Introduction

Earlier texts on permanent magnets have opened with historical reviews of these materials (Hadfield, 1962; McCaig, 1977; Parker, 1990; Parker and Studders, 1962). In this book the design of modern permanent magnets is emphasized, and so initially the development of the relationships that are required to model today's materials for a variety of common applications is considered. To the extent that a historical review is provided in this chapter, it is of those fundamental equations of electromagnetism that are needed to understand the performance of magnets in circuits and devices.

There are many properties of a permanent magnet that are considered in its design for a magnetic device, but most often it is the demagnetization curve that initially determines its suitability for the task. Its shape contains information on how the magnet will behave under static and dynamic operating conditions, and in this sense the material characteristic will constrain what can be achieved in the device design.

The B versus H loop of any permanent magnet has some portions which are almost linear, and others that are highly non-linear. The shapes of these B versus H loops, or at least the demagnetization portions of them, tell the designer a lot about the suitability of the material for a given application. A brief derivation of the B versus H loop is presented, to illustrate the microscopic mechanisms that determine the macroscopic performance of a magnet.