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

摘要

We report $^{31}$P and $^{7}$Li nuclear magnetic resonance (NMR) studies innew non-olivine LiZnPO$_4$-type LiCoPO$_4^\text{tetra}$ microcrystals, wherethe Co$^{2+}$ ions are tetrahedrally coordinated. Olivine LiCoPO$_4$, which wasdirectly transformed from LiCoPO$_4^\text{tetra}$ by an annealing process, wasalso studied and compared. The uniform bulk magnetic susceptibility and the$^{31}$P Knight shift obey the Curie-Weiss law for both materials with a highspin Co$^{2+}$ ($3d^7$, $S=3/2$), but the Weiss temperature $\Theta$ and theeffective magnetic moment $\mu_\text{eff}$ are considerably smaller inLiCoPO$_4^\text{tetra}$. The spin-lattice relaxation rate $T_1^{-1}$ reveals aquite different nature of the spin dynamics in the paramagnetic state of bothmaterials. Our NMR results imply that strong geometrical spin frustrationoccurs in tetrahedrally coordinated LiCoPO$_4$, which may lead to theincommensurate magnetic ordering.