中村 龍哉

Lithium-nickel layered oxides were prepared by the solid-state reaction at temperature range of 700-850℃ in O_2 atmosphere. The structural refinement with the help of Rietveld analysis showed that the crystal structure of these compounds were identified as α-NaFeO_2 type layered rock-salt, where some extra Ni ions were present in Li-layers. As the Ni content in the Li-layer increased, the lattice parameter a increased and the ratio of C/a decreased. From the magnetic susceptibility measurement, it was found that both the asymptotic Curie temperature and the spontaneous magnetization at low temperature are sensitive to the presence of extra Ni ions in Li-layers. The asymptotic Curie temperature, which was always positive, shifted toward higher temperature with an increase in the non-stoichiometry degree. It was seen that the spontaneous magnetization 4.2K is also enhanced with the non-stoichiometry. Considering these magnetic variation with the non-stoichiometry enabled us to propose the following model; Ni-Ni magnetic coupling within the transition metal layer was weak and ferromagnetic, whereas the inter-layer Ni-Ni coupling is relatively strong and antiferromagnetic. The increase in the asymptotic Curie temperature may be attributed to the development of magnetic correlation from two to three dimension, and the enhancement of the spontaneous magnetization is described by ferromagnetic cluster formation.