Magnetic Properties of Metastable Honeycomb KCoAsO4

Abstract

We present comprehensive neutron scattering data on polycrystalline samples of a new metastable honeycomb material KCoAsO4. Below 𝑇𝑁=14 K, the system orders into a zigzag antiferromagnetic state, with spins ordered into alternating ferromagnetic chains similar to the isostructural sister compound KNiAsO4. In the case of KCoAsO4, we find the moments are robustly canted out of plane closer to the crystallographic 𝑐 axis. A combination of weak interlayer coupling, lattice strain, and inhomogeneities lead to the coexistence of two magnetic ordering wave vectors 𝑘1 =(1.5 0 0) and 𝑘2 =(0.5 0 0.5), where the two structures differ only in their layer stacking. Inelastic data show the presence of a spin orbital mode at 24 meV, supporting a pseudospin ˜𝑆 =1/2 Kramer's doublet ground state of the Co2+ ions. We model the low energy excitations using both a conventional XXZ Hamiltonian and generalized Kitaev Heisenberg Hamiltonian within a linear spin wave limit. While either model can qualitatively reproduce the observed spectra, the lack of fine features and observation of disorder prevent a clear-cut determination of the low energy Hamiltonian. In the case of an XXZ-type model, a large easy-axis anisotropy is necessary to reproduce the gapped spectra and canting of the magnetic moments. For the generalized Kitaev model, despite the large canting of the moments away from the honeycomb layers, we find a noticeable if nondominant Kitaev term persists. The contrast of the magnetic properties of KCoAsO4 to other cobalt honeycombs highlights the sensitivity of the low energy magnetic properties of Co2+ to fine details of its crystalline environment.