Chiral orbital current driven topological Hall effect in Mn3Si2Te6

Chiral orbital current (COC) plays a crucial role in governing the magnetization and transport behaviour in the layered ferrimagnetic nodal-line semiconductor Mn3Si2Te6. Here, we observe that the topological Hall effect (THE), typically attributed to Berry curvature from chiral spin textures, originates from COC, which produces an emergent magnetic field for conduction electrons due to its real-space orbital textures. We find that the THE signal strengthens as we move down from bulk to nanoflakes, but tends to disappear with increasing current, along with the disappearance of the COC state. We also demonstrate a strong correlation between the colossal magnetoresistance (CMR) and the observed THE, suggesting that large Berry curvature and topological transport can arise purely from orbital degrees of freedom, providing a new platform for engineering dissipationless transport in 2D magnets.

💡 Research Summary

In this work the authors investigate the layered ferrimagnetic nodal‑line semiconductor Mn₃Si₂Te₆ (MST) and demonstrate that its topological Hall effect (THE) originates from chiral orbital currents (COC) rather than from conventional spin‑chirality mechanisms. High‑quality single crystals were grown by chemical vapor transport and characterized by X‑ray diffraction, energy‑dispersive spectroscopy, magnetization, and transport measurements. Magnetization data confirm that the a‑b plane is the easy plane (saturation ≈ 1.5 µB per Mn) while the c‑axis is hard, with a Curie temperature near 78 K. Electrical resistivity shows semiconducting behavior, and a colossal magnetoresistance (CMR) appears when the magnetic field is applied along the c‑axis.

Hall measurements at 5 K reveal a sharp low‑field peak superimposed on the ordinary Hall linear background and the anomalous Hall contribution that scales with the magnetization. By fitting the high‑field linear region to extract the ordinary Hall coefficient (R₀) and subtracting the anomalous term (R_S M_s) obtained from magnetization, the residual Hall signal is isolated as a topological Hall component (ρ_THE). This peak persists up to ~70 K in bulk crystals but only up to ~30 K in nanoflake devices, indicating a strong thickness dependence.

A key observation is the pronounced current‑density dependence of the THE. When the current density is increased from ~5 A cm⁻² to >10⁶ A cm⁻², the amplitude of ρ_THE diminishes dramatically and eventually vanishes. The critical current density for disappearance is much lower in nanoflakes, reflecting the fragility of the COC state in reduced dimensions. This behavior mirrors the known suppression of the COC with increasing current reported in earlier studies, where the orbital current flowing along Te edges generates an orbital moment M_COC and a real‑space emergent magnetic field B_c directed along the c‑axis. B_c adds to the external field and contributes to the Hall voltage, while M_COC couples to the Mn spins via spin‑orbit interaction, giving rise to the observed CMR.

To establish a direct link between THE and CMR, the authors compute the magnetoresistance percentage (MR %) and the relative topological Hall conductivity (σ_THE/σ_xy)_max as functions of temperature and current. Both quantities display nearly identical temperature and current dependence in bulk and nanoflake samples: they decrease as temperature rises (due to enhanced spin‑disorder scattering) and as current increases (because the COC is weakened). The simultaneous disappearance of CMR and THE at the same temperature/current thresholds strongly supports a common orbital origin.

Theoretical considerations suggest that each COC loop carries a topological orbital moment (TOM) that interacts with neighboring loops (chiral‑chiral interaction) and with the local Mn spins (spin‑chiral interaction). These interactions generate a finite scalar spin chirality without requiring Dzyaloshinskii‑Moriya interaction or non‑centrosymmetric crystal symmetry. Consequently, the emergent Berry curvature responsible for THE is rooted in the orbital texture rather than in a skyrmion‑like spin texture.

Overall, the study provides compelling experimental evidence that chiral orbital currents can generate an emergent magnetic field and associated Berry curvature, leading to a topological Hall response. The findings broaden the paradigm of topological transport: while most reported THEs rely on spin‑driven chiral textures, here the orbital degree of freedom alone suffices. This opens new avenues for engineering dissipationless transport in two‑dimensional magnets by manipulating orbital currents through thickness control, current biasing, or interface engineering, potentially enabling low‑power spintronic devices that exploit orbital topology.