Hard Ferrite (Ceramic) Magnets

Strontium hard ferrite (SrO·6Fe2O3) in three routes: wet-pressed and dry-pressed sintered ferrite, and injection-molded bonded ferrite. The sintered routes carry the higher magnetic properties of the family; the bonded route gives complex shapes, multipole magnetization and a tolerance class agreed per drawing, at a lower flux density. Magnetic orientation, the magnetization pattern and the values that apply are set per grade, per route and per drawing.

Assess the full operating temperature range at the magnet. Ferrite coercivity decreases as temperature falls, so cold-start conditions can be critical for demagnetization. At higher temperatures, magnetic output and the working point must also be checked. Injection-molded ferrite additionally requires review of the binder, moisture exposure and mechanical stability. The usable temperature follows the grade, the route and the demagnetization requirement of the design, so it is fixed per grade and per project rather than by one ferrite figure. Sintered ferrite is a brittle ceramic oxide: it chips at edges and does not tolerate clamping stress, so the assembly has to allow for that, and it normally needs no coating. A bonded part is less brittle and is shaped in the mold, but its temperature limit follows the binder system, and whether it needs a coating is a project decision.Worked example from the coefficients listed in the table below. Strontium ferrite carries a remanence coefficient αBr of −0.2 %/K over 0–100 °C, so the published band moves predictably with the temperature at the magnet. Taking Y30H-1 at 380–400 mT: at 100 °C that is roughly 319–336 mT, because 80 K at 0.2 %/K is about 16 % of remanence; at −20 °C it is roughly 410–432 mT, because 40 K at 0.2 %/K is about 8 %. Two consequences follow for the drawing. The hot end costs flux, so a design that only just closes its torque target at room temperature will not close it in a warm housing, and the check has to be run at the top of the range. The cold end adds flux but takes coercivity away, and that is the more dangerous direction: the flux margin rises while the resistance to demagnetization falls, so a cold start next to a strong opposing field is where a marginal grade fails. Below 0 °C the coefficient above is already an extrapolation, so the demagnetization check at that temperature — not the arithmetic — is what fixes the grade.

Choose ferrite when the design has enough volume for the flux it needs, when cost stability at volume matters, and when the working point of the magnetic circuit is defined well enough to fix a grade. Fix the route first - wet pressed, dry pressed or injection molded - because that decision sets the flux density band, the tolerance class and the tooling cost, and it is made on the drawing or the sample. Ferrite is a poor fit where the magnet has to sit in a very small envelope at high torque, where a very high (BH)max is required, or where parts must take impact without support. A first review needs dimensions, quantity, the magnetic or magnetization requirement, the working temperature and the tolerances; anything not fixed yet can be left open and settled in the evaluation.