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      Band structure evolution during the ultrafast ferromagnetic-paramagnetic phase transition in cobalt

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          Abstract

          Using spin- and time-resolved XUV photoemission, researchers monitor the band structure evolution of Co during its phase transition.

          Abstract

          The evolution of the electronic band structure of the simple ferromagnets Fe, Co, and Ni during their well-known ferromagnetic-paramagnetic phase transition has been under debate for decades, with no clear and even contradicting experimental observations so far. Using time- and spin-resolved photoelectron spectroscopy, we can make a movie on how the electronic properties change in real time after excitation with an ultrashort laser pulse. This allows us to monitor large transient changes in the spin-resolved electronic band structure of cobalt for the first time. We show that the loss of magnetization is not only found around the Fermi level, where the states are affected by the laser excitation, but also reaches much deeper into the electronic bands. We find that the ferromagnetic-paramagnetic phase transition cannot be explained by a loss of the exchange splitting of the spin-polarized bands but instead shows rapid band mirroring after the excitation, which is a clear signature of extremely efficient ultrafast magnon generation. Our result helps to understand band structure formation in these seemingly simple ferromagnetic systems and gives first clear evidence of the transient processes relevant to femtosecond demagnetization.

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          Most cited references51

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          Phase-matched generation of coherent soft X-rays

          Phase-matched harmonic conversion of visible laser light into soft x-rays was demonstrated. The recently developed technique of guided-wave frequency conversion was used to upshift light from 800 nanometers to the range from 17 to 32 nanometers. This process increased the coherent x-ray output by factors of 10(2) to 10(3) compared to the non-phase-matched case. This source uses a small-scale (sub-millijoule) high repetition-rate laser and will enable a wide variety of new experimental investigations in linear and nonlinear x-ray science.
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            Explaining the paradoxical diversity of ultrafast laser-induced demagnetization.

            Pulsed-laser-induced quenching of ferromagnetic order has intrigued researchers since pioneering works in the 1990s. It was reported that demagnetization in gadolinium proceeds within 100 ps, but three orders of magnitude faster in ferromagnetic transition metals such as nickel. Here we show that a model based on electron-phonon-mediated spin-flip scattering explains both timescales on equal footing. Our interpretation is supported by ab initio estimates of the spin-flip scattering probability, and experimental fluence dependencies are shown to agree perfectly with predictions. A phase diagram is constructed in which two classes of laser-induced magnetization dynamics can be distinguished, where the ratio of the Curie temperature to the atomic magnetic moment turns out to have a crucial role. We conclude that the ultrafast magnetization dynamics can be well described disregarding highly excited electronic states, merely considering the thermalized electron system.
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              Ultrafast spin transport as key to femtosecond demagnetization.

              Irradiating a ferromagnet with a femtosecond laser pulse is known to induce an ultrafast demagnetization within a few hundred femtoseconds. Here we demonstrate that direct laser irradiation is in fact not essential for ultrafast demagnetization, and that electron cascades caused by hot electron currents accomplish it very efficiently. We optically excite a Au/Ni layered structure in which the 30 nm Au capping layer absorbs the incident laser pump pulse and subsequently use the X-ray magnetic circular dichroism technique to probe the femtosecond demagnetization of the adjacent 15 nm Ni layer. A demagnetization effect corresponding to the scenario in which the laser directly excites the Ni film is observed, but with a slight temporal delay. We explain this unexpected observation by means of the demagnetizing effect of a superdiffusive current of non-equilibrium, non-spin-polarized electrons generated in the Au layer.
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                Author and article information

                Journal
                Sci Adv
                Sci Adv
                SciAdv
                advances
                Science Advances
                American Association for the Advancement of Science
                2375-2548
                March 2017
                24 March 2017
                : 3
                : 3
                : e1602094
                Affiliations
                [1 ]University of Kaiserslautern and Research Center OPTIMAS, 67663 Kaiserslautern, Germany.
                [2 ]Forschungszentrum Jülich GmbH, Peter Grünberg Institut (PGI 6), 52425 Jülich, Germany.
                [3 ]Experimentalphysik Universität Duisburg-Essen, Lotharstraße 1, 47057 Duisburg,Germany.
                [4 ]Graduate School MAINZ, Gottlieb-Daimler-Strasse 47, 67663 Kaiserslautern, Germany.
                [5 ]JILA, University of Colorado and National Institute of Standards and Technology, Boulder, CO 80309–0440, USA.
                [6 ]Georg-August-Universität Göttingen, I. Physikalisches Institut, 37077 Göttingen, Germany.
                [7 ]Experimentelle Physik VI, Technische Universität Dortmund, 44221 Dortmund, Germany.
                Author notes
                [* ]Corresponding author. Email: smathias@ 123456uni-goettingen.de
                Author information
                http://orcid.org/0000-0002-7341-7136
                http://orcid.org/0000-0002-7091-9878
                http://orcid.org/0000-0001-8386-6317
                http://orcid.org/0000-0001-7448-1167
                http://orcid.org/0000-0002-3920-6255
                Article
                1602094
                10.1126/sciadv.1602094
                5365247
                28378016
                adc84996-e4e4-4ddd-9a07-dfae278cc3c8
                Copyright © 2017, The Authors

                This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.

                History
                : 01 September 2016
                : 09 February 2017
                Categories
                Research Article
                Research Articles
                SciAdv r-articles
                Band Structures
                Custom metadata
                Ken Marvin Ortega

                femtomagnetism,band-structure renormalization,correlated materials,high-harmonic generation,time-resolved photoemission,stoner vs. heisenberg picture

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