An investigation of the aging of thermally prestabilized sintered samarium-cobalt magnets

The effects of long-term air exposure at 25°, 150°, 200° and 250° C on the stability of the open-circuit remanence at were studied. Magnets were prestabilized by heating at 150°-250° before room temperature aging, 50° above exposure level for the elevated-temperature tests, then they were exposed for periods of up to 8000 hours. Loss data for different operating temperatures and preheating conditions are reported. Preheating anticipates initial aging by thermally activated wall motion. This reduces aging losses at 25°C at least by a factor 2 to 4, at 150° by a factor 5-8. At operating temperatures above 200° the medium-term stability is strongly improved, but catastrophic losses set in after several 100 to 1000 hours. Prestabilized magnets show more uniform aging behavior than lots of samples tested without preheating. No significant differences were found between diepressed and isopressed magnets. Heating to at least 200°C and at least 50° above the use level for 2 hours is recommended for prestabilization.