Polarized-neutron investigation of magnetic ordering and spin dynamics in BaCo$_2$(AsO$_4$)$_2$ frustrated honeycomb-lattice magnet

TL;DR

This study uses polarized neutron scattering to elucidate the magnetic structure and spin dynamics of BaCo2(AsO4)2, revealing a quasi-collinear ground state, out-of-plane moments, and weak nuclear-magnetic interference, advancing understanding of frustrated honeycomb magnets.

Contribution

The paper provides the first detailed polarized neutron analysis of BaCo2(AsO4)2, uncovering its true magnetic structure and spin excitations, which differ from previous assumptions.

Findings

Ground state is quasi-collinear, not a simple helix.

Presence of a significant out-of-plane magnetic moment component.

Weak nuclear-magnetic interference indicating minimal cross-correlations.

Abstract

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.