Neutron diffraction study of mesoporous and bulk hematite, alpha-Fe2O3

Variable temperature neutron diffraction was carried out on mesoporous alpha-Fe2O3 (hematite) with a mean pore diameter of 38.5 angstrom. Data were directly compared to measurements carried out on bulk hematite. Unlike bulk hematite, the mesoporous material does not undergo a spin-flop transition from a weak ferromagnet to a pure antiferromagnet (Morin transition, T-M = 259.1(2)). Instead, the material remains a weak ferromagnet down to 2 K with the magnetic moments staying perpendicular to the R3c [111] ([003] in the hexagonal cell) direction rather than realigning (to) almost parallel to this direction. The angle of the magnetic Fe3+ moments to the [111] direction in the antiferromagnetic state also accurately was obtained for bulk hematite. Using magnetic hysteresis measurements, the canting angle responsible for weak ferromagnetism within the ab planes (hexagonal setting) was deduced for the mesoporous material at 12 K and compared to the angle made in bulk hematite above T-M.