Research ArticlePHYSICS

Ultrafast optically induced spin transfer in ferromagnetic alloys

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Science Advances  17 Jan 2020:
Vol. 6, no. 3, eaay8717
DOI: 10.1126/sciadv.aay8717
  • Fig. 1 Ultrafast OISTR in Fe50Ni50.

    (A) Schematic overview of the OISTR effect in Fe50Ni50. The optical excitation by the IR pump leads to an effective spin transfer from the occupied Ni minority channel into the Fe minority channel. Note that other excitations are also possible, and significant OISTR can only be expected if such a spin transfer transition dominates the full excitation process. (B) Projected density of states (DOS) calculation for Fe50Ni50 for Fe (green) and Ni (blue) demonstrating the favorable spin transfer from Ni to Fe in the minority channel. (C and D) TD-DFT calculations of the difference of the transient occupation compared with the unexcited case in the minority channels of Ni (C) and Fe (D) at characteristic time steps demonstrating the OISTR effect. In Ni at energies between 0.5 and 3 eV below the Fermi level, a negative signal arises corresponding to a loss of minority electrons, while a simultaneous positive signal correlating to minority spin gain is visible in Fe at equivalent energies above the Fermi level.

  • Fig. 2 Schematic of the conducted OISTR experiment.

    (A) Experimental setup. An EUV probe pulse investigates the element-specific magnetization dynamics in Fe50Ni50 triggered by an IR pump. (B) Magnetic asymmetry of Fe and Ni plotted as a function of the photon energy. The shaded green and blue energy ranges mark the energy range that is commonly integrated in an absorption edge experiment to obtain the total magnetization.

  • Fig. 3 Direct time-resolved verification of the OISTR effect on ultrashort time scales.

    (A) Static magnetic asymmetry of Fe and Ni plotted as a function of the energy relative to the Fermi level, i.e., the measured photon energy after subtracting the M3 core level energy. (B and C) Spectral dynamics of the magnetic asymmetry of Fe and Ni for different spectral regions. (B) In the energy regions marked in (A) by the shaded colored bars according to the calculations shown in Fig. 1, the spin dynamics show a clear fingerprint of OISTR, on ≈50-fs time scales. (C) In contrast, other spectral regions display only the conventional demagnetization caused by multistep relaxation processes. The inset shows the demagnetization dynamics on a longer time scale, revealing a quenching level of ~25% to which all spectral regions converge. Note that the integrated signals averaging over extended spectral regions show the typical delayed behavior between Fe and Ni (see the Supplementary Materials), as seen before (34). The characteristic energies that are analyzed are marked in (A) with the dashed lines.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/3/eaay8717/DC1

    Section S1. Spectrum

    Section S2. Majority dynamics

    Section S3. Two separated core levels for Ni

    Section S4. Energy integrated signals

    Fig. S1. High-harmonic spectrum reflected from the FeNi sample.

    Fig. S2. Transient occupations in Fe and Ni majority channels.

    Fig. S3. Schematic depiction on the influence of two separated core levels for Ni.

    Fig. S4. Energy integrated magnetization dynamics for Fe and Ni in Fe50Ni50.

  • Supplementary Materials

    This PDF file includes:

    • Section S1. Spectrum
    • Section S2. Majority dynamics
    • Section S3. Two separated core levels for Ni
    • Section S4. Energy integrated signals
    • Fig. S1. High-harmonic spectrum reflected from the FeNi sample.
    • Fig. S2. Transient occupations in Fe and Ni majority channels.
    • Fig. S3. Schematic depiction on the influence of two separated core levels for Ni.
    • Fig. S4. Energy integrated magnetization dynamics for Fe and Ni in Fe50Ni50.

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