„Magnetism of topological materials: insights from bulk and local probes”

Datum

31.01.2025

Zeit

14:15 - 15:15

Sprecher

Manaswini Sahoo

Zugehörigkeit

IFW Dresden

Sprache

en

Hauptthema

Materialien

Host

Beschreibung

Magnetism of topological materials: insights from bulk and local probes Magnetism in 2D materials promises many applications in future technologies; simul¬taneously, robust topological materials have opened a new path in the quantum world in recent decades. Then combining these two, the interplay between nontrivial topology and magnetism offers numerous applications in spin-based technologies, driven by innovative quantum phenomena such as the quantum anomalous Hall effect (QAHE) and axion insulators. These phenomena are manifested in materials like magnetic Weyl semimetals, magnetic topological insulators, skyrmions, or similar spin-textured materials, etc. This thesis mainly deals with the bulk and local magnetic properties of 2D/quasi-2D van der Waals materials which also host topologically non-trivial states. Importantly, it focuses on understanding the fundamental magnetic properties of these materials and on shedding light on the impact of the crystalline disorder on local and bulk magnetic properties. The thesis has been divided into two parts based on the materials. In the first part, the MnBi2 Te4 family of compounds holds promise for realizing the QAHE; however, achieving full quantization in antiferromagnetic MnBi2Te4 still requires an external magnetic field. Therefore, increasing the interlayer distance be¬tween the septuple layers of MnBi2 Te4 and substituting Bi with Sb are considered as viable strategies to achieve net spin polarization without a magnetic field. These intrinsic magnetic topological insulator candidates (MnBi2 Te4)(Bi2 Te3)n, n=O, 1,2 and MnSb2 Te4 are thoroughly investigated in the fourth and fifth chapters. The fourth chapter primarily focuses on the bulk magnetic characterization of the MnSb2 Te4 sin¬gle crystalline samples, aiming to verify the presence of a ferromagnetic ground state and explore different routes to enhance the magnetic transition temperature that is crucial for achieving QAHE at an elevated temperature. The major finding of this chapter is that Mn over-stoichiometry, combined with a specific Mn/Sb intermixing pattern and the resulting increasing three-dimensional nature of the magnetic order, drives the fer¬romagnetic transition temperature upwards. Further shifting the attention to the local magnetic ordering of Mn atoms in all the compounds, investigating the (Mn/Bi, Sb) site intermixing inherently present due to the similar ionic radii of the atoms. Powerful local techniques, such as nuclear magnetic resonance (NMR) and muon spin relaxation (µSR), are employed to identify these native defects and further confirm that they also undergo an order-disorder transition well below the magnetic ordering temperature. This comprehensive investigation provides a microscopic understanding of the crucial role played by intermixing and suggests pathways for optimizing the magnetic gap in its surface states necessary for the observation of the QAHE. In the second part of this thesis, the chiral soliton lattice candidates Cr113NbS2 and its analogous compound Mn113NbS2 are studied through NMR measurements in the sixth chapter. While topologically nontrivial spin textures have already been observed in Cr113NbS2 via Lorentz TEM, distinguishing between trivial domains and nontrivial periodic spin textures in Mn113NbS2 has proven to be challenging. In this thesis, NMR investigations of 53Cr and 55Mn nuclei, supported by theoretical calculations, provide an in-depth analysis of both compounds. The Cr113NbS2 compound exhibits all the signatures of a monoaxial chiral helimagnet (CHM) in a magnetic field. This detailed understanding allows for a comparison with Mn1;3NbS2 which, despite a much higher density of specific defects in its single crystal, demonstrates the presence of a predom¬inant CHM phase, characterized by significantly higher upper critical magnetic fields for the phase transition to a forced ferromagnetic state.

Links

• Seminars at the IFW

Letztmalig verändert: 31.01.2025, 07:37:23