Anomalous Magnetic Properties of Honeycomb Lattice Compounds -Neutron Scattering-

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Anomalous magnetic behaviors of honeycomb lattice compound Na_3M_2SbO_6 and Na_2M_2TeO_6 (M=Cu, Ni and Co) have been investigated by means of neutron scattering and measurements of magnetic susceptibility x and specific heat C. For M=Cu, spin-gap behaviors have been observed in the temperature dependences of x and C. In order to extract information of an origin of the spin-gap, magnetic excitation spectra have been measured by neutron inelastic scattering on a single crystal of Na_3Cu_2SbO_6. From the intensity distribution and the dispersion curves of the excitations, we have evaluated the values of spin-gap and exchange interactions. The results of x and C data of Na_3Ni_2SbO_6, Na_3Co_2SbO_6, Na_2Ni_2TeO_6, and Na_2Co_2TeO_6 indicate that the spin systems exhibit an antiferromagnetic transition at TN=18K, 6K, 34K, and 25K, respectively. To investigate the magnetic structure of these systems, neutron diffraction studies have been carried out for the polycrystalline samples. We can determine the magnetic structure of Na_3Ni_2SbO_6, and Na_2Co_2TeO_6.