LAUSR

The microscopic spin-spin correlations in the 2D layered spin-1 honeycomb lattice compound Na2Ni2TeO6 have been investigated by neutron diffraction and inelastic neutron scattering. The present study reveals a novel magnetic phenomenon where the magnetic symmetry is controlled by the nonmagnetic Na-layer which is a unique feature for the studied compound Na2Ni2TeO6 with respect to other Na based layered compounds, especially A2M2XO6 or A3M2XO6 compounds. The honeycomb lattice of spin-1 Ni ions, within the crystallographic ab planes, are well separated (~ 5.6 Å) along the c axis by an intermediate Na layer whose crystal structure contains chiral nuclear density distributions of Na ions. The chirality of the alternating Na layers is opposite. Such alternating chirality of the Na layer dictates the magnetic periodicity along the c axis where an up-up-down-down ( ) spin arrangement of the in-plane zigzag AFM structure [characterized by the propagation vector k = (1⁄2 1⁄2 1⁄2)] is found. Our results, thus, provide a strong correlation between the magnetic moments in the transition metal layers and the Na-chiral order in the adjacent nonmagnetic Na-layers. Besides, the above described commensurate (CM) zigzag AFM order state is found to coexist with an incommensurate (ICM) AFM state below the TN ~ 27.5 K. The ICM state is found to appear at much highere temperature ~ 50 K and persists down to lowest measured temperature of 1.7 K. Our reverse Monte Carlo (RMC) analysis divulges a two dimensional (2D) magnetic correlations (within the ab plane) of the ICM AFM state over the entire temperature range 1.7-50 K. Further, the spinHamiltonian has been determined by carrying out inelastic neutron scattering experiments and subsequent linear spin-wave theory analysis which reveals the presence of competing inplane exchange interactions up to 3 nearest neighbours consistent with the zigzag AFM ground state, and weak interplanar interaction as well as a weak single-ion-anisotropy. The values of the exchange constants yield that Na2Ni2TeO6 is situated well inside the zigzag AFM phase (spans over a wide ranges of J2/J1 and J3/J1 values) in the theoretical phase diagram. The present study, thus, provides a detailed microscopic understanding of the magnetic correlations and divulges the intertwining magneto-structural correlations.