Structural and magnetic phases in ball milled strontium ferrite

Prolonged milling of SrFe₁₂O₁₉ in air and vacuum results in mixtures of nanostructured iron oxides. The hexagonal ferrite structure is altered to form mixtures of nanoparticles of α-Fe₂O₃ and vacancy defected Fe_3-xO₄. Mechanical activation in air for 800 h results in the formation of ∼10 nm α-Fe₂O₃ particles with both ∼10 nm α-Fe₂O₃ (∼56%) and ∼3 nm Fe_3-xO₄ particles (∼44%) being obtained in the vacuum milled state. The vacancy defected magnetite particles are likely to form as a result of mechanochemical reduction of α-Fe₂O₃ in the oxygen free enviroment. The range of nanoparticle sizes obtained on milling is reflected by the relaxation effects observed at room temperature and 4.2 K in the Mössbauer spectra of the milled products. A spin glass-like irreversibility in magnetisation is observed below∼150 K.