摘要
Single-crystalline hematite （α-Fe2O3） nanorings （short nanotubes） and nanotubes were synthesized by a hydrothermal method. High-resolution transmission electron microscopy and selected-area electron diffraction confirm that the axial directions of both the nanorings and nanotubes are parallel to the crystalline c-axis. Intriguingly, the Morin transition occurs at about 210 K in the short nanotubes with a mean tube length of about 115 nm and a mean outer diameter of about 169 nm. However, it does not occur in the nanotubes with a mean tube length of about 317 nm and a mean outer diameter of about 148 nm. Detailed analysis of magnetization data, X-ray diffraction patterns, and room-temperature M6ssbauer spectra demonstrates that this very strong shape-dependence of Morin transition is intrinsic to hematite. We explain this intriguing shape- dependence quantitatively, in terms of the opposite signs of the surface mag- netic anisotropy constants of the surface planes parallel and perpendicular to the c-axis.
单人赛水晶的赤铁矿(-Fe<sub>2</sub>O<sub>3</sub>) nanorings (短 nanotubes ) 和 nanotubes 被一个热水的方法综合。高分辨率的传播电子显微镜学和精选区域的电子衍射证实 nanorings 和 nanotubes 的轴的方向与水晶的 c 轴平行。有趣地， Morin 转变与大约 115 nm 和大约 169 nm 的一条吝啬的外部直径的吝啬的试管长度在短 nanotubes 发生在大约 210 K。然而，它不与大约 317 nm 和大约 148 nm 的一条吝啬的外部直径的吝啬的试管长度发生在 nanotubes。磁化数据， X 光检查衍射模式，和房间温度 M 的详细分析 ? ssbauer 系列证明 Morin 转变的这很强壮的形状依赖对赤铁矿内在。我们向这吸引人的形状依赖份量上解释，以表面的相反的符号与 c 轴平行、垂直的表面飞机的磁性的 anisotropy 常数。
基金
This work was supported by the National Natural Science Foundation of China （No. 11174165）, the Natural Science Foundation of Ningbo （No. 2012A610051）, and the K. C. Wong Magna Foundation. VA acknowledges financial support from NIH and NIGMS under MBRS- RISE M.S.-to-Ph.D. Program （No. R25GM061331）.
