Magnetic structure, excitations and short-range order in honeycomb Na₂Ni₂TeO₆

Na2Ni2TeO6 has a layered hexagonal structure with a honeycomb lattice constituted by Ni2+ and a chiral charge distribution of Na+ that resides between the Ni layers. In the present work, the antiferromagnetic (AFM) transition temperature of Na2Ni2TeO6 is confirmed at T N ≈ 27 K, and further, it is found to be robust up to 8 T magnetic field and 1.2 GPa external pressure; and, without any frequency-dependence. Slight deviations from nominal Na-content (up to 5%) does not seem to influence the magnetic transition temperature, T N. Isothermal magnetization curves remain almost linear up to 13 T. Our analysis of neutron diffraction data shows that the magnetic structure of Na2Ni2TeO6 is faithfully described by a model consisting of two phases described by the commensurate wave vectors kc, (0.5 0 0) and (0.5 0 0.5), with an additional short-range order component incorporated in to the latter phase. Consequently, a zig-zag long-range ordered magnetic phase of Ni2+ results in the compound, mixed with a short-range ordered phase, which is supported by our specific heat data. Theoretical computations based on density functional theory predict predominantly in-plane magnetic exchange interactions that conform to a J 1-J 2-J 3 model with a strong J 3 term. The computationally predicted parameters lead to a reliable estimate for T N and the experimentally observed zig-zag magnetic structure. A spin wave excitation in Na2Ni2TeO6 at E ≈ 5 meV at T = 5 K is mapped out through inelastic neutron scattering experiments, which is reproduced by linear spin wave theory calculations using the J values from our computations. Our specific heat data and inelastic neutron scattering data strongly indicate the presence of short-range spin correlations, at T > T N, stemming from incipient AFM clusters.