Realization of ground-state artificial skyrmion lattices at room temperature

Abstract

The topological nature of magnetic skyrmions leads to extraordinary properties that provide new insights into fundamental problems of magnetism and exciting potentials for novel magnetic technologies. Prerequisite are systems exhibiting skyrmion lattices at ambient conditions, which have been elusive so far. Here, we demonstrate the realization of artificial Bloch skyrmion lattices over extended areas in their ground state at room temperature by patterning asymmetric magnetic nanodots with controlled circularity on an underlayer with perpendicular magnetic anisotropy (PMA). Polarity is controlled by a tailored magnetic field sequence and demonstrated in magnetometry measurements. The vortex structure is imprinted from the dots into the interfacial region of the underlayer via suppression of the PMA by a critical ion-irradiation step. The imprinted skyrmion lattices are identified directly with polarized neutron reflectometry and confirmed by magnetoresistance measurements. Our results demonstrate an exciting platform to explore room-temperature ground-state skyrmion lattices.

Figure 1. Schematic diagram of the artificial skyrmion lattices and measured magnetic hysteresis loops.

(a) The hybrid structure consists of Co dots (red) on top of Co/Pd PMA underlayer (grey) where the in-plane spin texture of the Co dots (purple arrows) is imprinted into an irradiated Co/Pd region (light blue) underneath the dots (tilted blue arrows). Green and yellow arrows indicate the moments in the Co/Pd underlayer and the core region of the (imprinted) vortex, respectively. Major in-plane (open symbols) and perpendicular (solid symbols) hysteresis loops are shown for (b) the Co/Pd underlayer as grown, (c) the irradiated Co/Pd witness sample and (d) the hybrid Co+Co/Pd sample.

Figure 2. Circularity control.

(a) Family of FORCs of the hybrid sample measured in the in-plane geometry. The zoomed-in view in (b) illustrates essentially two discrete vortex annihilation fields. Remanent-state (c) MFM and (d) SEMPA (superimposed onto a scanning electron microscopy image of the dots) images, after saturating the dots in an in-plane field parallel to the flat edge of the dots to the right, indicate circularity control. Scale bar, 2 μm. A key to the magnetization winding direction is shown in the insets.

Figure 3. Polarity control.

Top row shows schematic illustrations of the SL, VL and ML states. The yellow arrows mark the core direction in the vortex and the imprinted region, while the other arrows represent the magnetic moments in other parts of the structure. (a) Magnetization curves, with the field sweeping from zero to negative saturation, for the hybrid structure prepared into the SL (red), VL (black) and ML (blue) states at remanence. (b) The image highlights the magnetization difference between the VL and SL, and zoomed-in views of the magnetization curves in dashed boxes are shown in (c) near zero field and (d) ∼320 mT where the Co/Pd underlayer starts its reversal, respectively.

Figure 4. Polarized neutron reflectometry.

(a) Position-sensitive area-detector image of the (q_x, q_z) unpolarized neutron scattering, with polarized traces along (b) the specular (q_x=0) and (c) the first off-specular (q_x=3.6 μm⁻¹) reflections. The fitted depth-dependent nuclear (ρ_Ν, solid black and dashed blue curves) and magnetic (ρ_Μ, solid red and dashed green curves) scattering densities from the specular measurement are shown in (d), over the protected film (dashed curves) and the vortex region (solid curves). On top of the Si substrate (yellow region), at increasing depth, the film structure corresponds to the Pd seed (grey), Co/Pd underlayer (pink), Co dot (blue), Ta cap (green) and air (white). The calculated reflectivity from these depth profiles are shown as solid lines in (b). The Born-simulated reflectometry patterns from the OOMMF simulations are shown as dashed lines in (b) and (c). Error bars in q_z identify machine precision; error bars in normalized reflectivity are defined by the s.d. and scale with the square root of the number of measurements.

Figure 5. Anisotropic magnetoresistance (AMR).

(a) Perpendicular AMR, and (b) transverse AMR results (black and red curves for descending and ascending field sweeps, respectively), superimposed with (a) perpendicular hysteresis loop of the Co/Pd and (b) in-plane loop of the Co dots (blue curves), respectively, showing the AMR sensitivity to the imprinted spin texture in the Co/Pd underlayer.