Аннотации:
In this work, we characterize the hollow hematite (a-Fe2O3) micro spheres obtained by two nontemplate techniques: i) sol gel and ii) spray roasting process. Both techniques allow the production of
high yield hollow hematite spheres up to 100 g for the case of sol gel and up to 500 kg for the case of
spray roaster process. The samples were characterized by X-ray diffraction, scanning electron microscopy
and Mossbauer spectroscopy. The results indicate nearly uniform hollow spheres with diameters of €
around 1e1.5 mm and consisting of polycrystalline hematite. The Mӧssbauer spectroscopy reveals the
signal change of quadrupole shift values evidencing the occurrence of the Morin transition and that the
samples show an antiferromagnetic order at 77 K as in bulk hematite. The Morin temperature (TM) for
both samples was obtained from the measurements of the magnetic moments as a function of the
temperature in zero field cooling (ZFC) and field cooling (FC) modes. The values of TM for both samples
are lower than that reported for bulk hematite (TM(bulk) ¼ 263 K). Remarkably, the ZFC and FC loops do
not overlap in both samples, revealing irreversible Morin transition. However, the sample obtained by sol
gel presents thermal hysteresis with TM values of 260 K (in ZFC) and 248 K (FC). Whereas, the sample
obtained by spray roaster process presents complete irreversibility and TM values of 252 K (in ZFC) and
242 K (in FC)
