Fig. 3. Picking up two Fe atoms onto a Nb tip and demonstrating out-of-plane magnetic contrast.

(A) Constant-current STM images before (left two images) and after picking up the two indicated Fe atoms (right image) onto the tip (V = 6 mV and I = 2 nA). Scale bar, 2 nm. (B) dI/dV spectra taken on the normal conducting Nb(110) substrate in an external field of B_z = 0.4 T with (red) and without (gray) Fe atoms attached to the tip (V_stab = −10 mV, I_stab = 2 nA, and V_mod = 100 μV for the YSR tip and V_stab = −6 mV, I_stab = 1 nA, and V_mod = 20 μV for the clean Nb tip). (C) Constant-current STM topography (left) and the corresponding dI/dV map (right) of an antiferromagnetic 2a − [001] Mn₅ chain measured at B_z = 0.5 T and stabilized outside of the superconducting gap (V_stab = −10 mV, I_stab = 2 nA, and V_mod = 3 mV). Scale bar, 0.5 nm. (D) dI/dV spectra taken on the substrate and on the five atoms of the chain (V_stab = −10 mV, I_stab = 2 nA, and V_mod = 100 μV) with the YSR tip in an external field of B_z = 0.5 T. The spectra are vertically offset for clarity. (E) Asymmetry $A_{\text{YSR},\text{norm}}({\overrightarrow{\mathbf{r}}}_{\mathbf{t}})$ of the YSR peaks in (D) calculated from Eq. 3 (top) compared to the spin asymmetry $A_{\text{SP}}({\overrightarrow{\mathbf{r}}}_{\mathbf{t}})$ obtained with the Cr tip (bottom) using the data from Fig. 2B and averaging across the atoms and a substrate position. The off-zero dashed gray lines mark the average YSR asymmetry measured on ↑ and ↓ atoms.