Hard ferrites consist of oxidized metals and are thus included in the ceramic materials group. They consist of approx. 90% iron oxide (Fe2O3) and 10% alkaline earth oxide (BaO or SrO) – raw materials which are plentiful and inexpensive. Today, they represent the largest proportion of permanent magnets produced. As is typical of oxide ceramics, hard ferrite magnets display relatively resistant behavior towards moisture, solvents, alkaline solutions, weak acids, salts, lubricants and gas pollutants. Generally, hard ferrite magnets can therefore be used without additional corrosion protection. Isotropic magnets are shaped by compressing. Anisotropic magnets are compressed within a magnetic field. This provides the magnet with a preferential direction and triples its energy density.
In contrast to AlNiCo magnets, hard ferrites are characterized by flux densities but high coercive field strengths. This results in the generally flat shape of the materials. Barium ferrite and strontium ferrite are differentiated depending on the starting material.