标题： 关于赤铁矿纳米梭形结构与磁性相关性的洞察 显示英文标题

标题： Insight into the Correlation of Shape and Magnetism of Hematite Nanospindles

摘要： 已确定莫林转变（磁赤铁矿中的自旋重取向）随着纳米颗粒体积的减小而向低温移动。然而，我们的研究结果显示，在一系列磁赤铁矿纳米梭中出现了相反的效果：这些通过氯化铁溶液水热分解合成的颗粒，其纵横比$p$在$1.0$和$5.2$之间（长轴约为$70$至$290$纳米），显示出随$p$的增加，莫林转变温度$T_{\text{Morin}}$下降，尽管体积有所增加。通过(HR)STEM和XRD确定的它们的内部形态显示，它们是由初级粒子的外延融合形成，从晶体学取向上看，这些初级粒子排列得非常完美。 结合磁强计和 Mössbauer 光谱学，我们揭示了颗粒形状、磁性属性以及特别是 Morin 过渡之间的关联：虽然更接近球形的颗粒在大约$200$开尔文下经历所述过渡，$T_{\text{Morin}}$随着纳米梭状结构的增加而降低，同时变宽并显示出更宽的热迟滞。 我们的测量结果显示，在超过一个临界阈值$p \gtrapprox 1.5$后，Morin 过渡被完全抑制，表明尽管基于形状各向异性考虑这种行为是出乎意料的，但在赤铁矿基面内，颗粒净磁矩的弱铁磁（WFM）态得到了稳定。 对于与磁场相关的自旋翻转跃迁，检测到颗粒纵横比的类似趋势。 我们已经证明了存在偏离低温反铁磁（AFM）和高温 WFM 自旋结构的中间自旋取向状态，这些状态可能与$p \gtrapprox 1.5$下观察到的 Morin 过渡抑制有关。 显示英文摘要

摘要： It is established that the Morin transition, a spin reorientation in hematite, is shifted to lower temperatures with decreasing nanoparticle volume. However, our findings indicate an opposite effect in a series of hematite nanospindles: The particles, synthesized by hydrothermal decomposition of iron(III) chloride solution, with aspect ratios $p$ between $1.0$ and $5.2$ (long axis ca. $70$--$290$ nm) display decreasing Morin transition temperatures $T_{\text{Morin}}$ upon increasing $p$, despite the volume increase. Their inner morphology, determined via (HR)STEM and XRD, shows that they are formed by the epitactical fusion of primary particles, perfectly aligned in terms of crystallographic orientation. Combining magnetometry and M\"ossbauer spectroscopy, we uncover the correlation between particle shape, magnetic properties, and in particular the Morin transition: While more spherical particles undergo said transition at about $200$ K, $T_{\text{Morin}}$ decreases upon higher nanospindle elongation, while also being broadened and showing a wider thermal hysteresis. Our measurements reveal complete suppression of the Morin transition beyond a critical threshold $p \gtrapprox 1.5$, indicating stabilization of the weak ferromagnetic (WFM) state with net particle magnetic moment within the hematite basal plane, despite such behavior being unexpected based on shape anisotropy considerations. For the correlated, magnetic field-dependent spin-flop transition, a comparable trend in particle aspect ratio is detected. We have demonstrated the presence of intermediate spin alignment states that deviate both from the low-temperature antiferromagnetic (AFM) and high-temperature WFM spin structure for slightly elongated particles, likely being connected to the suppression of the Morin transition observed for $p \gtrapprox 1.5$.