Initial Irreversible Losses and Enhanced High-Temperature Performance of Rare-Earth Permanent Magnets

There is an increasing demand for permanent magnets with high-temperature stability. Normally, the magnets are heat treated above the operating temperature range to saturate the irreversible losses, thereby producing magnets that are stable, but with significantly degraded performance. In addition, uncertainty about the losses in differently shaped magnets causes difficulties for efficient magnetic circuit design. The variation of the initial irreversible losses with magnet shape, hysteresis loop, and operating temperature is presented here. Losses are not associated with thermal demagnetization of the whole magnet, but the damage is concentrated in an outer surface “skin” at the pole surfaces, where the demagnetizing field is nonuniform. The initial irreversible remanence loss at room temperature (Formula presented.) can be predicted as the product of the slope of the high-temperature magnetization curve k(T) and the effective demagnetizing factor N. Heat treatment with the vulnerable surfaces in contact with soft iron slabs moves the skin out of the permanent magnet and reduces the irreversible remanence loss by 30% or more, potentially improving the performance of permanent-magnet motors, generators, bearings, and thrusters. The results deepen the understanding of the magnetism of irreversible losses and will guide the applications of permanent magnets at higher temperatures.