Effect of particle agglomeration on the magnetization reversal of fine powders

The magnetization reversal of loose low-coercivity fine powders of magnetite or iron is independent of the value of the particle packing fraction. However, when these powders are compacted under pressure in a solid matrix of Perspex, the coercivity of the samples increases logarithmically with the increasing powder density. We assume that in these powders small superparamagnetic particles are agglomerated into clusters. Supposing the magnetization process to be due to a nucleation mechanism, the critical field at which this nucleation starts appears to be a function of the number of particles agglomerated in a cluster. This number varies with the density of the powder compacted in the matrix. By the suggested theoretical model, we have estimated the diameter of the superparamagnetic particles of the experimentally investigated fine iron powder to be approximately 46 Å. Valuable information about the magnetic structure of these powders has been obtained from the studies of the hyperfine splitting in their Mössbauer patterns and from those of the changes in the line intensities caused by the application of an external magnetic field.