Exchange Bias–Induced Giant Topological Hall Effect in Epitaxial NiFeMo/NiFeMoO Heterostructures

We report the realization of a large exchange bias–enhanced topological Hall effect in epitaxial NiFeMo/NiFeMoO heterostructures grown on c-cut Al₂O₃ (0001) substrates. Comprehensive structural characterization confirms coherent epitaxial growth with distinct strain relaxation behavior in single-layer NiFeMo and bilayer NiFeMo/NiFeMoO films. Magnetic measurements reveal that the metallic NiFeMo layer retains ferromagnetic ordering across the full temperature range, while the oxidized NiFeMoO layer exhibits a transition from paramagnetic to antiferromagnetic order near 175 K. This magnetic phase transition gives rise to a strong exchange bias in field-cooled heterostructures. Magnetic force microscopy, corroborated by micromagnetic simulations incorporating Dzyaloshinskii–Moriya interactions, demonstrates the formation of nanoscale skyrmion-like spin textures under moderate magnetic fields at room temperature. Magnetotransport experiments reveal a prominent topological Hall signal attributed to Berry curvature effects, which becomes significantly amplified in the presence of exchange bias at low temperatures. These findings highlight the cooperative role of interfacial exchange coupling and epitaxial strain in stabilizing nontrivial spin configurations and tuning the topological Hall response from 10 K to 300 K, underscoring the potential of oxide–metal interfaces for next-generation topological spintronic devices.