Polarized-neutron investigation of magnetic ordering and spin dynamics in BaCo₂(AsO₄)₂ frustrated honeycomb-lattice magnet

The magnetic properties of the cobaltite {\BCAO}, a good realization of the quasi two-dimensional frustrated honeycomb-lattice system with strong planar anisotropy, have been reinvestigated by means of spherical neutron polarimetry with CRYOPAD. From accurate measurements of polarization matrices both on elastic and inelastic contributions as a function of the scattering vector {\bf{Q}}, we have been able to determine the low-temperature magnetic structure of {\BCAO} and reveal its puzzling in-plane spin dynamics. Surprisingly, the ground-state structure (described by an incommensurate propagation vector ${\bf{k}}_{1}=(k_{x}, 0, k_{z}$), with $k_{x}=0.270{\pm}0.005$ and $k_{z} \approx -1.31$) appears to be a quasi-collinear structure, and not a simple helix, as previously determined. In addition, our results have revealed the existence of a non-negligible out-of-plane moment component $ \approx 0.25{\mu}_{B}$/Co$^{2+}$, representing about 10\% of the in-plane component, as demonstrated by the presence of finite off-diagonal elements $P_{yz}$ and $P_{zy}$ of the polarization matrix, both on elastic and inelastic magnetic contributions. Despite a clear evidence of the existence of a slightly inelastic contribution of structural origin superimposed to the magnetic excitations at the scattering vectors ${\bf{Q}}=(0.27, 0, 3.1)$ and ${\bf{Q}}=(0.73, 0, 0.8)$ (energy transfer ${\Delta}E \approx 2.3$ meV), no strong inelastic nuclear-magnetic interference terms could be detected so far, meaning that the nuclear and magnetic degrees of freedom have very weak cross-correlations. The strong inelastic $P_{yz}$ and $P_{zy}$ matrix elements can be understood by assuming that the magnetic excitations in {\BCAO} are spin waves associated with trivial anisotropic precessions of the magnetic moments involved in the canted incommensurate structure.