The Morin transition in nanostructured pseudocubic hematite: effect of the intercrystallite magnetic exchange

J. F. BENGOA; A. M. ALVAREZ; A. E. BIANCHI; G. PUNTE; R. E. VANDENBERGHE; R. C. MERCADER; S. G. MARCHETTI

MATERIALS CHEMISTRY AND PHYSICS

ELSEVIER SCIENCE SA

Año: 2009

0254-0584

The Morin transition has been studied in alpha-Fe2O3 particles of homogeneous pseudocubic morphology of about 1.8 mm side by different techniques. The particles were constituted by nanometric crystallites with a narrow size distribution. Samples annealed at 673, 773 and 873 K, yielded Morin transition temperatures, TM, of about 230, 241, and 245 K, respectively, which are lower than the bulk value, TM approx. 263 K. In addition, the temperature range ΔTM through which these samples undergo the transition is very narrow with respect to what is reported in the literature. Ranges of 20, 10 and 8 K, were obtained for the samples annealed at 673, 773 and 873 K. In this work, through the estimation of the values of the exchange magnetic energy, we have demonstrated that the existence of the Morin transition, and their shift and breadth are related to the presence of intercrystallite interactions inside the particles. These intercrystallite interactions are modified by non-stoichiometric hydroxyl groups and incorporated water molecules elimination, produced by the annealing treatments.