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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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Figure 1: The investigated layered structures that are optically excited by the femtosecond laser pulse.
Figure 2: Time evolution of the magnetization of the Ni layer, after excitation with a femtosecond laser pulse, as measured through the element-selective XMCD signal at the Ni L3 absorption edge and normalized to the value before time zero.
Figure 3: Calculated time evolution of the average magnetization of the Ni film, in the Au/Ni sample and in the Ni reference sample, for 33 and 13 mJ cm−2 incident pump fluence, respectively.
Figure 4: Calculated time- and depth-resolved magnetization change ΔM(z,t) induced by NEQ electron superdiffusion.

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We thank F. Radu for generous help during sample preparation and S. Valencia and K. Carva for fruitful discussions. Financial support by the German Ministry of Education and Research BMBF Grant 05K10PG2 FEMTOSPEX, by the Swedish Research Council (VR), the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreements No. 214810 FANTOMAS and No. 281043 FEMTOSPIN, and the Swedish National Infrastructure for Computing (SNIC) is gratefully acknowledged.

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A.E., N.P., T.K., K.H. R.M. and C.S. performed the time-resolved experiments; A.E. prepared the samples and analysed the data; M.B. and P.M. performed the calculations; C.S., M.B. and P.M.O. wrote the manuscript. All authors commented on, discussed and improved the manuscript.

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Correspondence to A. Eschenlohr, M. Battiato or C. Stamm.

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The authors declare no competing financial interests.

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Eschenlohr, A., Battiato, M., Maldonado, P. et al. Ultrafast spin transport as key to femtosecond demagnetization. Nature Mater 12, 332–336 (2013).

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