University of Illinois Urbana-Champaign

Neutron scattering studies of intercalated transition metal dichalcogenides

Lu, Kannan

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LU-DISSERTATION-2022.pdf

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https://hdl.handle.net/2142/117709

Description

Title
Neutron scattering studies of intercalated transition metal dichalcogenides
Author(s)
Lu, Kannan
Issue Date
2022-08-18
Director of Research (if dissertation) or Advisor (if thesis)
MacDougall, Gregory
Doctoral Committee Chair(s)
Madhavan, Vidya
Committee Member(s)
  • Wagner, Lucas K
  • Lorenz, Virginia
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
  • intercalated transition metal dichalcogenides
  • neutron scattering
Abstract
The magnetically intercalated transition metal dichalcogenides T$_{1/3}$MX$_{2}$ (T = 3d transition metals; M = \{Nb, Ta\}; X= S) has recently attracted significant interest due to its demonstrated peculiar chiral spin texture, the low-current-density magnetic switching, and the controversial anomalous Hall effect (AHE) in selected compounds of this family. Among other interesting observations, these phenomena not only pave the way for novel spintronics applications but also call for more thorough investigations of their magnetic structures. With this motivation in mind, I have grown single crystals and collected neutron scattering data on several compounds of this family with a particular interest in V$_{1/3}$\{Nb, Ta\}S$_{2}$, Fe$_{1/3}$NbS$_{2}$ and Co$_{1/3}$NbS$_{2}$. In this thesis, I will first discuss the measurements and results of our magnetic structure determination of V$_{1/3}$\{Nb, Ta\}S$_{2}$. Historically identified as ferromagnets, our diffraction results instead reveal that vanadium spins in these compounds are arranged into an A-type antiferromagnetic configuration at low temperatures. Refined moments are 1.37(6)$\mu_{B}$ and 1.50(9)$\mu_{B}$ for V$_{1/3}$TaS$_2$ and V$_{1/3}$NbS$_2$, respectively. Transition temperatures $T_{c}$~$=$~32K for V$_{1/3}$TaS$_{2}$ and 50K for V$_{1/3}$NbS$_{2}$ are obtained from the magnetization and neutron diffraction results. We attribute the small net magnetization observed in the low-temperature phases to a subtle ($\sim$2$^{\circ}$) canting of XY-spins in the out-of-plane direction. These new results are indicative of dominant antiferromagnetic exchange interactions between the vanadium moments in adjacent \textit{ab}-planes, likely eliminating the possibility of identifying stable chiral spin texture in these two materials. In the material Fe$_{1/3}$NbS$_{2}$, magnetic structures with two propagation vectors $\mathbf{k}_{1} = (0.5, 0, 0)$ and $\mathbf{k}_{2} = (0.5, 0.25, 0)$ are observed, consistent with a recent neutron diffraction result. The ordering temperatures of $T_{n, \mathbf{k}_{1}} = 45.64(5)$ K and $T_{n, \mathbf{k}_{2}} = 44.4(2)$ K, indicates a nearly ideal stoichiometry of the sample. The largest major development over previous neutron work is our ability to fully refine the direction of the ordered moments with using data collected at a large volume of reciprocal space, which shows a large deviation from the assumed easy-axis perpendicular to the hexagonal planes. The magnetic structure associated with $\mathbf{k}_{1}$ is a canted antiferromagnet with a cell doubling along a-axis. The magnetic structure associated with $\mathbf{k}_{2}$ is a noncollinear commensurate antiferromagnetic spiral. The in-plane component of the antiferromagnetic orders is compatible with the spin torque transfer mechanism responsible for the magnetic switching behavior. DFT calculations based on our magnetic models is able to reproduce the large Kerr rotation and ellipticity observed from static MOKE measurement below the N\'eel temperature. For the Co$_{1/3}$NbS$_{2}$, the ordered phase is widely believed to be a form of collinear antiferromagnetism which preserves $\mathcal{PT}$ symmetry, and there has been heated discussion about if and how such a state could lead to an AHE or whether the latter should be associated with a weak ferromagnetic moment seen in the same material. In this line of research, we address this controversy by presenting new neutron diffraction data on single crystals of Co$_{1/3}$NbS$_{2}$ and an analysis which suggests that the ordered phase in this material is actually a form of canted antiferromagnetism, which is also seen in Fe$_{1/3}$NbS$_{2}$ associated with the propagation vector $\mathbf{k}_{1} = (0.5, 0, 0)$. We further present new transport and magneto-optic Kerr measurements which demonstrate that the AHE persists in this material below $T_N$ to temperatures as low as T = 5 K. The success of the DFT calculations in describing MOKE in the related material Fe$_{1/3}$NbS$_{2}$ based on this $\mathbf{k}_{1} = (0.5, 0, 0)$ magnetic structure favors the crystal Hall origin for the AHE.
Graduation Semester
2022-12
Type of Resource
Thesis
Handle URL
https://hdl.handle.net/2142/117709
Copyright and License Information
Copyright 2022 Kannan Lu

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