Introduction

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, the designer requires the magnet's demagnetization curve, the second quadrant of the B versus 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. One should either avoid selecting a particular magnet type for an environment in which it will be exposed to conditions known to cause a permanent change, or provide protection for the magnet from this environment. Consider the case of alnico magnets, which, as described in Chapter 2, undergo a critical segregation of the ctl and a2 phases during their heat treatment between 550 and 650 ºC.