Large topological Hall effect in epitaxial NiFeMo/NiFeMoO films with low-temperature exchange bias

Abstract

We demonstrate exchange bias-driven large topological Hall effect in epitaxial NiFeMo/NiFeMoO heterostructures fabricated on Al2O3 (0001) substrates. Structural analyses confirm high-quality epitaxial growth with distinct strain states in single-layer NiFeMo and bilayer NiFeMo/NiFeMoO thin films. Magnetometry reveals that NiFeMo remains ferromagnetic over the entire temperature range, whereas NiFeMoO undergoes a suggested paramagnetic-to-antiferromagnetic transition near 175 K, inducing robust exchange bias in field cooled NiFeMo/NiFeMoO heterostructures. Magnetic force microscopy, supported by micromagnetic simulations incorporating Dzyaloshinskii–Moriya interactions, uncovers non-coplanar spin textures at room temperature under moderate magnetic fields. Magnetotransport studies identify a pronounced topological Hall-like signal, strongly enhanced by exchange bias at low temperatures. These results establish that epitaxial strain and interfacial exchange coupling act synergistically to stabilize non-coplanar spin textures and tailor the topological Hall response from 10 to 300 K, offering a promising pathway for oxide/metal interface-based topological spintronic applications.