Relevance of thermal disorder in the electronic and spin ultrafast dynamics of iron in the low-perturbation regime

G. M. Pierantozzi, A. De Vita, R. Cucini, A. M. Finardi, T. Pincelli, F. Sirotti, J. Fujii, C. Dri, G. Brajnik, R. Sergo, G. Cautero, G. Panaccione, and G. Rossi

Phys. Rev. B 109, 064411 – Published 13 February 2024

Abstract

Understanding the ultrafast demagnetization of transition metals requires pump-probe experiments sensitive to the time evolution of the electronic, spin, and lattice thermodynamic baths. By means of time-resolved photoelectron energy and spin-polarization measurements in the low-pump-fluence regime on iron, we disentangle the different dynamics of hot electrons and demagnetization in the subpicosecond and picosecond time range. We observe a broadening of the Fermi-Dirac distribution, following the excitation of nonthermal electrons at specific region of the iron valence band. The corresponding reduction of the spin polarization is remarkably delayed with respect to the dynamics of electronic temperature. The experimental results are corroborated with a microscopic 3-temperature model highlighting the role of thermal disorder in the quenching of the average spin magnetic moment, and indicating Elliot-Yafet type spin-flip scattering as the main mediation mechanism, with a spin-flip probability of 0.1 and a rate of energy exchange between electrons and lattice of $2.5 K {fs}^{- 1}$ .

Received 23 May 2023
Revised 18 September 2023
Accepted 10 January 2024

DOI:https://doi.org/10.1103/PhysRevB.109.064411

Physics Subject Headings (PhySH)

Light-induced magnetic effects Spin dynamics Spin polarization Ultrafast magnetization dynamics

Ferromagnets Magnetic thin films Transition metals

Spin-resolved photoemission spectroscopy Time & angle resolved photoemission spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

G. M. Pierantozzi ^1,*, A. De Vita ^1,2, R. Cucini ¹, A. M. Finardi ^1,2, T. Pincelli^1,3, F. Sirotti ⁴, J. Fujii¹, C. Dri ⁵, G. Brajnik ⁵, R. Sergo⁵, G. Cautero⁵, G. Panaccione ^1,†, and G. Rossi ^1,2

¹Istituto Officina dei Materiali (IOM), Consiglio Nazionale delle Ricerche (CNR), in Area Science Park, S.S. 14, km 163.5, I-34149 Trieste, Italy
²Dipartimento di Fisica, Universitá degli Studi di Milano, Via Celoria 16, I-20133 Milano, Italy
³Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, Berlin 14195, Germany
⁴Physique de la Matiére Condensée, CNRS and École Polytechnique, Institut Polytechnique de Paris, Palaiseau 91120, France
⁵Elettra Sincrotrone Trieste S.C.p.A, in Area Science Park, S.S.14 - km 163.5, I-34149 Trieste, Italy

^*Corresponding author: pierantozzi@iom.cnr.it
^†Corresponding author: panaccione@iom.cnr.it

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Issue

Vol. 109, Iss. 6 — 1 February 2024

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Images

Figure 1
(a) Density functional theory (DFT) calculations in the local density approximation (LDA) for Fe(001), showing the dispersion of the band structure at $Γ$ as a function of $k_{z}$ along the $Γ − H$ direction. The available optical transitions to empty states close to the vacuum level for $4.8 eV$ are highlighted by arrows. (b) and (c) Angle-resolved photoemission spectroscopy (ARPES) spectra along the $\bar{Γ} − \bar{X}$ direction on clean Fe(001) and Fe(001)- $p (1 \times 1) O$ , measured with $25 eV, p$ -polarized synchrotron radiation. The parabola encloses the maximum E-k region accessible by $4.8 eV$ photons according to conservation rules, given a work function of $4.7 eV$ .
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Figure 2
Time-resolved relative variation of total electron yield (TEY; top) and spin polarization (SP; bottom) at $1.3 {mJ / cm}^{2}$ pump fluence, using $4.8 eV$ probe and $1.55 eV$ pump. Inset: Experimental geometry.
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Figure 3
Time-resolved relative variation of spin polarization (SP) as a function of pump fluence.
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Figure 4
(a) Cartoon representation of the time-resolved (TR) angle-resolved photoemission spectroscopy (ARPES) experimental geometry. (b) ARPES static measurement at $21.7 eV$ , s-polarized. (c)–(e) TR-ARPES with $1.55 eV$ pump and $21.7 eV$ probe, measured in the regions of (b) highlighted by the colored rectangles. Main panel: Spectrum averaged over the temporal delays before $t_{0}$ . Insets: In the top (bottom), the difference between the spectrum at $t_{0} \pm 60 fs$ (300– $500 fs$ ) and the average before $t_{0}$ (blue positive, red negative). (f) $k$ -integrated spectrum of (e) vs delay. Top: Difference map after subtraction of the average before $t_{0}$ (blue positive, red negative). Bottom: Delay cuts in the energy ranges indicated by the colored rods in the top panel, multiplied by arbitrary factors to compare their line shapes. (g) Comparison of the behavior of thermal electrons in the three regions of momentum space highlighted in (b), with corresponding colors: The intensity difference integrated from 0–0.2 eV is rescaled to compensate different photoemission intensities due to pump fluence dependence on the incidence angle.
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Figure 5
Electronic temperature [top, from the time-resolved (TR) angle-resolved photoemission spectroscopy (ARPES) experiment] and relative magnetic moment [bottom, from the TR spin-polarization (SP) experiment]. Black lines are fits based on the microscopic 3-temperature model (m-3TM). The temporal alignment has been retrieved using $t_{0}$ values obtained by the two independent fit procedures.
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