Geometrical and Kitaev Frustration in Spin-Orbit Coupled Magnets
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Date
2023-03-30
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Johns Hopkins University
Abstract
In materials with significant interactions between magnetic spins, the reduction of temperature below the scale of the magnetic interaction of the spins $J\approx k_BT$ generally results in the formation of static spin textures. Frustration between interactions can result in either the significant reduction of the ordering temperature or the formation of an exotic and long sought after spin-liquid state. The origin of the frustration originates from the arrangement of spins on lattices with geometric frustration, such as the triangular and kagome lattices, where no ordered state can satisfy all interactions. Exchange anisotropy induced by strong spin-orbit coupling has emerged as another route towards frustration in so-called Kitaev materials.
In this dissertation, the low energy properties of five different strongly spin-orbit coupled frustrated magnets are investigated experimentally. Through a variety of measurements and theoretical studies, we achieve an understanding of the unusual forms of magnetism in Ba$_4$Nb(Ru,Ir)$_3$O$_{12}$, $\beta$-Li$_2$IrO$_3$, D$_3$LiIr$_2$O$_6$, and BaCo$_2$(AsO$_4$)$_2$, all of which are investigated for potential spin-liquid behaviour. Ba$_4$NbRu$_3$O$_{12}$ and Ba$_4$NbIr$_3$O$_{12}$ are "traditional" quantum spin-liquid candidates in that the origin of their frustrated interactions lies in the triangular lattice upon which (Ru,Ir)$_3$O$_{12}$ J$_{eff}=\frac{1}{2}$ trimers sit. Neutron scattering measurements and bulk characterization are used to characterize the magnetic ground states and excitations in each material.
The lithium iridates $\beta$-Li$_2$IrO$_3$ and D$_3$LiIr$_2$O$_6$ are Kitaev materials in which strong ferromagnetic bond-dependent interactions are the result of the strong spin-orbit coupled Ir$^{4+}$ ions in octahedral crystal fields on a honeycomb lattice. Isotope enriched powder samples are examined by inelastic neutron scattering, and both materials show evidence of Kitaev interactions. D$_3$LiIr$_2$O$_6$ in particular does not show any evidence of magnetic order, and polarized inelastic neutron scattering combined with THz spectroscopy reveal low energy excitations suggestive of a Kitaev spin-liquid ground state.
The final part of the thesis describes extensive work on BaCo$_2$(AsO$_4$)$_2$, a material recently examined for its potential realization of Kitaev physics. Detailed analysis of single crystal inelastic neutron spectroscopy in an in-plane field is performed by the application of linear spin-wave theory and molecular dynamics simulations, and we rigorously show that BCAO is not a Kitaev material but instead is driven by competing interactions between first and third neighbors with an XXZ type planar anisotropy.
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Condensed Matter, Neutron Scattering, Magnetism