Professor Thorsten Hesjedal FInstP

Abstract:

The collective precession of magnetization manifests itself as magnon modes. These modes are governed by complex-valued vectorial eigenfunctions, which have remained experimentally challenging to observe. Here we introduce X-ray magnetic vector chronoscopy (XMVC), a time-resolved resonant scattering method that reconstructs the full magnetization dynamics with angular resolution of 0.1° (±0.01°). Applied to a synthetic antiferromagnetic multilayer (Si/NiFe (8 nm)/Ru (0.8 nm)/CoFeB (5.5 nm)), XMVC enables magnon state tomography, by directly measuring the nanoscale vectorial eigenfunctions of hybridized modes arising from magnon–magnon coupling. This approach provides full access to the system’s non-Hermitian Hamiltonian, revealing the complex-valued coupling strengths and non-orthogonal eigenbases. These results establish XMVC as an experimental platform for studying nanoscale spin systems by extracting the eigenfunctions of the system. 

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.