Hard Ferrite Magnets
are manufactured from iron oxide and either barium carbonate or strontium carbonate. The raw materials are mixed in the correct proportions, granulated, and calcined (presintered). After going through several intermediate phases, a hexaferrite phase (BaFe12O19 or SrFe12O19) is achieved. The presintered granulate is ground to a powder. It can then be pressed wet or dry in a magnetic field (anisotropic) or in the absence of a magnetic field (isotropic) and sintered. Hard ferrites are ceramic materials, with the mechanical hardness and brittleness typical of ceramics. A typical way of processing them is by grinding with diamond discs.
Hard Ferrite magnets have excellent corrosion resistance and have normal operating capabilities between -40°C and +250°C. As temperature increases, remanence decreases by 0.2%/°C whereas coercivity increases by 0.3%/°C. At very low temperatures there is a risk of permanent demagnetization in magnet systems with low working points.