The design and application of high energy rare earth permanent magnets

When a designer specifies the use of a permanent magnet, he certainly hopes that its magnetization will indeed remain permanent, or at least a close approximation to this. Specifically, he requires the magnet´s demagnetization curve, the second quadrant of the B vs. H characteristic, to remain unchanged under normal operating conditions. Unfortunately, this is never the case, so it is important to understand the nature of the changes that may occur, so that any degradation of the magnetic properties reflected in the demagnetization curve may be accounted for in the design. Changes in a magnet after it has been manufactured and fully magnetized may be caused by any combination of external influences, such as temperature, pressure and applied field. These changes fall into three categories. The first category comprises those effects that result in a permanent change in the demagnetization curve, which persist even if the magnet is fully remagnetized. The other two categories are “irreversible” changes which persist even after the cause has been removed, but the original demagnetization curve can be restored by fully remagnetizing the material. Depending upon the shape of the demagnetization curve, an externally applied field may cause the magnet to operate on an “internal” curve with a lower (BH)_max. The other principal cause of irreversible change in the demagnetization curve is thermal fluctuations