Fig. 2: Magnetic nature of the impurity bound state.

a Dependence of the impurity state with Ni tip under a weak magnetic field. We apply +0.5 T, + 0.1 T, −0.1 T, −0.5 T, −0.1 T, + 0.1 T fields to systematically flip the magnetization of the tip and sample. The inset schematics illustrate the respective spins of the tip and the impurity state that is anti-aligned with bulk magnetization direction. b Dependence of the impurity state with a strong magnetic field. Under both +8 T and −8T, the peak exhibits a magnetization-polarized Zeeman energy shift of 2.2 meV, which amounts to an effective moment of −5µ_B. The inset data shows the energy shift for different magnetic field magnitudes. Inset schematic illustrates the magnetization-polarized Zeeman effect. The applied field aligns the spins of the impurity state in the same orientation, hence +z and -z orientation fields lead the energy to shift in the same direction.