The stability of permanent magnets

The paper describes tests made to compare the magnetic stability of representative martensitic and precipitation-hardening (isotropic and anisotropic) permanent-magnet materials. The weakening of magnets with time after magnetization, when left as free from disturbance as possible, was measured over a period of about three years, both on unstabilized magnets and on magnets artificially stabilized by weakening them 1% or 5% by applying demagnetizing fields. Anisotropic alloys were found to be much more stable than isotropic alloys, the stability of Columax being outstanding. Alnico, in spite of its much higher coercive force, was not more stable than the martensitic steels. The effect on stability of the working-point of the permanent-magnet material was also examined and found to be small above the (BH)max point on the demagnetization curve. Below the (BH)max point, however, the stability of both isotropic and anisotropic materials was considerably reduced. Further tests were made on the effects of heating at various temperatures up to 220° C and of mechanical stress and impact. Stability in high-temperature treatment is only roughly related to stability in time tests. The results of the mechanical tests were inconclusive as the magnet assemblies available, designed primarily for long-term tests, were not suitable for subjection to severe mechanical treatment.