Relation between coercive force and microstructure of sintered SmCo5 permanent magnets

Abstract

The coercive force of a sintered SmCo5 permanent magnet is found to depend reproducibly on the temperature of heat treatment. In order to find out the origin of this, the mechanism of the demagnetization was studied by magnetic-domain observation, using the polar Kerr effect. It appears that the highest coercive forces which are obtained by heat treating at 850–950 °C and quenching are due to difficult nucleation of reverse domains and strong pinning of domain walls by the grain boundaries. After quenching from higher temperature (∼1100 °C) the coercive force decreases owing to weaker pinning of the domain walls. The drop of the coercive force which is found to occur upon heating at lower temperature (∼700 °C) comes from an easy nucleation of reverse domains. Indirect evidence is presented which supports the assumption that this easy reverse-domain nucleation occurs on Sm2Co17 precipitates. The Sm2Co17 precipitation is governed by the Co-rich SmCo5 phase boundary and its metastable extension below the eutectoid temperature of SmCo5. The state of the grain boundary which may be responsible for the strong pinning is discussed.