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

TL;DR

This study investigates the magnetic phases of Li$_2$NiW$_2$O$_8$, revealing successive magnetic transitions, collinear and incommensurate orders, and the influence of anisotropy on magnetic structure in a frustrated spin-1 chain system.

Contribution

It provides the first detailed microscopic magnetic model of Li$_2$NiW$_2$O$_8$, highlighting the role of single-ion anisotropy in determining magnetic order on a frustrated lattice.

Findings

Successive magnetic transitions at 18 K and 12.5 K.

Collinear and commensurate magnetic order below 12.5 K.

Incommensurate order between 12.5 K and 18 K.

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 $T_{\rm N1}\simeq 18$ K and $T_{\rm N2}\simeq 12.5$ K in zero field. Nuclear magnetic resonance and neutron diffraction reveal collinear and commensurate magnetic order with the propagation vector $\mathbf k=(\frac12,0,\frac12)$ below $T_{\rm N2}$. The ordered moment of 1.8 $\mu_B$ 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 $T_{\rm N2}$ and $T_{\rm N1}$. 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$^{2+}$.