We report inelastic neutron-scattering measurements of the spin dynamics in the layered hexagonal magnet $2H{\text{-AgNiO}}_2$, which has stacked triangular layers of antiferromagnetically coupled ${\text{Ni}}^{2+}$ spins $\left(S=1\right)$ ordered in a collinear alternating stripe pattern. We observe a broad band of magnetic excitations above a small gap of 1.8 meV and extending up to 7.5 meV, indicating strongly dispersive excitations. The measured dispersions of the boundaries of the powder-averaged spectrum can be quantitatively explained by a linear spin-wave dispersion for triangular layers with antiferromagnetic nearest- and weak next-nearest-neighbor couplings, a strong easy-axis anisotropy, and additional weak interlayer couplings. The resulting dispersion relation has global minima not at magnetic Bragg wave vectors but at symmetry-related soft points and we attribute this anomalous feature to the strong competition between the easy-axis anisotropy and the frustrated antiferromagnetic couplings. We have also calculated the quantum corrections to the dispersion relation to order $1/S$ in spin-wave theory by extending the work of Chubukov and Jolicoeur [Phys. Rev. B 46, 11137 (1992)] and find that the presence of easy-axis anisotropy significantly reduces the quantum renormalizations predicted for the isotropic model.

• Received 5 September 2008

DOI:https://doi.org/10.1103/PhysRevB.79.104421