Research ArticleBAND STRUCTURES

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

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Science Advances  24 Mar 2017:
Vol. 3, no. 3, e1602094
DOI: 10.1126/sciadv.1602094
  • Fig. 1 Schematic of the time- and spin-resolved XUV photoemission spectroscopy experiment and the potential response of the electronic and spin systems to laser-induced demagnetization.

    (A) The thin, in-plane-magnetized Co film (30 ML) is excited with near-infrared (NIR) laser pulses (74 ± 1 fs, 1.6 eV) that induce demagnetization. The evolution of the band structure is measured via spin- and time-resolved photoemission using XUV pulses (33 ± 7 fs, 22 eV) from high-harmonic generation (HHG). (B) Exchange split density of states for a 3d ferromagnet (left). Reduced magnetization in the Stoner-like picture via a potential collapse of the exchange splitting (middle) and in the localized spin picture via band mirroring (right).

  • Fig. 2 Investigated region of the band structure.

    (A) Bulk (bottom) and surface Brillouin zones (BZ; top) of the Co fct lattice. The red shaded spherical section illustrates the observed region in the Brillouin zone. Note that the central point of the sphere is the Γ point in the Brillouin zone above (not shown) because the value of k = 2.9 Å−1 for our experimental conditions exceeds the size of the first Brillouin zone with k = 1.8 Å−1. (B) Cut through the ΓKUX plane of one side of the bulk Brillouin zone and the projection to the surface. The red line represents the region in reciprocal space over which we integrate with our spin detector. (C) Calculated band dispersion for the majority Δ2,up band and minority Δ5,down band by a tight-binding method based on the work of Miyamoto et al. (32).

  • Fig. 3 Spin-resolved photoemission spectra.

    (A) Spin-integrated photoemission spectra of Co/Cu(001) (30 ML) before (−100 fs) and after (100 fs) optical excitation. (B) Spin dynamics extracted from the measured spin polarization at EB = 2.3 eV. (C and D) Partial intensities of majority- and minority-spin photoemission spectra as a function of time. Lines correspond to the fits, as described in the text, whereas the arrows indicate a decrease/increase in spectral weight. (E) Transient quenching of the spin polarization. (F) Transient spin polarization extracted from energies around the Fermi level (red squares) and at higher binding energies (black circles), together with the appearance of hot electrons (violet open squares). All lines in (F) are guides to the eye.

  • Fig. 4 Analysis of possible exchange collapse versus band mirroring.

    (A) Extracted energetic shifts of the majority and minority bands as a function of time. (B) Modeled majority and minority spectra (top) and spin polarization (bottom), if only energetic shifts are considered, in comparison to the measured experimental data at t = 100 fs. (C) Extracted amount of band mirroring. The scaling prefactors AMaj and BMaj (blue solid and red dashed lines) that were multiplied with the unpumped (“initial”) majority and minority spectra, respectively, to fit the data of the majority channel after excitation are shown. One sees that the majority channel loses spectral weight from its initial majority spectrum and gains spectral weight from the initial minority spectrum accordingly. The same was carried out for AMin and BMin in the minority channel (blue dashed and red solid lines). (D) Same as (B), if only band mirroring is considered.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/3/e1602094/DC1

    section S1. Time- and angle-resolved photoemission spectroscopy.

    section S2. Time- and angle-resolved photoemission spectroscopy: Difference spectra.

    fig. S1. Time- and angle-resolved photoemission spectroscopy.

    fig. S2. Time- and angle-resolved photoemission spectroscopy: Difference spectra.

  • Supplementary Materials

    This PDF file includes:

    • section S1. Time- and angle-resolved photoemission spectroscopy.
    • section S2. Time- and angle-resolved photoemission spectroscopy: Difference spectra.
    • fig. S1. Time- and angle-resolved photoemission spectroscopy.
    • fig. S2. Time- and angle-resolved photoemission spectroscopy: Difference spectra.

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