Artificial Dense Lattices of Magnetic Skyrmions

Abstract

Multilayer Co/Pt films with perpendicular magnetic anisotropy are irradiated by focused a He⁺ ion beam to locally reduce the anisotropy value. The irradiated spots with the diameters of 100 and 200 nm are arranged in square lattices with the periods of 200 and 300 nm. The formation of nonuniform magnetic states within the spots was observed by magnetic force microscopy methods. We use the concentric distribution of the irradiation fluence within the spot to obtain the radial modulation of the anisotropy constant. This allows us to induce magnetic skyrmions during magnetization reversal of the system. The skyrmions remained stable at zero external magnetic field at room temperature. Magnetization hysteresis loops of the samples were investigated by magnetooptical methods and the results are in good agreement with micromagnetic simulations.

Keywords: Co/Pt films; He ion nanomodification; magnetic skyrmions.

Figure 1

(Color online) (a) Typical polar MOKE hysteresis loop of the initial Co/Pt film. It demonstrates easy-axis perpendicular anisotropy. (b) 3 × 3 μm² MFM image of labyrinth domain structure in the initial Co/Pt film in a remnant state. (c) The geometry of nanomodification: the irradiated spots with the reduced anisotropy form a rectangular lattice with the period of 200 and 300 nm). D₁ and D₂ are the diameters of the regions with different irradiation fluence (f₁ and f₂, correspondingly) and anisotropy (K₁, K₂).

Figure 2

(Color online) (a) 2 × 2 μm² MFM image of the remnant magnetic state of the sample irradiated with the uniform He⁺ ion fluence (2.5 × 10¹⁵ ions/cm²) within a spot. (b) Magnetization hysteresis loop of the same structure. Crosses are the experimental data. The orange solid line is the loop obtained using numerical simulations.

Figure 3

(Color online) (a) Hysteresis magnetization loop of the sample with inhomogeneous concentric He⁺ ion fluence (sample No. 2, spot diameters are D₁ = 80 nm, D₂ = 100 nm). Crosses show the experimental data on the polar magneto-optical Kerr effect and the solid red line is the magnetization loop obtained using numerical simulations. (b) MFM image of the remnant magnetic state of the sample No. 2. The scan width is 1.6 μm. (c) Hysteresis magnetization loop of the sample No. 3 (spot diameters are D₁ = 180 nm, D₂ = 200 nm). Crosses are the experimental MOKE data. The solid orange line is the magnetization loop obtained using numerical simulations. (d) MFM image of the remnant magnetic state of the sample No. 3. The scan width is 1.6 μm. Inset represents AFM image with the size of 0.8 μm. (e) MFM image of the reversed remnant magnetic state of the sample No. 3. The scan width is 1.6 μm. (f) The close view of a single spot. (g) The profile of the MFM signal along the line in (f). The black curve is the experimental data and the red dashed line is the averaged signal.

Figure 4

(Color online) The schematic representations of the magnetization state transformations in the unit cell of the spots lattice during the magnetization reversal (results of numerical simulations). The upper line is for the system with uniform anisotropy within a spot (sample 1, D₁ = 100 nm, a = 200 nm). ON stands for onion state. The middle line is for the sample with concentric distribution of the anisotropy (sample 2, D₂ = 100 nm, a = 200 nm). The bottom line corresponds to the sample 3 (D₁ = 200 nm, a = 300 nm). CW means circular wall, SKM stands for skyrmionium, and SK is for skyrmion. The field values shown above the arrow correspond to the transition between different states. The field at which a particular magnetization distribution is obtained is shown below the corresponding distribution. The image surrounded by a dotted line does not show a stable state but an instant snapshot of the magnetization turnover in the uniformly irradiated spot.