Two-layer Heisenberg model of quasi-2D triangular lattice antiferromagnet and study of the magnetoelastic coupling in Ba₃CoSb₂O₉ using sound velocity measurements

Li, Ming (2020) Two-layer Heisenberg model of quasi-2D triangular lattice antiferromagnet and study of the magnetoelastic coupling in Ba₃CoSb₂O₉ using sound velocity measurements. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The magnetic field evolution of ground spin states of the stacked planar triangular antiferromagnet with antiferromagnetic interlayer interaction Jc is explored using a minimal 3D classical Heisenberg model (published in Ref. [1]). A bi-quadratic coupling is also used to mimic the effect of spin fluctuations [2] which are known to stabilize the magnetization plateau. A single ion anisotropy is included and states with a magnetic field applied in the ab-plane and along the c-axis are determined. For H || ab-plane, an additional state, in contrast to the 2D model [2], is obtained with weak interlayer interaction. Meanwhile the magnetization plateau decreases with the increment of Jc and vanishes at medium values of Jc. Moreover, two new states with a small z components of spins emerge with large Jc. For H || c-axis, an extra state, compared with the 2D model, is obtained with a weak interlayer interaction. When Jc is large enough, only the state corresponding to the Umbrella phase in the 2D model exists. High-resolution ultrasonic measurements are used to study magnetoelastic coupling as a function of the inplane magnetic field orientation in the spin-1/2 triangular lattice antiferromagnet Ba₃CoSb₂O₉ (published in Ref. [3]). Via these measurements, the relevance of this coupling in stabilizing the 1/3 magnetization plateau (up-up-down state) is explored. The analysis indicates that, while the magnetoelastic coupling in Ba₃CoSb₂O₉ is large, in comparison to other triangular lattice antiferromagnets, the strength of this coupling is still too small to fully account for the magnetization plateau width in Ba₃CoSb₂O₉. Spin fluctuations are therefore the dominant mechanism inducing and stabilizing the magnetization plateau. Our results also show that the amplitude of the spin fluctuations suddenly drops as the V phase is induced at higher field. Furthermore, as the temperature approaches the uud phase boundary from the paramagnetic state, the short range spin correlation, responsible for the softening of the acoustic modes, are also observed. Comparing the experimental results in ordered states at different temperatures, our results indicate that the effect of the thermal fluctuations on the magnetoelastic coupling are negligible at low temperatures in comparison to that of the quantum fluctuations.

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