  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 honeycomb lattice of spin-1 Ni2+ ions, within
the crystallographic ab planes, are well separated 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 is found. 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 higher 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 spin-Hamiltonian
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 rd 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.
