用$^{31}$ P和$^7$ Li核磁共振研究橄榄石和非橄榄石LiCoPO $_4$的反常自旋涨落和磁阻挫
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：
我们报告了新的非橄榄石LiZnPO $_4$型LiCoPO $_4^\text{tetra}$微晶中的$^{31}$ P和$^{7}$ Li核磁共振（NMR）研究，其中Co $^{2+}$离子是四面体配位的。还研究和比较了通过退火过程从LiCoPO $_4^\text{tetra}$直接转化的橄榄石LiCoPO $_4$。对于具有高自旋Co $^{2+}$（$3d^7$，$S=3/2$）的两种材料，均匀的体磁化率和$^{31}$ P奈特位移服从居里-外斯定律，但外斯温度$Θ$和有效磁矩$μ_\text{eff}$在LiCoPO $_4^\text{tetra}$中相当小。自旋-晶格弛豫速率$T_1^{-1}$揭示了两种材料在顺磁状态下的自旋动力学的完全不同的性质。我们的NMR结果表明，在四面体配位的LiCoPO $_4$中发生了强烈的几何自旋阻挫，这可能导致无公度的磁有序。
英文摘要：
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 $Θ$ and the effective magnetic moment $μ_\text{eff}$ 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.
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