Extreme Domain Wall Speeds under Ultrafast Optical Excitation

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Extreme Domain Wall Speeds under Ultrafast Optical Excitation

Rahul Jangid, Nanna Zhou Hagström, Meera Madhavi, Kyle Rockwell, Justin M. Shaw, Jeffrey A. Brock, Matteo Pancaldi, Dario De Angelis, Flavio Capotondi, Emanuele Pedersoli, Hans T. Nembach, Mark W. Keller, Stefano Bonetti, Eric E. Fullerton, Ezio Iacocca, Roopali Kukreja, and Thomas J. Silva

Phys. Rev. Lett. 131, 256702 – Published 19 December 2023

See synopsis: Domain Walls Break the Sound Barrier

Abstract

Time-resolved ultrafast EUV magnetic scattering was used to test a recent prediction of $> 10 km / s$ domain wall speeds by optically exciting a magnetic sample with a nanoscale labyrinthine domain pattern. Ultrafast distortion of the diffraction pattern was observed at markedly different timescales compared to the magnetization quenching. The diffraction pattern distortion shows a threshold dependence with laser fluence, not seen for magnetization quenching, consistent with a picture of domain wall motion with pinning sites. Supported by simulations, we show that a speed of $\approx 66 km / s$ for highly curved domain walls can explain the experimental data. While our data agree with the prediction of extreme, nonequilibrium wall speeds locally, it differs from the details of the theory, suggesting that additional mechanisms are required to fully understand these effects.

Received 28 April 2023
Revised 5 July 2023
Accepted 8 November 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.256702

Physics Subject Headings (PhySH)

Demagnetization Domain wall motion Domain walls Domains Laser-induced demagnetization Magnetic order Magnetic texture Magnetization dynamics Magnons Micromagnetism Optical generation of spin carriers Spin current Spintronics Ultrafast demagnetization Ultrafast magnetic effects

Free-electron lasers

Micromagnetic modeling Sputtering Ultrafast femtosecond pump probe

Condensed Matter, Materials & Applied PhysicsQuantum Information, Science & Technology

synopsis

Domain Walls Break the Sound Barrier

Published 19 December 2023

Experiments reveal that the boundaries between magnetic domains in a multilayered magnetic metal can move faster than sound, confirming a previous prediction.

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Authors & Affiliations

Rahul Jangid ^1,2, Nanna Zhou Hagström ^3,1, Meera Madhavi ¹, Kyle Rockwell ⁴, Justin M. Shaw ⁵, Jeffrey A. Brock⁶, Matteo Pancaldi ⁷, Dario De Angelis ⁷, Flavio Capotondi⁷, Emanuele Pedersoli ⁷, Hans T. Nembach^8,9, Mark W. Keller⁵, Stefano Bonetti^3,10, Eric E. Fullerton⁶, Ezio Iacocca⁴, Roopali Kukreja ^1,*, and Thomas J. Silva ^5,†

¹Department of Materials Science and Engineering, University of California Davis, Davis, California, USA
²National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, New York 11973, USA
³Department of Physics, Stockholm University, 106 91 Stockholm, Sweden
⁴Center for Magnetism and Magnetic Nanostructures, University of Colorado Colorado Springs, Colorado Springs, Colorado, USA
⁵Quantum Electromagnetics Division, National Institute of Standards and Technology, Boulder, Colorado, USA
⁶Center for Memory and Recording Research, University of California San Diego, La Jolla, California, USA
⁷Elettra Sincrotrone Trieste S.C.p.A., Area Science Park, S.S. 14 km 163.5, 34149 Trieste, Italy
⁸Department of Physics, University of Colorado, Boulder, Colorado 80309, USA
⁹Associate, Physical Measurement Laboratory, National Institute of Standards and Technology, Boulder, Colorado 80305, USA
¹⁰Department of Molecular Sciences and Nanosystems, Ca’ Foscari University of Venice, 30172 Venezia, Italy

^*Corresponding author: rkukreja@ucdavis.edu
^†Corresponding author: thomas.silva@nist.gov

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Vol. 131, Iss. 25 — 22 December 2023

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Images

Figure 1
Experimental schematic and evolution of labyrinthine domain pattern as a function of delay time. (a) Optical pump EUV magnetic scattering probe setup with MFM image of the sample. The white arrow highlights the direction of the linear texture of the domain pattern. Magnetic diffraction scattering on the CCD is fitted with a 2D phenomenological model described in the Supplemental Material Sec. A [28], from which we separate the ring and lobe components. (b) Isolated isotropic (ring) and anisotropic (lobes) fit components with arrows indicating the radius ( $q_{R}$ , $q_{L}$ ) and full-width half maximum ( $Γ_{R}$ , $Γ_{L}$ ) of scattering. Time-resolved (c) amplitude ( $A_{R}$ ), (d) ring radius ( $q_{R}$ ), and (e) width ( $Γ_{R}$ ) obtained from the fit of the isotropic scattering (ring). Delay curves are plotted for a range of measured fluence values from 0.8 to $13.4 mJ / {cm}^{2}$ . The scattering amplitude which is proportional to magnetization, decays immediately following laser excitation indicating demagnetization which recovers on ps timescales. The ring radius ( $q_{R}$ ) and width ( $Γ_{R}$ ) of the isotropic scattering approximate the average real-space domain size and correlation length of the labyrinthine domains, respectively. Note that the plotted data for $A_{R}$ , $q_{R}$ and $Γ_{R}$ are relative to the before $t = 0$ value.
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Figure 2
Laser fluence dependence of isotropic and anisotropic scattering from labyrinthine and stripe domains. (a) Normalized scattered amplitude dependence on fluence for both the isotropic ( $A_{R}$ ) and anisotropic scattering ( $A_{L}$ ). Fluence dependence of (b) ring shift and (c) width for both the ring ( $Δ q_{R} / q_{R}$ and $Δ Γ_{R} / Γ_{R}$ ) and lobes ( $Δ q_{L} / q_{L}$ and $Δ Γ_{L} / Γ_{L}$ ). The dashed lines indicate the results of linear error-weighted fits of the data. For $A_{R}$ and $A_{L}$ , the fits extend over the entire range of pump fluence. For $Δ q_{R} / q_{R}$ and $Δ Γ_{R} / Γ_{R}$ , two fits were performed below and above the threshold fluence of $7.8 mJ / {cm}^{2}$ .
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Figure 3
Laser fluence dependence of quench and recovery time. (a) Quench and (b) recovery time constants obtained from the temporal fits (see the Supplemental Material, Sec. B SM) for $A_{R}$ , $A_{L}$ , $q_{R}$ , and $Γ_{R}$ . The magnetization quench is 2 times faster than the change in radial ring position and ring width ( $τ_{m} \approx 0.3 ps$ ) irrespective of the fluence value. The recovery time constants ( $τ_{rec}$ ) for magnetization quench ( $A_{R}$ and $A_{L}$ ) are also distinct from $τ_{rec}$ for ring shift ( $q_{R}$ ) and width ( $Γ_{R}$ ).
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Figure 4
Simulated modification of domain pattern and calculated domain wall velocity. (a) Simulated modified domain pattern (black and white domains) and initial state (colored outline). The modified state was simulated assuming a 40% reduction in the saturation magnetization as discussed in the text. The color of the outline denotes the initial wall curvature which was estimated using the inverse of the radius of local circle fit. The comparison clearly shows that regions with high curvature (dark red and blue) undergo noticeable domain wall motion. (b) Fluence dependence of calculated domain wall velocity for labyrinthine domains estimated using experimentally measured and simulated contraction of diffraction ring radius.
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