Abstract
Saturation magnetization and coercive force of Alnico 5 and other magnet alloys with similar compositions are measured at elevated temperatures by means of an astatic magnetometer. The I-T curves obtained reveal that these alloys consist of two kinds of ferromagnetic phases, α1 and α2, after appropriate heat-treatment. It is found that Curie point of the less ferromagnetic phase α2 changes considerably with aging temperature. These results are also discussed from the standpoint of an equilibrium diagram and the phase relation. Each of the Hc-T curves obtained shows a striking maximum at a certain temperature, which corresponds to the Curie point of the α2 phase contained in the alloy. Particularly for a commercial alloy of Alnico 5 aged at about 600°C, the Curie point of the α2 phase lies in the neighbourhood of room temperature, so that a maximum value of the coercive force can conveniently be obtained at room temperature. Furthermore, a theoretical approach is proposed to estimate the coercive force for such a two-phase alloy. This may satisfactorily explain the characteristic feature of the observed Hc–T curve, and may elucidate that the coercive force of Alnico 5 largely depends upon the magnetization of the less ferromagnetic matrix.
