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Title: Laser-induced ultrafast transport and demagnetization at the earliest time: first-principles and real-time investigation

Abstract

It is generally believed that there are at least two ways to use an ultrafast laser pulse to demagnetize a magnetic sample. One is to directly photo-demagnetize the system through spin-orbit coupling (SOC), and the other is to utilize ultrafast hot electron transport without SOC. The challenge is that these two processes are entangled on the same time scale. While the experimental results have been inconclusive, theoretical investigations are even scarcer, beyond those earlier studies based on spin superdiffusion. For instance, we even do not know how fast electrons move under laser excitation and how far they move. Here we carry out a first-principles time-dependent calculation to investigate how fast electrons actually move under laser excitation and how large the electron transport affects demagnetization on the shortest time scale. To take into account the transport effect, we implement the intraband transition in our theory. In the bulk fcc Ni, we find the effect of the spin transport on the demagnetization is extremely small, no more than 1%. The collective electron velocity in Ni is 0.4 Å/fs, much smaller than the Fermi velocity, and the collective displacement is no more than 0.1 Å. But this does not mean that electrons domore » not travel fast; instead we find that electron velocities at two opposite crystal momenta cancel each other. We follow the Γ-X line and find a huge dispersion in the velocities in the crystal momentum space. In the Fe/W(110) thin film, the overall demagnetization is larger than Ni, and the Fermi velocity is higher than Ni. However, the effect of the spin transport is still small in the Fe/W(110) thin film. Based on our numerical results and existing experimental findings, we propose a different mechanism that can explain two latest experimental results. Our finding sheds new light on the effect of ballistic transport on demagnetization.« less

Authors:
ORCiD logo [1];  [1];  [1];  [2]
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  1. Indiana State Univ., Terre Haute, IN (United States)
  2. Univ. of Missouri, St. Louis, MO (United States)
Publication Date:
Research Org.:
Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States). National Energy Research Scientific Computing Center (NERSC)
Sponsoring Org.:
USDOE
OSTI Identifier:
1544160
Resource Type:
Accepted Manuscript
Journal Name:
Journal of Physics. Condensed Matter
Additional Journal Information:
Journal Volume: 30; Journal Issue: 46; Journal ID: ISSN 0953-8984
Publisher:
IOP Publishing
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY

Citation Formats

Zhang, G. P., Bai, Y. H., Jenkins, Tyler, and George, Thomas F. Laser-induced ultrafast transport and demagnetization at the earliest time: first-principles and real-time investigation. United States: N. p., 2018. Web. doi:10.1088/1361-648X/aae5a9.
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Zhang, G. P., Bai, Y. H., Jenkins, Tyler, & George, Thomas F. Laser-induced ultrafast transport and demagnetization at the earliest time: first-principles and real-time investigation. United States. https://doi.org/10.1088/1361-648X/aae5a9
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Zhang, G. P., Bai, Y. H., Jenkins, Tyler, and George, Thomas F. Thu . "Laser-induced ultrafast transport and demagnetization at the earliest time: first-principles and real-time investigation". United States. https://doi.org/10.1088/1361-648X/aae5a9. https://www.osti.gov/servlets/purl/1544160.
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@article{osti_1544160,
title = {Laser-induced ultrafast transport and demagnetization at the earliest time: first-principles and real-time investigation},
author = {Zhang, G. P. and Bai, Y. H. and Jenkins, Tyler and George, Thomas F.},
abstractNote = {It is generally believed that there are at least two ways to use an ultrafast laser pulse to demagnetize a magnetic sample. One is to directly photo-demagnetize the system through spin-orbit coupling (SOC), and the other is to utilize ultrafast hot electron transport without SOC. The challenge is that these two processes are entangled on the same time scale. While the experimental results have been inconclusive, theoretical investigations are even scarcer, beyond those earlier studies based on spin superdiffusion. For instance, we even do not know how fast electrons move under laser excitation and how far they move. Here we carry out a first-principles time-dependent calculation to investigate how fast electrons actually move under laser excitation and how large the electron transport affects demagnetization on the shortest time scale. To take into account the transport effect, we implement the intraband transition in our theory. In the bulk fcc Ni, we find the effect of the spin transport on the demagnetization is extremely small, no more than 1%. The collective electron velocity in Ni is 0.4 Å/fs, much smaller than the Fermi velocity, and the collective displacement is no more than 0.1 Å. But this does not mean that electrons do not travel fast; instead we find that electron velocities at two opposite crystal momenta cancel each other. We follow the Γ-X line and find a huge dispersion in the velocities in the crystal momentum space. In the Fe/W(110) thin film, the overall demagnetization is larger than Ni, and the Fermi velocity is higher than Ni. However, the effect of the spin transport is still small in the Fe/W(110) thin film. Based on our numerical results and existing experimental findings, we propose a different mechanism that can explain two latest experimental results. Our finding sheds new light on the effect of ballistic transport on demagnetization.},
doi = {10.1088/1361-648X/aae5a9},
journal = {Journal of Physics. Condensed Matter},
number = 46,
volume = 30,
place = {United States},
year = {Thu Oct 25 00:00:00 EDT 2018},
month = {Thu Oct 25 00:00:00 EDT 2018}
}
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Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record
https://doi.org/10.1088/1361-648X/aae5a9
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Cited by: 7 works
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Figures / Tables:

FIG. 1 FIG. 1: (a) Spin transport geometry under electric current. The bias is applied longitudinally, so electrons move in the opposite direction of the electric field. (b) Laser-induced spin transport. Here the laser electric field is perpendicular to the light propagation direction. The initial motion of the electron is vertical. (c)more » If the interband transition is ignored, the Fermi sphere shifts under an external field. However, in our simulation, we do not use this approach. (d) Supercell of one layer of Fe on three layers of W.« less
All figures and tables (6 total)
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    Spin-orbit torque-mediated spin-wave excitation as an alternative paradigm for femtomagnetism
    journal, September 2019

    • Zhang, G. P.; Murakami, M.; Bai, Y. H.
    • Journal of Applied Physics, Vol. 126, Issue 10
    • DOI: 10.1063/1.5110522
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