The spin-$\frac{1}{2}$ kagome antiferromagnet is an archetypal frustrated system predicted to host a variety of exotic magnetic states. We show using neutron scattering measurements that deuterated vesignieite ${\mathrm{BaCu}}_3{\mathrm{V}}_2{\mathrm{O}}_8(\mathrm{OD}{)}_2$, a fully stoichiometric $S=1/2$ kagome magnet with $<1\%$ lattice distortion, orders magnetically at $T_N=9\mathrm{K}$ into a multi-$\mathbf{k}$ coplanar variant of the predicted triple-$\mathbf{k}$ octahedral structure. We find that this structure is stabilized by a dominant antiferromagnetic third-neighbor exchange $J_3$ with minor first- or second-neighbor exchanges. The spin-wave spectrum is well described by a $J_3$-only model including a tiny symmetric exchange anisotropy.

• Received 1 June 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.107203