Preparation and magnetic properties of high coercivity strontium ferrite micropowders obtained by extended wet milling

Micropowders of strontium ferrite were obtained by extended wet milling (over 1500 h) of an ordinary SrFe12O19powder prepared by a chemical process from oxalates. The room temperature intrinsic coercive force of the micropowder drastically increased from about 300 Oe (as milled samples) to 6000 - 6500 Oe after removing the stresses and the defects, introduced by milling, by a suitable annealing at900 - 1000\degC. This is one of the highest coercive force reported up to the present for isotropic strontium ferrite powder and it nearly approaches the theoretical value for this material. The magnetic behaviour of these powders is well described by the coherent rotation model of Stoner and Wohlfarth for an assembly of single domain particles oriented at random.Isotropic magnet samples were sintered from these micro-powders at 1150°C. The main magnetic properties of the sintered body were:H_{ci}= 4100Oe andB_{r} = 2000G. The energy product maximum was estimated at approximately 0.85 MGO.