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. 2018 Jul 20;4(7):eaat3077.
doi: 10.1126/sciadv.aat3077. eCollection 2018 Jul.

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

Xuewen Fu  1 , Shawn D Pollard  2 , Bin Chen  3 , Byung-Kuk Yoo  4 , Hyunsoo Yang  2 , Yimei Zhu  1
Affiliations

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

Xuewen Fu et al. Sci Adv. .

Abstract

Understanding the fundamental dynamics of topological vortex and antivortex naturally formed in microscale/nanoscale ferromagnetic building blocks under external perturbations is crucial to magnetic vortex-based information processing and spintronic devices. All previous studies have focused on magnetic vortex-core switching via external magnetic fields, spin-polarized currents, or spin waves, which have largely prohibited the investigation of novel spin configurations that could emerge from the ground states in ferromagnetic disks and their underlying dynamics. We report in situ visualization of femtosecond laser quenching-induced magnetic vortex changes in various symmetric ferromagnetic Permalloy disks by using Lorentz phase imaging of four-dimensional electron microscopy that enables in situ laser excitation. Besides the switching of magnetic vortex chirality and polarity, we observed with distinct occurrence frequencies a plenitude of complex magnetic structures that have never been observed by magnetic field- or current-assisted switching. These complex magnetic structures consist of a number of newly created topological magnetic defects (vortex and antivortex) strictly conserving the topological winding number, demonstrating the direct impact of topological invariants on magnetization dynamics in ferromagnetic disks. Their spin configurations show mirror or rotation symmetry due to the geometrical confinement of the disks. Combined micromagnetic simulations with the experimental observations reveal the underlying magnetization dynamics and formation mechanism of the optical quenching-induced complex magnetic structures. Their distinct occurrence rates are pertinent to their formation-growth energetics and pinning effects at the disk edge. On the basis of these findings, we propose a paradigm of optical quenching-assisted fast switching of vortex cores for the control of magnetic vortex-based information recording and spintronic devices.

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Figures

Fig. 1
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
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
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
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
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.
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References

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