Imaging the Ultrafast Coherent Control of a Skyrmion Crystal

Exotic magnetic textures emerging from the subtle interplay between thermodynamic and topological fluctuation have attracted intense interest due to their potential applications in spintronic devices. Recent advances in electron microscopy enable the imaging of random photogenerated individual skyrmions. However, their deterministic and dynamical manipulation is hampered by the chaotic nature of such fluctuations and the intrinsically irreversible switching between different minima in the magnetic energy landscape. Here, we demonstrate a method to coherently control the rotation of a skyrmion crystal by discrete amounts at speeds which are much faster than previously observed. By employing circularly polarized femtosecond laser pulses with an energy below the band gap of the Mott insulator ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$, we excite a collective magnon mode via the inverse Faraday effect. This triggers coherent magnetic oscillations that directly control the rotation of a skyrmion crystal imaged by cryo-Lorentz transmission electron microscopy. The manipulation of topological order via ultrafast laser pulses shown here can be used to engineer fast spin-based logical devices.

Popular Summary

Technological advancements in computation, data storage, and sensing all require new techniques to control the nanoscale magnetic properties of materials. Particularly important is the magnetic orientation of individual atoms, known as spin. Here, we provide a new protocol for controlling nanoscale magnetic textures on ultrafast timescales, offering exciting new opportunities for ultrafast spin switches in high-density next-generation information storage devices.

The visualization and control of very few spins has not yet been achieved on ultrafast timescales. Recently, researchers developed a new technique that can visualize and control the rotation of a handful of spins arranged in a vortexlike texture, called a skyrmion. We demonstrate a method to coherently control the rotation of a skyrmion crystal by discrete amounts at speeds that are orders of magnitude faster than those previously observed. By employing circularly polarized femtosecond laser pulses, we excite a collective magnon mode in the Mott insulator ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$. This triggers coherent magnetic oscillations that directly control the rotation of a skyrmion crystal, which we image with a type of electron microscopy. Additionally, we harness the insulating properties of this special skyrmion host material. This avoids heating effects and enables low-energy consumption devices.

The manipulation of topological order via ultrafast laser pulses shown here can be used to engineer fast spin-based logical devices.

Figure 1

Illustration and schematic of laser-driven skyrmion crystal rotation process. (a) Real-space images of skyrmion crystal in ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ taken before excitation with the laser pulse and (b) after six successive near-infrared laser pulses each rotate the skyrmion crystal by a discrete amount. Note that the angle (depicted in blue) to the horizontal changes. (c) Fourier transforms of LTEM images following successive pulses of $8\mathrm{mJ}/{\mathrm{cm}}^2$ of near-infrared femtosecond laser excitation. The angle of the hexagonal ordering of the skyrmion crystal changes as a function of the number of pulses applied to the sample. (d) Tracking the position of a single peak in the FT of an image while pumping the sample with individual femtosecond laser pulses. Note that ${\sigma }^{+}$ and ${\sigma }^{-}$ polarizations both rotate the skyrmion crystal in the same direction, while linear polarization does not rotate the skyrmion crystal. The error bars are 95% confidence intervals calculated from the data and multiplied by an uncertainty factor determined from the noise level in the data.

Figure 2

Fluence and time dependence of skyrmion crystal rotation. (a) Fluence dependence of skyrmion crystal rotation. The dashed line is a linear fit to the data points with fluences from 1.6 to $8\mathrm{mJ}/{\mathrm{cm}}^2$. (b) Threshold value of fluence needed to drive rotation in the skyrmion crystal. This value of fluence ($2.2\mathrm{mJ}/{\mathrm{cm}}^2$) is split into two pulses and used to perform the double pulse time-resolved experiments described in (c). Two laser pulses, each with a fluence of $1.1\mathrm{mJ}/{\mathrm{cm}}^2$, are separated by a controlled delay. After the pulses excite the sample, a Lorentz image of the magnetization is recorded. The change in the Fourier transform of the images is shown for various time delays between pulses, illustrating that this time delay between the two pulses directly influences the observed rotation. For clarity, the observed changes are shown after the rotation is driven by 120 pairs of pulses.

Figure 3

Detailed time dependence of skyrmion crystal rotation. (a) Double pulse timed experiments showing the rotation of the skyrmion crystal observed as a function of the delay between the two pump pulses. The pulses either have both ${\sigma }^{+}$ polarization (blue points) or a sequence of ${\sigma }^{+}/{\sigma }^{-}$ polarization (red points). For the blue points, we use pulses of equal amplitude (each with $1.1\mathrm{mJ}/{\mathrm{cm}}^2$) and observe a coherent oscillation in the amplitude of rotation with a period of 175 ps that damps out progressively over a nanosecond. For the case of ${\sigma }^{+}/{\sigma }^{-}$ polarization, the second pulse of ${\sigma }^{-}$ polarization has half the amplitude ($0.6\mathrm{mJ}/{\mathrm{cm}}^2$) of the first one ($1.1\mathrm{mJ}/{\mathrm{cm}}^2$ with ${\sigma }^{+}$ polarization). Here, we observe rotation with approximately half the amplitude out of phase with the previous oscillation. The blue and red dashed lines are a guide for the eye. Error bars (95% confidence intervals) are within the markers used. (b) Theoretical prediction of the rotation angle $\theta /{\theta }_0$ for such pulse sequences in a clean system where ${\theta }_0$ is the rotation angle for a single pulse. The theory is based on the calculation of rotational torques arising from breathing-mode oscillations; see Supplemental Material [26].

Figure 4

Skyrmion crystal rotation in a real-space image from the TEM. (a) Real-space LTEM image of the magnetic structure in ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$. The scale bar (bottom right) is 500 nm. Response of the skyrmion crystal after excitation by a femtosecond laser pulse train is shown for specific regions of the film in (b)–(d). Each pulse has an energy of $8\mathrm{mJ}/{\mathrm{cm}}^2$. We take the FT of each subsection of the image and plot the angle of a single peak in the FT as a function of the number of pulses applied to the sample. The intensity corresponds to the intensity of the peak in the Fourier transform within a region of angles. See Supplemental Material [26] for rotational maps of the entire film.