  Single-crystalline alpha-Fe2O3 nanorings (short nanotubes) and nanotubes were
synthesized by a hydrothermal method. High-resolution transmission electron
microscope and selected-area electron diffraction confirm that the axial
directions of both nanorings and nanotubes are parallel to the crystalline
c-axis. What is intriguing is that 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 169 nm while it disappears in the nanotubes with a mean tube length
of about 317 nm and a mean outer diameter of 148 nm. Detailed analyses of
magnetization data, x-ray diffraction spectra, and room-temperature Mossbauer
spectra demonstrate that this very strong shape dependence of the Morin
transition is intrinsic to hematite. We can quantitatively explain this
intriguing shape dependence in terms of opposite signs of the surface magnetic
anisotropy constants in the surface planes parallel and perpendicular to the
c-axis (that is, K_parallel = -0.37 erg/cm^2 and K_perp = 0.42 erg/cm^{2}).
