Insight into the Correlation of Shape and Magnetism of Hematite Nanospindles

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 $p \gtrapprox 1.5$.