Magnetic phase diagram ofBa3CoSb2O9as determined by ultrasound velocity measurements

Using high-resolution sound velocity measurements we have obtained a very precise magnetic phase diagram of ${\mathrm{Ba}}_{3}{\mathrm{CoSb}}_{2}{\mathrm{O}}_{9}$, a material that is considered to be an archetype of the spin-$\frac{1}{2}$ triangular-lattice antiferromagnet. Results obtained for the field parallel to the basal plane (up to 18 T) show three phase transitions, consistent with predictions based on simple two-dimensional isotropic Heisenberg models and previous experimental investigations. The phase diagram obtained for the field perpendicular to the basal plane clearly reveals an easy-plane character of this compound and, in particular, our measurements show a single first-order phase transition at ${H}_{c1}=12.0$ T which can be attributed to a spin flop between an umbrella-type configuration and a coplanar $V$-type order where spins lie in a plane perpendicular to the $ab$ plane. At low temperatures, softening of the lattice within some of the ordered phases is also observed and may be a result of residual spin fluctuations.