Research ArticleCONDENSED MATTER PHYSICS

Optical manipulation of magnetic vortices visualized in situ by Lorentz electron microscopy

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Science Advances  20 Jul 2018:
Vol. 4, no. 7, eaat3077
DOI: 10.1126/sciadv.aat3077
  • Fig. 1 Femtosecond laser pulse quenching of a magnetic vortex in Py disks.

    (A) Sketch of imaging the femtosecond laser pulse–induced change of spin configuration in a ferromagnetic Py disk by 4D EM operated in Lorentz phase mode with a continuous electron beam. The green femtosecond laser pulse (520 nm, 350 fs pulse duration) is focused to 40 μm on the sample. (B) Schematic Lorentz contrast reverse mechanism of a magnetic vortex in a circular Py disk before and after a femtosecond laser pulse excitation due to the change of spin chirality. Because of the opposite Lorentz force of the imaging electrons impinging on the sample, the Lorentz contrast of a vortex core can be either black or white. The inset depicts the typical transient temperature evolutions after a femtosecond laser excitation (see Materials and Methods) in both the Py disk and the silicon nitride substrate (TC is the Curie point of the Py disk, TR is the room temperature, and laser fluence is at 12 mJ/cm2). (C) Femtosecond laser pulse–induced variation of a magnetic vortex in circular, square, and regularly triangular Py disks. The right panel of each Fresnel image schematically depicts the corresponding spin configuration. The blue and red dashed lines correspond to the white and black Lorentz contrasts, respectively, while the blue and red dots correspond to the counterclockwise and clockwise vortices, individually. The green dots mark the magnetic antivortex. The same notes are used in all subsequent figures.

  • Fig. 2 Occurrence frequency distribution of the femtosecond laser pulse–induced magnetic structures in three geometrical Py disks.

    (A to C) Frequency distribution of the femtosecond laser pulse (fluence of 12 mJ/cm2)–induced spin configurations in circular, square, and triangular Py disks, respectively. The bottom panel in each subfigure shows the typical Fresnel images of the experiments, and the middle panel schematically shows their corresponding spin configurations. The most frequent single clockwise and counterclockwise vortex structures with opposite Lorentz imaging contrast were counted separately, while the other magnetic structures with opposite Lorentz imaging contrast but with the same spin configuration were added together. The inset in each subfigure denotes the femtosecond laser pulse quenching process in the Py disks.

  • Fig. 3 Comparison of micromagnetic simulations with experimental observations.

    (A to C) Simulation results of the occurrence frequency distribution and energies of the femtosecond laser pulse (fluence of 12 mJ/cm2)–induced magnetic structures in circular, square, and triangular Py disks, respectively. The bottom panel in each subfigure shows the typical results of possible magnetic structures obtained by the micromagnetic simulations (pink bars). The corresponding experiment-determined occurrence frequency distribution of the femtosecond laser pulse–induced magnetic structures is also plotted in blue bars for comparison. The simulation results reproduce the experimental results well (the statistical errors of the histograms are below 5%), with the exception of one magnetic structure in the triangle disk [indicated by the dashed red circle in (C)].

  • Fig. 4 Typical magnetization dynamics in a Py disk after a femtosecond laser pulse quenching by micromagnetic simulation.

    Snapshots of the magnetization dynamics during the formation of different magnetic structures in a triangular Py disk at different times after a femtosecond laser pulse excitation: (A) formation a single magnetic vortex state; (B) formation of a magnetic structure with two vortices; (C) formation of a magnetic structure with three vortices. The laser fluence is 12 mJ/cm2. Note that the precise time scale of the magnetization relaxation process may vary (from hundreds of picoseconds to nanoseconds) with real ferromagnetic disk systems, which exhibit even more complex pinning mechanisms as well as temperature-dependent damping. The vortices and antivortices (including the half-antivortices indicated by the green half dots at the disk edge) in the different colored circles indicate the magnetic vortex-antivortex pairs that annihilate during the magnetization relaxation process. The blue and pink arrows indicate the spin pinning sites at the disk edge.

  • Fig. 5 A paradigm of the optical quenching–assisted, magnetic vortex–based information recording process.

    (Left) Schematic of the optical quenching–assisted, magnetic vortex–based information recording system, where a linear polarized femtosecond laser pulse is used to transiently demagnetize the initial magnetic vortex and another synchronized orthogonal small magnetic field pulse is used to set the polarization of the newly formed magnetic vortex. (Right) Sketch for the working mechanism of the optical quenching–assisted, magnetic vortex–based information recording process. The data information “1” and “0” are recorded by the polarity (up and down) of the magnetic vortex. The fluence of the femtosecond laser pulse should be controlled above the threshold for spin melting, but below the threshold for changes in the ferromagnetic disk’s crystallites.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/7/eaat3077/DC1

    Fig. S1. Occurrence frequency distribution of femtosecond laser pulse–induced magnetic structures in a circular Py disk at a fluence of 16 mJ/cm2.

    Fig. S2. Occurrence frequency distribution of femtosecond laser pulse–induced magnetic structures in a square Py disk at a fluence of 16 mJ/cm2.

    Fig. S3. Occurrence frequency distribution of femtosecond laser pulse–induced magnetic structures in a triangular Py disk at a fluence of 16 mJ/cm2.

    Fig. S4. Schematic of relative change to the sum of demagnetization and exchange energies associated with the transformation of the indicated state to a lower-energy state estimated by application of a magnetic field applied along the ±x or ±y directions, whichever is lowest.

    Fig. S5. Typical magnetic structures in triangular Py disks (edge length of 1.7 μm) determined by micromagnetic simulation to show the pinning sites at the disk edge.

    Fig. S6. Annular bright-field images of a circular Py disk after a femtosecond laser pulse quenching with different fluences to show the change of the inside crystallites.

    Movie S1. Fresnel imaging of femtosecond laser pulse quenching–induced magnetic structure change in a circular Py disk (diameter of 3 μm) at a fluence of 12 mJ/cm2.

    Movie S2. Fresnel imaging of femtosecond laser pulse quenching–induced magnetic structure change in a square Py disk (edge length of 3 μm) at a fluence of 12 mJ/cm2.

    Movie S3. Fresnel imaging of femtosecond laser pulse quenching–induced magnetic structure change in a triangular Py disk (edge length of 1.7 μm) at a fluence of 12 mJ/cm2.

    Movie S4. Fresnel imaging of femtosecond laser pulse quenching–induced magnetic structure change in a circular Py disk (diameter of 1.7 μm) at a fluence of 12 mJ/cm2.

    Movie S5. Micromagnetic simulation on the magnetization relaxation dynamics of the formation of a single magnetic vortex structure in the triangular Py disk (edge length of 1.7 μm) after a femtosecond pulse quenching (fluence of 12 mJ/cm2).

    Movie S6. Micromagnetic simulation on the magnetization relaxation dynamics of the formation of a magnetic structure with two magnetic vortices in the triangular Py disk (edge length of 1.7 μm) after a femtosecond pulse quenching (fluence of 12 mJ/cm2).

    Movie S7. Micromagnetic simulation on the magnetization relaxation dynamics of the formation of a magnetic structure with three magnetic vortices in the triangular Py disk (diameter of 3.0 μm) after a femtosecond pulse quenching (fluence of 12 mJ/cm2).

    Movie S8. Micromagnetic simulation on the magnetization relaxation dynamics of the formation of a single magnetic vortex structure in the circular Py disk (diameter of 3.0 μm) after a femtosecond pulse quenching (fluence of 12 mJ/cm2).

    Movie S9. Micromagnetic simulation on the magnetization relaxation dynamics of the formation of a magnetic structure with four magnetic vortices in the circular Py disk (diameter of 3.0 μm) after a femtosecond pulse quenching (fluence of 12 mJ/cm2).

    Movie S10. Micromagnetic simulation on the magnetization relaxation dynamics of the formation of a magnetic structure with four magnetic vortices in the circular Py disk (diameter of 3.0 μm) after a femtosecond pulse quenching (fluence of 12 mJ/cm2).

    Movie S11. Micromagnetic simulation on the magnetization relaxation dynamics of the formation of a single magnetic vortex structure in the square Py disk (edge length of 3 μm) after a femtosecond pulse quenching (fluence of 12 mJ/cm2).

    Movie S12. Micromagnetic simulation on the magnetization relaxation dynamics of the formation of a magnetic structure with three magnetic vortices in the square Py disk (edge length of 3 μm) after a femtosecond pulse quenching (fluence of 12 mJ/cm2).

    Movie S13. Micromagnetic simulation on the magnetization relaxation dynamics of the formation of a magnetic structure with four magnetic vortices in the square Py disk (edge length of 3 μm) after a femtosecond pulse quenching (fluence of 12 mJ/cm2).

  • Supplementary Materials

    The PDF file includes:

    • Fig. S1. Occurrence frequency distribution of femtosecond laser pulse–induced magnetic structures in a circular Py disk at a fluence of 16 mJ/cm2.
    • Fig. S2. Occurrence frequency distribution of femtosecond laser pulse–induced magnetic structures in a square Py disk at a fluence of 16 mJ/cm2.
    • Fig. S3. Occurrence frequency distribution of femtosecond laser pulse–induced magnetic structures in a triangular Py disk at a fluence of 16 mJ/cm2.
    • Fig. S4. Schematic of relative change to the sum of demagnetization and exchange energies associated with the transformation of the indicated state to a lower-energy state estimated by application of a magnetic field applied along the ±x or ±y directions, whichever is lowest.
    • Fig. S5. Typical magnetic structures in triangular Py disks (edge length of 1.7 μm) determined by micromagnetic simulation to show the pinning sites at the disk edge.
    • Fig. S6. Annular bright-field images of a circular Py disk after a femtosecond laser pulse quenching with different fluences to show the change of the inside crystallites.
    • Legends for movies S1 to S13

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.avi format). Fresnel imaging of femtosecond laser pulse quenching–induced magnetic structure change in a circular Py disk (diameter of 3 μm) at a fluence of 12 mJ/cm2.
    • Movie S2 (.avi format). Fresnel imaging of femtosecond laser pulse quenching–induced magnetic structure change in a square Py disk (edge length of 3 μm) at a fluence of 12 mJ/cm2.
    • Movie S3 (.avi format). Fresnel imaging of femtosecond laser pulse quenching–induced magnetic structure change in a triangular Py disk (edge length of 1.7 μm) at a fluence of 12 mJ/cm2.
    • Movie S4 (.avi format). Fresnel imaging of femtosecond laser pulse quenching–induced magnetic structure change in a circular Py disk (diameter of 1.7 μm) at a fluence of 12 mJ/cm2.
    • Movie S5 (.avi format). Micromagnetic simulation on the magnetization relaxation dynamics of the formation of a single magnetic vortex structure in the triangular Py disk (edge length of 1.7 μm) after a femtosecond pulse quenching (fluence of 12 mJ/cm2).
    • Movie S6 (.avi format). Micromagnetic simulation on the magnetization relaxation dynamics of the formation of a magnetic structure with two magnetic vortices in the triangular Py disk (edge length of 1.7 μm) after a femtosecond pulse quenching (fluence of 12 mJ/cm2).
    • Movie S7 (.avi format). Micromagnetic simulation on the magnetization relaxation dynamics of the formation of a magnetic structure with three magnetic vortices in the triangular Py disk (diameter of 3.0 μm) after a femtosecond pulse quenching (fluence of 12 mJ/cm2).
    • Movie S8 (.avi format). Micromagnetic simulation on the magnetization relaxation dynamics of the formation of a single magnetic vortex structure in the circular Py disk (diameter of 3.0 μm) after a femtosecond pulse quenching (fluence of 12 mJ/cm2).
    • Movie S9 (.avi format). Micromagnetic simulation on the magnetization relaxation dynamics of the formation of a magnetic structure with four magnetic vortices in the circular Py disk (diameter of 3.0 μm) after a femtosecond pulse quenching (fluence of 12 mJ/cm2).
    • Movie S10 (.avi format). Micromagnetic simulation on the magnetization relaxation dynamics of the formation of a magnetic structure with four magnetic vortices in the circular Py disk (diameter of 3.0 μm) after a femtosecond pulse quenching (fluence of 12 mJ/cm2).
    • Movie S11 (.avi format). Micromagnetic simulation on the magnetization relaxation dynamics of the formation of a single magnetic vortex structure in the square Py disk (edge length of 3 μm) after a femtosecond pulse quenching (fluence of 12 mJ/cm2).
    • Movie S12 (.avi format). Micromagnetic simulation on the magnetization relaxation dynamics of the formation of a magnetic structure with three magnetic vortices in the square Py disk (edge length of 3 μm) after a femtosecond pulse quenching (fluence of 12 mJ/cm2).
    • Movie S13 (.avi format). Micromagnetic simulation on the magnetization relaxation dynamics of the formation of a magnetic structure with four magnetic vortices in the square Py disk (edge length of 3 μm) after a femtosecond pulse quenching (fluence of 12 mJ/cm2).

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