Zero-field deterministic all-optical writing and annihilation of nanometer-scale skyrmion bubbles

Abstract

Skyrmions are highly stable chiral magnetic spin textures with non-trivial topology. They can act as quasi-particles that can be generated, manipulated and annihilated, and hold promise for future memory and logic devices. As of now, all-optical stochastic nucleation of skyrmion ensembles, mostly in small applied magnetic fields, has been shown. However, to research their true potential, the ability to selectively toggle switch individual single skyrmions would be highly beneficial. In this paper, we demonstrate the field-free optical control of single stable skyrmions via single femtosecond laser pulses with diameters down to 175 nm, containing a fixed chirality. By engineering ferrimagnetic Co/Gd-based multilayers, we resolve the competition between deterministic and stochastic processes, and thereby overcome the challenge of optically writing and annihilating sub-micron skyrmions on demand. Our work is envisioned to fuel applications of skyrmion-based applications and opens up further endeavors in research related to the behaviour of more complicated skyrmion-based textures.

Figures (6)

• Figure : Figure 1: Schematic illustration of the essence of the experiment performed in this research. At the bottom, a magnetic thin-film stack composed of 3 components: heatsink, a Co/Gd-based ferrimagnet for all-optical switching, and a Pt/Co-based extension. At the top, femtosecond laser pulses are directed at the sample, the inset showcasing an MFM image of a grid switched with said pulses. An uneven number of pulses ($2N_{\text{P}}-1$) results in a switched domain, whereas an even number of pulses ($2N_{\text{P}}$) brings it back to the original state
• Figure : Figure 2: Kerr microscopy and analysis of magnetic stack with a single Pt/Co extension with and without the inclusion of a heat sink. (a) Kerr image of a domain generated after a single femtosecond laser pulse for $t_{\text{Co}}=1.05$ nm and $E_p=264$ nJ. Light and dark contrast correspond to magnetization up (original direction) and down, respectively. (b-c) Two insets further illustrating the roughness $\sigma_D$ of the domain wall and the scale of the generated multidomain in (a). (d) $\sigma_D$ plotted against $t_{\text{Co}}$ for the magnetic stack with and without a heat sink, including a phenomenological fit ($a+be^{ct_{\text{Co}}}$) for guidance. (e) $A_{\text{min}}$ plotted against $t_{\text{Co}}$ without a heat sink, also including a phenomenological fit ($(a+be^{ct_{\text{Co}}})^{-1}$) for guidance. The errorbars represent the first and third quartile linked to each data point. Both (d) and (e) contain insets at different $t_{\text{Co}}$ for illustration. (f) Threshold fluence $F_0$ for AOS and multidomain formation for different $t_{\text{Co}}$. The grayed-out area for $t_{\text{Co}}>1.2$ nm indicates in-plane magnetization. All scalebars are 5 micron.
• Figure : Figure 3: SEMPA images of a magnetic stack with a single Pt/Co extension without a heat sink. (a) raw SEMPA image where up-magnetization is indicated in yellow, while down-magnetization is indicated in blue, superposed on the in-plane contrast as indicated by the color wheel. (b) SEMPA image of the same domain with further data processing for clearer visualization of the domain wall.
• Figure : Figure 4: MFM images illustrating the result of firing $N_\text{P}$ laser pulses at different stack compositions. For the compositions, the thickness of the Co layer is indicated in picometers in brackets and the number next to the square brackets indicates the number of Pt(1.25)/Co($t_{\text{Co}}$) extensions. The addition of "Cu" next to the square brackets indicates the inclusion of a heat sink. The pulse energies for each composition are 43, 69, 78 and 89 pJ, respectively. The dotted white circle represents the laser spot size of 1 $\mu$m. The scalebar is 500 nm.
• Figure : Figure S.I.1: Kerr images of the full dataset of switching single skyrmions on the magnetic stack with a double Pt(1.25)/Co(0.75) repeat. (a), (b) and (c) contain laser pulse energies varying from 3.2-94.9, 98.1-189.8 and 193.0-284.7 pJ, respectively. The lighter grey indicates the initial magnetization state, whereas the darker grey represents the magnetization opposite to the initial state. (d) contains a schematical illustration of a single 2$\times$5 grid of pulses, which a number indicating the number of pulses at each position. The spacing between each point in both the x- and y-direction is 2 $\mu$m. The scale bar is 8 $\mu$m.