Phase evolution and magnetic properties of a high-energy ball-milled hematite–alumina system

The system $(α-Fe2O3)x(α-Al2O3)1−x$ was subjected to 24 h of high-energy ball-milling varying its nominal concentration, $x.$ The milled samples were structurally and magnetically characterized at room temperature by x-ray diffraction, Mössbauer spectroscopy, and magnetic measurements. Mössbauer studies were also performed in the temperature range 250–6 K. As a result of the earlier analyses, it was observed that the milling products were extremely dependent on the hematite starting concentration. In samples with low $α-Fe2O3$ initial concentration (i.e., $x⩽0.12),$ the paramagnetic solid solution $α-(FeΔYAl1−ΔY)2O3,$ the α-Fe and the $FeAl2O4$ phases were identified, along with alumina, which was always residual. The presence of spinel and metallic iron was attributed to the stainless-steel vial and balls abrasion. For $x>0.12,$ the iron component was no longer present but another magnetic component, corresponding to an aluminum-substituted hematite phase, $α-(Fe1−ΔWAlΔW)2O3,$ could be seen to increase with increasing $x.$ This solid solution was shown to be transitional, at room temperature, between two ordering states, weak ferromagnetic and superparamagnetic, the latter resulting from the milling induced particle size reduction. Low temperature Mössbauer spectra revealed the magnetic ordering of both solid solutions, $α-(FeΔYAl1−ΔY)2O3$ and $α-(Fe1−ΔWAlΔW)2O3,$ and indicated the suppression of the Morin transition for the iron-rich solid solution. The magnetization versus magnetic field curves obtained for samples with $x⩾0.12$ revealed, besides a general superparamagnetic character, some hysteretic behavior due to the magnetic phases eventually existing.