Structure and magnetic properties of nanocrystalline SrFe ₁₂O₁₉ alloy produced by high-energy ball milling and annealing

The structure of SrFe₁₂O₁₉ powders, subjected to mechanochemical treatment in a high-energy ball mill and subsequent annealing, were studied by X-ray diffraction technique, high-resolution scanning electron microscopy and sedimentation analysis. The magnetic properties were measured in a vibrating sample magnetometer. As a result of the milling under Ar or air atmosphere an amorphous phase was formed in the powder and its amount rised with increase of the milling time. In addition, the milling resulted in a reduction of powder particles and SrFe₁₂O₁₉ crystallite size as well as in increase of microstrain in the SrFe₁₂O₁₉ lattice. The milling caused a deterioration of the magnetic properties. The annealing led to a sharp improvement of the magnetic properties due to crystallization and formation of the SrFe₁₂O₁₉ phase with fine crystallites. The best properties (μ₀H_ci≈0.42 T, B _r≈0.24 T and (BH)_max≈9.6 kJ/m³) were reached after the milling and annealing of the powder at 950-1000 °C for 1 hr. In this case, the average particles size was about 1 μm and the average crystallite size was 100-200 nm. The high magnetic properties of the annealed powder may be explained by the formation of SrFe₁₂O₁₉ nanocrystallites with a size smaller than the critical dimension of the single-domain particles.