Unusual spin fluctuations and magnetic frustration in olivine and non-olivine LiCoPO_4 detected by ~(31)P and ~7Li nuclear magnetic resonance

摘要

We report ~(31)P and ~7Li nuclear magnetic resonance (NMR) studies in new non-olivine LiZnPO_4-type LiCoPO_4~(tetra) microcrystals, where the Co~(2+) ions are tetrahedrally coordinated. Olivine LiCoPO_4, which was directly transformed from LiCoPO_4~(tetra) by an annealing process, was also studied and compared. The uniform bulk magnetic susceptibility and the ~(31)P Knight shift obey the Curie-Weiss law for both materials with a high spin Co~(2+) (3d~7, S = 3/2), but the Weiss temperature 0 and the effective magnetic moment μ_(eff) are considerably smaller in LiCoPO_4~(tetra). The spin-lattice relaxation rate T_1~(-1) reveals a quite different nature of the spin dynamics in the paramagnetic state of both materials. Our NMR results imply that strong geometrical spin frustration occurs in tetrahedrally coordinated LiCoPO_4, which may lead to the incommensurate magnetic ordering.