Distinct Magnetic Response to Isoelectronic Substitution in (Fe_1-δ)₃GeTe₂: A Mössbauer Spectroscopy Study

Isoelectronic substitution is a powerful tool for disentangling charge-carrier and lattice-driven effects in van der Waals (vdW) magnets, yet its microscopic impact is still poorly explored. Here, we systematically investigate the effect of isoelectronic Ni and Co substitution on the magnetic properties of (Fe_1-δ)₃GeTe₂ single crystals using a combination of macroscopic magnetometry and microscopic ⁵⁷Fe Mössbauer spectroscopy. Although substituting Fe with specific concentrations of Ni and Co was designed to introduce the same total number of excess 3d electrons, these substituents induce markedly different magnetic responses, with Co leading to a more pronounced suppression of the Curie temperature and saturation magnetization. Mössbauer analysis reveals that these differences originate from dopant-specific site occupancy and the resulting local modification of Fe magnetic exchange pathways. In particular, Co preferentially substitutes at the (Fe1)_A and Fe2 sites, whereas Ni exhibits partial interstitial incorporation, introducing additional structural disorder. These local effects alter hyperfine interactions, suppress Fe-Fe exchange, and promote magnetic moment localization. Our findings highlight the critical role of dopant identity beyond electron count, emphasizing that local atomic environments govern the tuning of magnetic order in vdW ferromagnets. This work provides microscopic insight into substitution-driven magnetism and guides future strategies for tailoring spintronic functionalities in two-dimensional materials.