Static and dynamic magnetism of topologically non-trivial magnetic materials

• Applicant: Dr. Alexey Alfonsov, Ph.D.
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 499461434

The topology of the electronic bands has shown its importance in the field of condensed matter physics by paving a way to a discovery of new states of matter such as topological insulators (TI). Strong spin-orbit coupling present in this class of materials causes a non-triviality of the bulk band topology, which, in turn, yields a massless Dirac dispersion with spin-momentum locking at the surface. The recent discovery of magnetic topological insulators (MTI) has opened new avenues to explore exotic phenomena, such as the quantum anomalous Hall effect, the topological magnetoelectric effect and chiral Majorana fermions, which can emerge from the interplay between topological electronic states and magnetic degrees of freedom, be they ordered or strongly fluctuating. A very informative method to study the latter is high-field high-frequency electron spin resonance (ESR) spectroscopy complemented with static magnetometry technique. In our project, we propose to use ESR spectroscopy to obtain new comprehensive insights into the fundamental origin and the details of the magnetism in MTIs, and to establish a relation of the magnetic properties such as magnetic anisotropy, interactions and ordering with observed signatures of the non-trivial band topology. We will use the advantages of the high-field high-frequency ESR instrumentation at the IFW Dresden enabling a high-sensitivity detection of ESR signals in a very broad frequency range from 0.01 to 1 THz, in fields up to 16 T, and in a temperature range 0.3 - 300 K. Systematic studies of frequency, magnetic field, and angular dependences of the ESR modes in single-crystalline samples will allow an accurate determination of the magnetic anisotropies, the spin structures in the ground state, and parameters of the spin dynamics. The accurate evaluation of the temperature dependences of the ESR response grants access to the dynamics of the magnetic moments, thereby allowing the investigation of the interplay between electronic structure and fluctuating magnetic moments in the paramagnetic state. We will relate all these parameters with the chemical composition, crystallographic structure, and with the observed features of the non-trivial band topology in the studied MTIs. An important aspect of the project will be studies of exfoliated samples, which will enable the investigation of the thickness dependent evolution of the surface dominated magnetism. The obtained information should be of a paramount importance for a comprehensive understanding of the nature of the static and dynamic magnetism in both, ordered and not ordered, states of magnetic topological insulators, which is necessary for further assessment of the interplay between magnetism and non-trivial band topology.

• DFG Programme: Research Grants