Mössbauer effect study of the spin structure in natural hematites

Three natural hematites, α-Fe₂O₃, from the region of Elba have been investigated by means of ⁵⁷Fe Mössbauer spectroscopy at variable temperatures between 80 and 400K. The samples were selected on the basis of their different morphology as observed from powder X-ray diffraction and transmission electron microscopy. The mean crystallite diameters (MCD) along [104] are 1000, 280 and 40 nm respectively. Energy-dispersive analyses of X-rays revealed the presence of minor amounts of Si impurities in those two hematites having the largest MCD. All three hematites show the coexistence at low temperatures of antiferromagnetic-like (AF) and weakly-ferromagnetic-like (WF) spin states. The saturation values of the AF and WF magnetic hyperfine fields and quadrupole shifts have been determined, from which conclusions are drawn concerning the spin structure in relation to the crystallinity of the samples. The variations of the hyperfine parameters in the Morin-transition region indicate a gradual reorientation of both AF and WF spins towards the basal plane. As expected, the Morin transition itself is affected by the particle size. The two hematites exhibiting the largest particle dimensions still show an AF contribution for T>270K. It is suggested, and argued that this unusual behaviour is due to the presence of Si⁴⁺, and hence Fe²⁺, in the lattice. The characteristic Mössbauer temperatures and the intrinsic isomer shifts were evaluated from the temperature variation of the observed isomer shifts. Both parameters are not significantly affected by the morphology and are in excellent agreement with data obtained for synthetic hematites.