Strong shape-dependence of Morin transition in α-Fe2O3 single-crystalline nanostructures

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 Mossbauer 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 magnetic anisotropy constants of the surface planes parallel and perpendicular to the c-axis.