Topological and Planar Hall Effect in Monoclinic van der Waals Ferromagnet NbFeTe$_2$

Two-dimensional (2D) van der Waals (vdW) ferromagnets have emerged as a critical class of quantum materials for next-generation, low-dimensional spintronic devices. In this study, we report a comprehensive study of the transport properties of the layered soft ferromagnet $\text{NbFeTe}_2$. We report the first observation of the topological Hall effect (THE) and the planar Hall effect (PHE) in metallic $\text{NbFeTe}_2$. THE signatures persist up to 45 K, while PHE remains evident well above Curie temperature ($T_C$). The observed negative longitudinal magnetoresistance, along with the PHE, provides strong evidence for a nontrivial electronic band structure. The coexistence of perpendicular magnetic anisotropy and a substantial THE: two key properties that are highly desirable for future spintronics applications, makes monoclinic vdW ferromagnetic $\text{NbFeTe}_2$ a promising platform to advance spintronics applications.

💡 Research Summary

In this work the authors present a comprehensive investigation of the structural, magnetic, and transport properties of monoclinic NbFeTe₂ single crystals grown by chemical vapor transport. X‑ray diffraction and high‑resolution transmission electron microscopy confirm the high crystalline quality of the P2₁/c phase, while energy‑dispersive X‑ray spectroscopy verifies the stoichiometry (Fe : Nb : Te ≈ 1.00 : 0.98 : 2.02). Magnetization measurements reveal a ferromagnetic transition near 78 K with the crystallographic a‑axis as the easy axis. Zero‑field‑cooled and field‑cooled curves show a pronounced irreversibility below the transition, and an additional drop in magnetization around 40 K suggests the possible coexistence of a low‑temperature spin‑glass‑like component. AC susceptibility peaks are frequency‑independent, further supporting a conventional ferromagnetic ground state.

Electrical resistivity is metallic over the entire 2–300 K range. Detailed analysis shows that the temperature dependence can be described by a sum of linear (electron‑phonon) and quadratic (electron‑magnon or electron‑electron) terms, with distinct coefficients in the low‑temperature (< 23 K) and intermediate (23–80 K) regimes. Application of a 3 T magnetic field enhances the linear term, indicating that electron‑magnon scattering dominates.

Magnetoresistance measurements performed with the magnetic field parallel (longitudinal MR, LMR) and perpendicular (transverse MR, TMR) to the current direction reveal a predominantly negative MR from 5 K up to 120 K. The magnitude reaches about –6 % at 8.5 T near 80 K. Notably, LMR is consistently larger in absolute value than TMR at low temperatures, hinting at additional contributions beyond simple ferromagnetic suppression of spin‑disorder scattering, such as a possible chiral‑anomaly‑related charge pumping associated with Weyl nodes.

Hall effect studies uncover two unconventional components. First, a topological Hall effect (THE) appears below ~45 K, superimposed on the ordinary and anomalous Hall signals. The presence of THE is a hallmark of non‑coplanar spin textures (e.g., skyrmions) that generate a real‑space Berry curvature and act as emergent magnetic fields on charge carriers. Second, a robust planar Hall effect (PHE) is observed well above the Curie temperature, persisting up to at least 100 K. The PHE indicates strong anisotropic scattering linked to spin‑orbit coupling and an underlying non‑trivial band topology, possibly involving Dirac‑ or Weyl‑type dispersion in the two‑dimensional layers.

Collectively, NbFeTe₂ exhibits three key attributes desirable for spin‑tronic applications: (i) perpendicular magnetic anisotropy with a clear easy axis, (ii) a sizable topological Hall response indicating emergent spin textures, and (iii) a planar Hall effect that survives in the paramagnetic regime, reflecting persistent spin‑orbit‑driven anisotropy. These findings position monoclinic NbFeTe₂ as a promising platform for low‑dimensional, topological spin‑tronic devices. Future work focusing on exfoliation to few‑layer thicknesses, electric‑field gating, and strain engineering could enable deterministic control of the skyrmion‑like textures and the associated Berry‑phase phenomena, paving the way toward practical topological spin‑logic and memory technologies.