Unusual spin fluctuations and magnetic frustration in olivine and non-olivine LiCoPO${}_4$ detected by ${}^{31}$P and ${}^7$Li nuclear magnetic resonance

Abstract

We report ${}^{31}$P and ${}^7$Li nuclear magnetic resonance (NMR) studies in new non-olivine LiZnPO${}_4$-type LiCoPO${}_4^{\text{tetra}}$ microcrystals, where the Co${}^{2+}$ ions are tetrahedrally coordinated. Olivine LiCoPO${}_4$, which was directly transformed from LiCoPO${}_4^{\text{tetra}}$ 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 $\hat{I}˜$ and the effective magnetic moment $\hat{I}{¼}_{\text{eff}}$ are considerably smaller in LiCoPO${}_4^{\text{tetra}}$. 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.

6 pages, 7 figures, published in PRB