Fig. 2: Mapping the strength of vortex core pinning.
From: Probing the pinning strength of magnetic vortex cores with sub-nanometer resolution
a Measurement scheme: B∥ is stepped equidistantly (B1 to B5) to move the vortex core from r1 to r5, while dI/dV is recorded at fixed tip position. The resulting dI/dV(B∥) displays the core shape in case of a constant core displacement rate. b STM topography overlaid with vortex core center positions at two different B⊥ for the B∥ highlighted by grey dashed lines in d, e. The positions are deduced from dI/dV images at the corresponding B∥. Dotted lines connect start and end point illustrating the target paths. The grayscale colormap represents local height values from −25 pm (black) to 25 pm (white). c–e dI/dV recorded at the tip position marked by “B” in Fig. 1e for c, “A” in b for d and “B” in b for e while sweeping B∥ at B⊥ = 0 T, −1.2 T, and −1.5 T, respectively. The B∥ sweep moves the vortex core from “A” to “C” in Fig. 1e, as well as from leftmost to rightmost square or circle in b, respectively. The real space dI/dV profiles recorded along the dashed lines in Fig. 1b, 1c, and 1d, respectively, are plotted in gray. The upper and lower axis are linked by the measured average displacement rate, i.e., (rC − rA)/(B∥,C − B∥,A) for c and, respectively, for d, e. f–h Deduced core positions from c–e assuming a rigid vortex core profile and a straight path (symbols). Solid black lines are micromagnetically simulated core positions for an Fe cylinder (diameter: 280 nm, thickness: 10 nm) with a single pinning site exhibiting Aex = 0 for a volume of 1.1 × 1.1 × 0.5 nm3 at the surface center. Dotted gray lines show simulated displacement without pinning center. The violet line in g marks the displacement at rate χpinned.
