Direct Visualization of the Impurity Occupancy Road Map in Ni-Substituted van der Waals Ferromagnet Fe₃GaTe₂

Impurity substitution is effective for studying the intrinsic properties of a quantum material. When the target element has multiple Wyckoff positions, it is challenging but essential to know the exact position and occupancy order of the impurity atoms. Via comprehensive experimental and theoretical investigations, we establish the Ni substitution road map in van der Waals ferromagnet Fe₃GaTe₂. The results unambiguously reveal that in (Fe_1-xNi_x)₃GaTe₂, Ni atoms initially form interlayer gap Ni3 sites when x < 0.1 and then gradually occupy Fe2 sites. When x > 0.75, they start to substitute for Fe1 sites and eventually realize full occupation. Accordingly, T_C and saturation moments both show nonlinear decreases tied to the different roles of Ni3, Fe1, and Fe2 sites in the spin Hamiltonian. The results not only yield fruitful insights into the roles of different Fe sites in Fe₃GaTe₂ but also set a paradigm for the future study of impurity substitution on other quantum materials.