Aerospace and military required magnets that operated at temperatures higher than 400 degrees Celsius. In response to this requirement, magnet manufacturers were developing a new class of ultra high temperature magnets for use at temperatures up to 550 degrees Celsius.
High temperature stability of permanent magnets has two implications, including low temperature coefficient of remanence and low irreversible flux loss of open circuit. For low αBr magnet, the varies of magnetic flux was little with the increase of temperature. For the latter, magnetic flux under room temperature and high temperature might vary widely, but both must change little with time. The Maximum Working Temperature Tw is belonging to the second implication, then have the strong connection as to whether magnets have high intrinsic coercivity and low temperature coefficient of intrinsic coercivity or not. The combination of these two parameters will ensure magnet to maintain high enough intrinsic coercivity under high temperature and avoid knee point appears in B-H curve. Reduce absolute value of αHcj can improve Tw effectively compared with the enhancement of intrinsic coercivity when the composition is basically determined.