Topological Magnetism Group

Abstract:

<jats:title>Abstract</jats:title> <jats:p> Chiral soliton lattices (CSLs) emerge from the competition between Dzyaloshinskii–Moriya interaction, anisotropy, and magnetic fields. While well established in monoaxial helimagnets, their role in materials with anisotropic, direction-dependent chirality remains poorly understood. Here, we report the direct observation of a crossover from π to 2π soliton lattices in the non-centrosymmetric Heusler compound Mn <jats:sub>1.4</jats:sub> PtSn. Combining Lorentz transmission electron microscopy, resonant elastic X-ray scattering, and micromagnetic simulations, we identify a π-CSL as the magnetic ground state—rather than the expected spiral phase—which evolves into a classical 2π-CSL under increasing out-of-plane fields. This transition is governed by an interplay between uniaxial magnetocrystalline anisotropy and magnetostatic interactions, qualitatively captured by a double sine-Gordon model. Our framework extends to materials with <jats:italic>D</jats:italic> <jats:sub> 2 <jats:italic>d</jats:italic> </jats:sub> , <jats:italic>S</jats:italic> <jats:sub>4</jats:sub> , <jats:italic>C</jats:italic> <jats:sub> <jats:italic>n</jats:italic> <jats:italic>v</jats:italic> </jats:sub> , or <jats:italic>C</jats:italic> <jats:sub> <jats:italic>n</jats:italic> </jats:sub> symmetries in the thin-film limit, providing a unifying route to engineer magnetic phase diagrams in chiral systems with implications for soliton-based spintronics and topological transport. </jats:p>

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.