Magnetoelastic coupling and the magnetization plateau in Ba3CoSb2O9

High-precision ultrasonic measurements are used to study magnetoelastic coupling as a function of the in-plane magnetic field orientation in the spin-$1/2$ triangular lattice antiferromagnet ${\mathrm{Ba}}₃{\mathrm{CoSb}}₂{\mathrm{O}}₉$. The relevance of this coupling in stabilizing the 1/3 magnetization plateau is explored. The analysis indicates that, while the magnetoelastic coupling in ${\mathrm{Ba}}₃{\mathrm{CoSb}}₂{\mathrm{O}}₉$ 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 ${\mathrm{Ba}}₃{\mathrm{CoSb}}₂{\mathrm{O}}₉$. 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 drop as the V phase is induced at higher field.