Static and dynamic properties of the quasi-two-dimensional antiferromagnet ${\mathrm{K}}_2{\mathrm{V}}_3{\mathrm{O}}_8$ have been investigated by ${}^{51}\mathrm{V}$-NMR experiments on nonmagnetic ${\mathrm{V}}^{5+}$ sites. Above the structural transition temperature $T_S=115$ K, NMR spectra are fully compatible with the $P4bm$ space-group symmetry. The formation of superstructure below $T_S$ causes splitting of the NMR lines, which get broadened at lower temperatures so that individual peaks are not well resolved. Evolution of NMR spectra with magnetic field along the $c$ axis below the magnetic transition temperature $T_N\sim 4\mathrm{K}$ is qualitatively consistent with a simple Néel order and a spin-flop transition. However, a broad feature of the spectra does not rule out possible incommensurate spin structure. The spin-lattice relaxation rate $1/T_1$ below $T_N$ shows huge enhancement for a certain range of magnetic field, which is independent of temperature and attributed to cross relaxation due to anomalously large nuclear spin-spin coupling between ${\mathrm{V}}^{5+}$ and magnetic ${\mathrm{V}}^{4+}$ sites. The results indicate strong gapless spin fluctuations, which could arise from incommensurate orders or complex spin textures.

• Received 1 April 2019

DOI:https://doi.org/10.1103/PhysRevB.100.054406