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      Temperature Dependence of Laser-Induced Demagnetization in Ni: A Key for Identifying the Underlying Mechanism

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          Coherent ultrafast magnetism induced by femtosecond laser pulses

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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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              Super-Diffusive Spin-Transport as a Mechanism of Ultrafast Demagnetization

              We propose a semi-classical model for femtosecond-laser induced demagnetization due to spin-polarized excited electron diffusion in the super-diffusive regime. Our approach treats the finite elapsed time and transport in space between multiple electronic collisions exactly, as well as the presence of several metal films in the sample. Solving the derived transport equation numerically we show that this mechanism accounts for the experimentally observed demagnetization within 200fs in Ni, without the need to invoke any angular momentum dissipation channel.
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                Author and article information

                Journal
                PRXHAE
                Physical Review X
                Phys. Rev. X
                American Physical Society (APS)
                2160-3308
                May 2012
                May 16 2012
                : 2
                : 2
                Article
                10.1103/PhysRevX.2.021006
                © 2012

                http://creativecommons.org/licenses/by/3.0/

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