The effect of crystallinity and Al substitution on the magnetic structure and morin transition in hematite

• Published in: Journal of magnetism and magnetic materials 1988-04, Vol.72 (2), p.141-151
• Main Authors: De Grave, E., Bowen, L.H., Vochten, R., Vandenberghe, R.E.
• Format: Article
• Language: English
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Exact sciences and technology; Magnetic properties and materials; Physics; Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)
• ISSN: 0304-8853
• DOI: 10.1016/0304-8853(88)90182-5

An aluminous hematite with 6.5 at% substitution and obtained by thermal decomposition at 500°C of an aluminous goethite, has been annealed at many different temperatures between 550 and 900°C. The resulting compounds have been characterized by X-ray diffraction and electron microscopy. XPS and zeta-potential measurements indicate an enrichment in Al of the surface of the particles at high sintering temperatures. Mössbauer spectra were recorded as a function of temperature and the results compared with those obtained for as-prepared aluminous hematites as a function of the Al substitution. This shows that the amount of Al leaving the hematite lattice at high temperatures is insignificant. A careful evaluation of the Mössbauer data of thermally treated, substituted and non-substituted hematite has allowed to distinguish the effects of crystallinity from those of the Al substitution. It is further suggested that the two low-temperature phases observed in the Mössbauer spectra, coexist within the particles and that the magnetic exchange interaction is on the basis of the canted spin directions.