Professor Thorsten Hesjedal FInstP

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.

Abstract:

Berry curvature and spin texture are representative tuning parameters that govern spin-orbit coupling-related physics and are also the foundation for future device applications. Here, we investigate the impact of the Sb-to-Bi ratio on shaping the electronic band structure and its correlated first- and second-harmonic magneto-transport signals in the intrinsic magnetic topological insulator Mn(Bi_1-xSb_x)₂Te₄. First-principles calculations reveal that the introduction of Sb not only triggers a topological phase transition but also changes the integral of the Berry curvature at the shifted Fermi level, which leads to the reversal of the anomalous Hall resistance polarity for Sb fractions x > 0.67. Moreover, it also induces the opposite spin splitting of the valence bands compared to the Sb-free host, and the resulting clockwise/counterclockwise spin chirality gives rise to a tunable unidirectional second-harmonic anomalous Hall response. Our findings pave the way for constructing chiral spin-orbitronic devices through band structure engineering.

Abstract:

The physical properties of magnetic topological materials are strongly influenced by their nontrivial band topology coupled with the magnetic structure. Co₃Sn₂S₂ is a ferromagnetic kagome Weyl semimetal displaying giant intrinsic anomalous Hall effect which can be further tuned via elemental doping, such as Ni substitution for Co. Despite significant interest, the exact valency of Co and the magnetic order of the Ni dopants remained unclear. Here, we report a study of Ni-doped Co₃Sn₂S₂ single crystals using synchrotron-based X-ray magnetic circular dichroism (XMCD), X-ray photoelectron emission microscopy (XPEEM), and hard/soft X-ray photoemission spectroscopy (XPS) techniques. We confirm the presence of spin-dominated magnetism from Co in the host material, and also the establishment of ferromagnetic order from the Ni dopant. The oxygen-free photoemission spectrum of the Co 2p core levels in the crystal well resembles that of a metallic Co film, indicating a Co⁰⁺ valency. Surprisingly, we find the electron filling in the Co 3d state can reach 8.7-9.0 electrons in these single crystals. Our results highlight the importance of element-specific X-ray spectroscopy in understanding the electronic and magnetic properties that are fundamental to a heavily studied Weyl semimetal, which could aid in developing future spintronic applications based on magnetic topological materials.

Abstract:

We present a detailed x-ray magnetic circular dichroism (XMCD) study of the magnetic properties of Gd-doped EuO thin films, synthesized via molecular-beam epitaxy with Gd doping levels up to over 12%. The impact of Gd doping on the electronic and magnetic behavior of EuO is studied using XMCD and magnetometry. Gd doping significantly enhances the Curie temperature (𝑇C) from 69 K in undoped EuO to over 120 K, driven by increased carrier density, while preserving the high quality of the single-crystalline films. At higher doping levels, a plateau in 𝑇C is observed, which is attributed to the formation of Eu-Gd nearest-neighbor pairs that limit dopant activation. We also observe a distinctive “double-dome” structure in the temperature-dependent magnetization, which we attribute to both the ferromagnetic ordering of Eu 4⁢𝑓 moments at lower temperatures and the influence of conduction electrons via 4⁢𝑓−5⁢𝑑 exchange interactions at higher temperatures. These findings provide key insights into the mechanisms of carrier-induced magnetic transitions.