Structure and magnetic properties of nanocrystalline SrFe12O19 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 (mu H-0(ci)approximate to 0.42 T, B-r approximate to 0.24 T and (BH)(max)approximate to 9.6 kJ/m(3)) were reached after the milling and annealing of the powder at 950-1000 degrees C for 1 hr. In this case, the average particles size was about 1 mu m 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. (C) 2005 Elsevier B.V. All rights reserved.