Morin Transition Temperature Studies in Ą-Fe2O3 Pseudocubic Particles.

J.F. BENGOA, M.S. MORENO, S.G. MARCHETTI, R.E. VANDENBERGHE AND R.C. MERCADER

Mexido DF

Congreso; Latin American Conference On Applications Of The Mössbauer Effect; 2004

Bulk hematite (a-Fe2O3) undergoes a rotation of its antiferromagnetically coupled magnetic moments (Morin transition) at a temperature TM»260 K. Above TM, the magnetic sublattice directions are slightly canted so bringing about a weak-ferromagnetism (WF). Below TM both sublattices are exactly antiparallel and the solid is antiferromagnetic (AF). The transition is suitably studied by Mössbauer spectroscopy because of the significant change in the quadrupole shift (2e) from @-0.20 mm/s for WF to @+0.38 mm/s for AF [1]. TM can be modified or suppressed altogether by different causes. The presence of impurities, Fe substitution for different ions, or too small crystallite size, can alter the TM (when the average diameter is smaller than @20 nm, TM is not observed). The role of these, and several other variables, like the existence of water or HO in the samples, is still not fully understood. In addition, there are evidences that the rotation of the magnetic moments is not gradual, but passes rather by some intermediate states [2].   Toward investigating the effect of  the shape of the magnetic particle on TM, in the present work we have studied the Morin transition in nanostructured pseudocubic a-Fe2O3 particles of about 0.8 mm sides. The preparation was carefully chosen to obtain a system with a very narrow crystallite size distribution and particles of homogeneous morphology. We followed the hydrothermal synthesis methodology proposed by Sugimoto et al. [3]. We studied the crystalline structure by DRX, the size and morphology of the particles by SEM, and the size and morphology of the crystallites by HRTEM.