Recent Ferrite Magnet Developments

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Abstract
Currently used ferrite magnets (also called ceramic magnets) are modifications of the magnetoplumbites MO·6 Fe2O3 where M stands for Ba or Sr—with additions of other compounds, off‐stoichiometric ratios, flexible binders, etc. The first commercial magnetoplumbite magnets were of the barium ferrite type, but the newer ones are modified strontium ferrites which are about 30% higher in coercive force at the same level of remanence and 4% lower in weight than the barium ferrites. They are also easier to manufacture. The results of hysteresis loop measurements on some 20 000 anisotropic different ferrite magnets are summarized. The highest values, which are all obtained on modified strontium ferrites, range from (BH)max = 4.8 MGOe at IHc = 2.4 kOe to (BH)max = 0.8 MGOe at IHc = 11 kOe depending on process and chemical composition. The permanent magnets with the highest presently known energy/cost ratios are obtained by subdividing a green body of sulfate‐modified strontium ferrite using a ``cookie‐cutting'' technique and by quick‐firing the magnets so that they contain nonequilibrium structures.

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