Zigzag antiferromagnetic ground state with anisotropic correlation lengths in the quasi-two-dimensional honeycomb lattice compound Na2Co2TeO6

The crystal structure, magnetic ground state, and the temperature-dependent microscopic spin-spin correlations of the frustrated honeycomb lattice antiferromagnet Na2Co2TeO6 have been investigated by powder neutron diffraction. A long-range antiferromagnetic (AFM) ordering has been found below TN similar to 24.8 K. The magnetic ground state, determined to be zigzag antiferromagnetic and characterized by a propagation vector k = (1/200), occurs due to the competing exchange interactions up to third-nearest neighbors within the honeycomb lattice. The exceptional existence of a limited magnetic correlation length along the c axis (perpendicular to the honeycomb layers in the ab planes) has been found even at 1.8 K, well below the TN similar to 24.8 K. The observed limited correlation along the c axis is explained by the disorder distribution of the Na ions within the intermediate layers between honeycomb planes. The reduced ordered moments m(Co(1)) = 2.77(3) mu(B)/Co2+ and m(Co(2)) = 2.45(2) mu(B)/Co2+ at 1.8 K reflect the persistence of spin fluctuations in the ordered state. Above T-N similar to 24.8 K, the presence of short-range magnetic correlations, manifested by broad diffuse magnetic peaks in the diffraction patterns, has been found. Reverse Monte Carlo analysis of the experimental diffuse magnetic scattering data reveals that the spin correlations are mainly confined within the two-dimensional honeycomb layers (ab plane) with a correlation length of similar to 12 angstrom at 25 K. The nature of the spin arrangements is found to be similar in both the short-range and long-range ordered magnetic states. This implies that the short-range correlation grows with decreasing temperature and leads to the zigzag AFM ordering at T <= T-N. The present study provides a comprehensive picture of the magnetic correlations over the temperature range above and below the T-N and their relation to the crystal structure. The role of intermediate soft Na layers on the magnetic coupling between honeycomb planes is discussed.