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

The magnetic properties of the cobaltite BaCo2(AsO4)(2), 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 Q, we have been able to determine the low-temperature magnetic structure of BaCo2(AsO4)(2) and reveal its puzzling in-plane spin dynamics. Surprisingly, the ground-state structure (described by an incommensurate propagation vector k(1) = (k(x), 0, k(z)), with k(x) = 0.270 +/- 0.005 and k(z) approximate to -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 approximate to 0.25 mu(B)/Co2+, 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 Q = (0.27, 0, 3.1) and Q = (0.73, 0, 0.8) (energy transfer Delta E approximate to 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 BaCo2(AsO4)(2) are spin waves associated with trivial anisotropic precessions of the magnetic moments involved in the canted incommensurate structure.