Long Term Magnetic Stability of Alnico and Barium Ferrite Magnets

Abstract

Alnico and barium ferrite materials do not age at room temperature. The decreases in remanence, occurring with time, are adjustments by the magnet to its environment. Remanence adjustment proceeds with the logarithm of time and amounts to 2% or less one year after magnetization. Remanences in magnets with $H_{\mathrm{ci}}\text{\hspace{0.17em}>\hspace{0.17em}2200\hspace{0.17em}}\mathrm{oe}$ show no changes. Also, the smaller the irreversible susceptibility at the operating point, the more stable is remance. Remanences can be stabilized completely by preadjustment. Alnico V magnets are more stable than similar Alnico III magnets, even when the Alnico V magnets operate at a higher susceptibility. Néel's theory suggests that the oriented material is more stable than the isotropic material because larger volumes are involved in each magnetization reversal. To explain relatively large domain-like regions of reversal, we have to assume that magnetic regions cooperate during the magnetization reversal.