Probing local structure, intercalant disorder, and magnetism in Fe_xNbSe₂

Magnetically intercalated transition metal dichalcogenides (TMDs) represent a promising class of emergent quantum materials for ultralow-power applications based on the manipulation of electron spin. In particular, iron-intercalated niobium-based dichalcogenides (i.e. Fe_xNbS₂ and Fe_xNbSe₂) pose as exciting materials platforms for antiferromagnetic spintronics, yet insight into how the intercalant distribution varies across compositions remains unclear. Here, we investigate the interplay between composition, local structure, and the onset of intercalant superlattice formation in Fe_xNbSe₂ (x ≤ 0.25) using energy-dispersive X-ray spectroscopy, confocal Raman spectroscopy and x-ray diffraction. We observe the emergence of new Raman-active phonon modes with increasing iron intercalation and, in particular, prominent ultralow-frequency modes as x approaches 0.25, attributed to iron superlattice formation. Moreover, we probe the symmetries and temperature-dependent behavior of these new modes via polarization- and temperature-dependent Raman measurements and find an anomalous temperature response for one of the associated superlattice modes. Furthermore, we track the evolution of the Néel transition across iron concentrations and find a maximum T_N ~ 130K as x approaches 0.25, signaling the importance of long-range intercalant superstructure on the robustness of magnetic ordering.