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

Na₂Ni₂TeO₆has 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 Na₂Ni₂TeO₆is confirmed atT_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 Na₂Ni₂TeO₆is faithfully described by a model consisting of two phases described by the commensurate wave vectorsk→c,0.500and0.500.5, 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 aJ₁-J₂-J₃model with a strongJ₃term. The computationally predicted parameters lead to a reliable estimate forT_Nand the experimentally observed zig-zag magnetic structure. A spin wave excitation in Na₂Ni₂TeO₆atE≈ 5 meV atT= 5 K is mapped out through inelastic neutron scattering experiments, which is reproduced by linear spin wave theory calculations using theJvalues from our computations. Our specific heat data and inelastic neutron scattering data strongly indicate the presence of short-range spin correlations, atT>T_N, stemming from incipient AFM clusters.