
                Scientific Reports
                Nature Publishing Group
                2045-2322
                11 April 2014
                2014
                : 4
                Affiliations
                [1 ]Laboratoire de Physique des Solides, Université Paris-Sud , CNRS UMR 8502, 91405 Orsay, France
                [2 ]Sorbonne Universités, UPMC Univ Paris 06 , UMR 7614, LCPMR, 75005 Paris, France
                [3 ]Institut Jean Lamour, CNRS UMR 7198, Université de Lorraine , 54506 Vandoeuvre-lès-Nancy, France
                [4 ]CNRS, UMR 7614 , LCPMR, 75005 Paris, France
                [5 ]Center for Magnetic recording Research, University of California San Diego , La Jolla, CA 92093-0401, USA
                Author notes
                Article
                srep04658
                10.1038/srep04658
                3983600
                24722395
                Copyright © 2014, Macmillan Publishers Limited. All rights reserved

                This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. The images in this article are included in the article's Creative Commons license, unless indicated otherwise in the image credit; if the image is not included under the Creative Commons license, users will need to obtain permission from the license holder in order to reproduce the image. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/

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