Formation of Néel-type skyrmions in an antidot lattice with perpendicular magnetic anisotropy

Magnetic skyrmions are particlelike chiral spin textures found in magnetic films with out-of-plane anisotropy and are considered to be potential candidates as information carriers in next generation data storage devices. Despite intense research into the nature of skyrmions and their dynamic properties, there are several key challenges that still need to be addressed. In particular, the outstanding issues are the reproducible generation, stabilization, and confinement of skyrmions at room temperature. Here, we present a method for the capture of magnetic skyrmions in an array of defects in the form of an antidot lattice. We find that inhomogeneity in the total effective field produced by the antidot lattice is important for the formation of skyrmions which are mainly stabilized by the dipolar interaction. With micromagnetic simulations and scanning transmission x-ray microscopy we elucidate that the formation of skyrmions within the antidot lattice depends on the lattice constant and that, below a certain lattice constant, the skyrmion formation is suppressed. Based on our results we propose that, by varying the lattice constant, we can modify the probability of skyrmion formation in different parts of a sample by specific patterning. This provides another platform for experimental investigations of skyrmions and skyrmion-based devices.

Figure 1

(a) Visualization of the 3D spin orientation for skyrmions inside the antidot lattice in an applied magnetic field of $-35$ mT taken from micromagnetic simulations. (b) Schematic of the multilayer stack with an antidot fabricated using focused Ga ion beam milling (FIB). (c) Scanning electron microscope image of the antidot lattice with antidot diameter $d=250\mathrm{nm}$ and lattice constant $\text{LC}=550$ nm patterned in the ${[\mathrm{Pt}/\mathrm{Co}/\mathrm{Ta}]}_{12}$ film.

Figure 2

(a) Out-of-plane hysteresis loop of the ${[\mathrm{Pt}/\mathrm{Co}/\mathrm{Ta}]}_{12}$ film. (b) The frequency asymmetry ($\mathrm{\Delta }f$) extracted from the BLS measurement of the ${[\mathrm{Pt}/\mathrm{Co}/\mathrm{Ta}]}_{12}$ film for an in-plane applied bias magnetic field of 0.9 and $-0.9$ T. Black dots are the experimental results, while the red solid line is the linear fit to the experimental data. $\mathrm{\Delta }f$ is the difference between the frequency of the Stokes and the anti-Stokes peaks present in the BLS spectra. A schematic of the experimental configuration is shown in the inset. (c)–(e) XMCD-STXM images of magnetic states in the multilayer film at three different magnetic fields indicated with blue points on the hysteresis loop in (a). (f)–(h) Simulated images of similar magnetic configurations in the multilayer film. In all images, the dark and bright contrast corresponds to domains with magnetization pointing up and down, respectively. The skyrmions are indicated by solid red circles and the domain ends from which the skyrmions are formed are indicated by dashed red circles.

Figure 3

Simulated images of the evolution of the magnetic configuration in antidot lattices with different lattice constants on increasing the magnetic field. The dark and bright contrast corresponds to domains with magnetization pointing up and down, respectively. Pinned domains are indicated by red dashed circles and the skyrmions are marked with circles with red solid lines.

Figure 4

(a)–(d) Contour plots of the simulated effective field distributions overlaid on the color map for the different lattices at 5 mT. The center point (CP), indicated by the gray dot, is the intersection of the diagonal lines connecting the centers of the antidots. (e), (f) Simulated effective field from different lattices obtained by taking a line scan across the two different regions marked by horizontal dashed and solid lines in Figs. 4–4, respectively. Different line colors are used for the different lattices.

Figure 5

Snapshot of a micromagnetic simulation showing the magnetic configuration of a composite structure combining two antidot lattices with $\text{LC}=800$ and 400 nm with an unpatterned film at $H=-50$ mT. Symbols R, F, and L indicate three different regimes of the structure, where skyrmion formation is at random locations (R), forbidden (F), and localized at the center points in the effective magnetic field (L). For these simulations, the random distribution of the anisotropy of individual grains has not been included.

Figure 6

XMCD-STXM images of the evolution of the magnetic configuration in the antidot lattices with magnetic field for anitdot lattices with (a)–(d) $\text{LC}=550$ nm and (e)–(h) $\text{LC}=1000$ nm. The dark and bright contrast corresponds to magnetic domains with magnetization pointing up and down, respectively. The skyrmions are indicated by solid circles and the domains from which the skyrmions are formed are indicated by red dashed circles and ellipses.

Figure 7

Line profile of the magnetic contrast across the skyrmion marked with a yellow solid circle in Fig. 6. The profile is fitted with a Gaussian curve and has a full width half maximum of 147 nm.