Professor Thorsten Hesjedal FInstP

Abstract:

Topological insulators become functional magnetic quantum materials once time-reversal symmetry is broken, enabling phenomena such as the quantum anomalous Hall effect and related chiral transport states. Over the past decade, we have pursued a systematic program to understand how magnetic order can be introduced, controlled, and quantitatively characterized in topological materials grown by molecular beam epitaxy. This contribution reviews our work on magnetic doping and magnetic proximity effects in (Bi,Sb)2 Te3based systems, with a particular emphasis on depth-resolved and local probes of magnetism. Using a combination of polarized neutron reflectometry, muon spin spectroscopy, and element-specific x-ray techniques, we have established where magnetic order resides, how homogeneous it is, and how it couples across interfaces. We show that magnetic doping often leads to intrinsically inhomogeneous magnetic states, while carefully engineered heterostructures can imprint or enhance magnetism in a controlled manner. Recent results on $\text{CrTe}_{2} / \text{Bi}_{2} \text{Te}_{3}$ heterostructures provide direct evidence for proximity-induced magnetism in a topological insulator without chemical doping. Together, these studies demonstrate how neutrons and muons provide essential insight into magnetic topological materials and guide the design of platforms for quantum and spintronic devices.

Abstract:

Magnetic/superconducting heterostructures represent a frontier in condensed matter physics, offering pathways to realize unconventional pairing mechanisms such as topological superconductivity, spin-triplet pairing, and Majorana zero modes for fault-tolerant quantum computing. In this work, we integrate the magnetic van der Waals material MnBi₂Te₄ (MBT) with a superconducting NbN thin film, achieving ultralow-disorder interfaces through Ti buffer layer engineering. Temperature- and field-dependent critical currents, extracted from differential resistance spectra, reveal robust coupling between the MnBi₂Te₄ and the superconducting order of NbN, enabling proximity-induced superconductivity within MnBi₂Te₄. Notably, the proximity-induced critical currents remain invariant under in-plane field rotation, in contrast to the anisotropic response observed in pristine NbN. Moreover, the hysteretic behavior observed in the interfacial magnetoresistance curves confirms the proximity-induced spin polarization at the MBT interface, which is consistent with Andreev reflection results. These findings demonstrate a platform for fabricating high-quality heterointerfaces and enable targeted exploration of exotic quantum states.

Abstract:

Full size CCD camera videos for https://www.nature.com/articles/s41467-018-04563-4

Abstract:

Non-collinear spin textures, such as spin spirals and skyrmions, exhibit rich emergent physics in their spin dynamics. Nevertheless, the potential to utilize their distinctive spin resonance characteristics for on-chip microwave magnonic applications is rarely explored. Here we demonstrate microwave emission and mode coupling from the resonating spin spiral lattice in a Cu₂OSeO₃/Pt/NiFe heterostructure. We use time-resolved resonant elastic X-ray scattering to visualize the exact vectorial spin precession modes from the two magnetic species in real time. Our results show that the ferromagnetic NiFe layer dynamically captures the excitation modes of the conical order in helimagnet Cu₂OSeO₃. The off-resonance NiFe spin precession is phase locked to the helimagnet with a fixed offset, thereby presenting distinct chiral dynamics. This demonstrates that the magnons produced in the process—referred to as helimagnons—can wirelessly transmit spin information at gigahertz frequencies, opening new avenues for on-chip microwave magnonics.