Commensurate and incommensurate magnetic order in spin-1 chains stacked on the triangular lattice in Li${}_2$NiW${}_2$O${}_8$

Abstract

We report thermodynamic properties, magnetic ground state, and microscopic magnetic model of the spin-1 frustrated antiferromaget Li${}_2$NiW${}_2$O${}_8$ showing successive transitions at $T_{N1}\simeq 18$ K and $T_{N2}\simeq 12.5$ K in zero field. Nuclear magnetic resonance and neutron diffraction reveal collinear and commensurate magnetic order with the propagation vector $\mathbf{k}=(\frac{1}{2},0,\frac{1}{2})$ below $T_{N2}$. The ordered moment of 1.8 $\hat{I}{¼}_B$ at 1.5 K is directed along $[0.89(9),-0.10(5),-0.49(6)]$ and matches the magnetic easy axis of spin-1 Ni${}^{2+}$ ions, which is determined by the scissor-like distortion of the NiO${}_6$ octahedra. Incommensurate magnetic order, presumably of spin-density-wave type, is observed in the region between $T_{N2}$ and $T_{N1}$. Density-functional band-structure calculations put forward a three-dimensional spin lattice with spin-1 chains running along the $[01\bar{1}]$ direction and stacked on a spatially anisotropic triangular lattice in the $ab$ plane. We show that the collinear magnetic order in Li${}_2$NiW${}_2$O${}_8$ is incompatible with the triangular lattice geometry and thus driven by a pronounced easy-axis single-ion anisotropy of Ni${}^{2+}$.

Published version: 12+ pages, 11 figures, 3 tables