title: Synthesis And Magnetotransport Study of Kagome Lattice Magnets
Creators: name: Jones, David Connor
Publisher: name: George Mason University
Year: 2023
Subjects: subject: Physics
subject: Materials
subject: Magnetism
subject: Experimental
subject: Topology
subject: Condensed matter physics

description: This thesis has been embargoed for 2 years. It will not be available until July 2023 at the earliest.
description: The investigation and manipulation of novel magnetic textures within topological quantum materials is emerging as a new frontier for future spin-based electronic devices. Recently, transition-metal based kagome magnets have shown to provide a natural platform to study the interplay between complex magnetism and electronic topology. Of particular interest is the family of RMn6Sn6 (R = rare-earth) compounds with kagome lattices, shown to host complex magnetic textures and topological states, both strongly dependent on the choice of R atom. This master's thesis is an experimental study on role of the rare-earth elements R = Y, Tb on the magnetic and magnetotransport properties within single crystals of RMn6Sn6 compounds using magnetometry and magnetotransport measurements combined with first-principles calculations. The framework of this study largely focuses on the magnetic rare-earth compound TbMn6Sn6 and compares the magnetic and electronic properties to the non-magnetic rare-earth parent compound YMn6Sn6. We first present the methods and review the crystal growth and characterization of single crystals of YMn6Sn6 and TbMn6Sn6. The Sn self-flux method was employed for crystal growth, yielding thick plate-like shaped single crystals of YMn6Sn6 and TbMn6Sn6. Once trimmed to adequate dimensions, magnetic susceptibility and electrical transport measurements were performed. The DC magnetic susceptibility (𝜒) of YMn6Sn6 with the applied field 𝐻 ⊥ 𝑐 and TbMn6Sn6 with 𝐻 ⊥ 𝑐 and 𝐻 ∥ 𝑐 reveal high ordering temperatures of 𝑇! ≈ 345 and 423 K, respectively, as well as distinct features consistent with transformations to their magnetic structure. For TbMn6Sn6, a spin-reorientation transition is observed at 𝑇"# ≈ 308 K where the collinear moments on Tb and Mn reorient along the ab-plane to the c-axis upon decreasing temperature. The electrical resistivity (𝜌$$) of YMn6Sn6 and TbMn6Sn6 with the electrical current perpendicular to the c-axis (𝐼 ⊥ 𝑐) reveal high metallicity for the samples. We next perform comprehensive magnetization measurements on the YMn6Sn6 and TbMn6Sn6 samples in combination with first-principal calculations to describe the microscopic nature of the role of the rare-earth element Tb within the collinear ferrimagnetic (FiM) structure in TbMn6Sn6. By considering a simplified description of the collinear configuration on magnetic anisotropy energy (𝑀𝐴𝐸), by lumping the Heisenberg exchange and single-site anisotropy terms, our analysis describes how the spinreorientation magnetic phase diagram for TbMn6Sn6 is quantitatively described by the temperature dependencies of magnetic moments on the Mn- and Tb-sublattices. An enhanced magnetic state on Tb at low temperatures leads to a strong out-of-plane magnetization which likely enhances the stability of the intrinsic topological Chern gap state previously observed by Yin et. al. [1] within TbMn6Sn6 in the presence of a modest out-of-plane applied magnetic field of 𝜇%𝐻 ≈ 2 T at 4.2 K. Moreover, the significance of the antiferromagnetic (AFM) coupling between the Mn- and Tb-sublattices is revealed through a comparison between the estimated ground state magnetic anisotropy energies of the Mn-sublattices for the magnetic rare-earth TbMn6Sn6 compound [𝑀𝐴𝐸&amp;'(0) ≈ − 0.47 meV per Mn] and the non-magnetic rare-earth compound YMn6Sn6 [𝑀𝐴𝐸&amp;'(0) ≈ − 0.12 meV per Mn]. We then explore to what extent the magnetic state of Tb affects the electronic properties in TbMn6Sn6, especially near the Fermi surface, and therefore the transport properties, such as the anomalous Hall effect (AHE), through magnetotransport measurements on YMn6Sn6 and TbMn6Sn6 with 𝐼 ⊥ 𝑐 and 𝐻 ∥ 𝑐. By comparing the measured magnetoresistance (𝑀𝑅) to that of YMn6Sn6 the role of the magnetic rare-earth Tb on the electronic properties is clearly significant low temperatures. The 𝑀𝑅 for TbMn6Sn6 transitions from negative to large and positive below around 100 K and is likely attributed to the enhanced magnetic state on Tb at low temperatures. The Hall resistivity (𝜌($) with 𝐻 dependence and its calculated anomalous Hall resistivity (𝜌$( )*) with 𝜌$$ + dependence for TbMn6Sn6 provides evidence of a likely dominating extrinsic contribution below around 100 K and a leading intrinsic contribution above 100 K that is likely not generated by a field-induced topological Chern gap. At the end, we summarize the key findings of this study and outline future work to expand the scope of this study. Ultimately, the goal following this project would be to construct a comprehensive understanding of the role of the rare-earth elements among the nine available RMn6Sn6 compounds to simultaneously engineer desirable magnetic and topological states potentially valuable for future spin-based electronic devices.