Magnetic-field excited domain wall motion in antiferromagnetically-coupled bubble skyrmion pairs

Antiferromagnetically (AFM) coupled skyrmion pairs, compared to ferromagnetic skyrmions, have richer physics and offer technological advantages for spintronic device applications due to their compensation of skyrmion Hall effects and expected higher mobility. Here we report a direct imaging study on the magnetic-field excited motion of the chiral domain walls (DWs) located at the boundary of the AFM-coupled bubble skyrmion pairs using element-specific x-ray photoemission electron microscopy. These AFM-coupled skyrmions were created across the interfaces of the AFM-coupled Co and Gd layers in a sputtered Ta/Pt/[Co/Gd/Pt] 10 /Al multilayer film. With in-situ pulsed magnetic fields of different magnitudes applied slightly off the film plane (<5 degrees), we observed the motion of DWs, causing a decrease or increase in the size of bubble skyrmions. When excited by the fields H>50 Oe, DWs move slightly towards the center of the bubble skyrmions causing skyrmions to shrink. When 50 Oe<H<140 Oe, DWs move outward without showing any preferred direction. However, at fields H>140 Oe, the asymmetric motion of DWs favoring the in-plane field direction was observed, which may be attributed to the different spin reorientations direction within the chiral DWs with respect to the applied field.