Professor Thorsten Hesjedal FInstP

Abstract:

The synthesis and magnetic properties of the intrinsic magnetic topological insulator MnBi2Te4, grown by magnetron sputtering, are investigated. While this growth method enables smoother morphologies than molecular beam epitaxy and is compatible with scalable processing, the metastable nature of MnBi2Te4 presents considerable challenges in phase control and magnetic uniformity. By systematically varying the relative sputter powers of Mn, Bi2Te3, and Te targets, conditions that favor the formation of near‐stoichiometric MnBi2Te4, as supported by X‐ray diffraction, atomic force microscopy, and energy‐dispersive X‐ray spectroscopy, are identified. These films exhibit reduce surface roughness and lower twin‐domain density compared to Mn‐rich counterparts, which show evidence of phase separation and structural disorder. Magnetometry and X‐ray magnetic circular dichroism reveal that both film types exhibit sizable Mn moments, although signatures of antiferromagnetic order are only weakly expressed and appear sensitive to composition and morphology. Despite producing structurally well‐ordered films, clear linear dichroism attributable to A‐type antiferromagnetic ordering is not observed, suggesting magnetic inhomogeneity or suppression of interlayer coupling. These results highlight the compositional sensitivity of sputtered MnBi2Te4 and underline the difficulties in stabilizing the intrinsic magnetic topological phase in thin‐film form.

Abstract:

This study investigates the intrinsic magnetism and field‐driven spin alignment in NiI2 using X‐ray absorption spectroscopy and X‐ray magnetic circular dichroism (XMCD). NiI2, a van der Waals material, exhibits helimagnetic and type‐II multiferroic behavior. This study reveals robust XMCD signals across paramagnetic, antiferromagnetic, and helimagnetic phases under applied out‐of‐plane fields up to 6 T, while no net moment emerges at zero field. Atomic multiplet calculations confirm a covalent Ni 3d ground state with a significantly reduced spin moment. The results establish the intrinsic nature of NiI2's magnetism and clarify its field‐driven spin alignment mechanism. This comprehensive spectroscopic characterization lays the foundation for future applications of NiI2 in advanced spintronic and multiferroic devices, despite challenges posed by its low transition temperature in the monolayer limit. Future research should focus on enhancing its critical temperature through doping, strain engineering, or heterostructure fabrication.