The magnetic anisotropy of a three-dimensional honeycomb iridate
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One of the most sought-after, yet elusive ground states in condensed matter physics is the quantum spin liquid. The spin liquid is a strongly interacting spin system with exchange frustration and quantum fluctuations that prevent it from ordering magnetically as usual. Despite the theoretical introduction of this intriguing state of matter in the 1970's, it has lacked experimental backing until insights by Kitaev flourished in 2006. By considering a model honeycomb lattice with directionally dependent exchange anisotropy, he showed that a spin liquid ground state emerges as the limiting case of purely anisotropic exchange interactions. The design and availability of new strongly correlated materials has since lead to a persuasive experimental advance towards realization of this new state of matter, with the results presented herein at the forefront. In this thesis, one of the most promising spin liquid candidates, Li₂IrO₃, is studied in magnetic fields up to 100 tesla. A highly-sensitive torque magnetometry technique is developed to directly probe the magnetic anisotropy of Li₂IrO₃ in both pulsed and DC magnets. The torque measurements confirm a transition to a complex magnetic order at T = 38 K, in agreement with susceptibility data. At low magnetic fields, the ratio of the torque to the applied magnetic field has a linear response with an angle dependence that yields the principle components of magnetic anisotropy. At temperatures greater than ~150 K, the observed magnetic anisotropy can be described by a g-factor anisotropy that is constrained by the crystal structure, with the easy axis along the crystallographic c direction. At ~ 75 K, the torque reveals a crossover to an exchange-dominated anisotropy, highlighted by an order of magnitude increase in the b component of susceptibility at temperatures within the magnetically ordered state. The temperature evolution of the magnetic anisotropy suggests that the extreme magnetic anisotropy observed at low temperatures must be driven by spin-anisotropy in the exchange interactions, as opposed to spatial anisotropy. This thorough investigation of the anisotropic susceptibility provided the first experimental evidence for highly spin-anisotropic exchange interactions to exist in Li₂IrO₃ -- the essential ingredient for realizing Kitaev's quantum spin liquid ground state.
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