Effects of the composition, sintering temperature and heat treating temperature on magnetic properties of permanent magnets were investigated. An attempt was made to correlate the effects on magnetic properties with the microstructures of sintered $SmCo_5$ magnets.
The heat treating temperature range studied was between 700 and 1,100℃ for 1 hr in purified argon atmosphere. $SmCo_5$ magnets heat treated at 900℃ and quenched exhibited the highest coercive force. This is believed to be due to the difficulty in nucleation of reverse domains without the $Sm_2Co_{17}$ phase in the microstructure of the magnets and strong pinning of domain walls at grain boundaries where oxygen is segregated. The coercive force of the magnets decreased as the heat treating temperature was raised to higher temperatures (＞900℃). This is attributed to the weaker pinning of domain walls at grain boundaries. The $SmCo_5$ magnets heat treated at lower temperatures (＜900℃) exhibited lower coercive force values than those exhibited by the magnets heat treated at 900℃. This lower coercive values are due to easier nucleation of reverse domains at the $Sm_2Co_{17}$ phase that is formed as an eutectoid decomposition product below 800℃.
The coercive force and shrinkage of $SmCo_5$ magnets were measured, varying their total Sm content that ranges between 35 and 38 wt%. Magnets whose total Sm content was 36 wt% exhibited the highest coercive force and also the greatest shrinkage. The $SmCo_5$ magnets with 36 wt% Sm content are of a hyperstoichiometric composition at which Co vacancy concentration is the maximum. The highest coercive force exhibited by the magnets containing 36 wt% Sm is attributed to the fact that the more the Co vacancy exists, the more the segregation of oxygen occurs at grain boundaries and the stronger the domain wall pinning is.
The coercive force decreased as the sintering temperature was raised to higher temperatures. On the other hand, the density increased as the sintering temperature was raised. This decrease of the coercive force is attributed to the grain growth and ripening of inclusions and pores. Since new grain boundaries are also thought to contain fewer inclusions and pores, domain wall pinning at new grain boundaries become weaker and this results in a lower coercive force.
