6 - Magnetizing and testing

Summary

Introduction

In previous Chapters, transfer of energy to and from a permanent magnet, and associated changes in energy of the external field have been discussed. For initial magnetization, the objective is to apply sufficient energy to the material to align its internal magnetization vectors Min a unique direction, for which the magnet is said to saturate at M_sat. Optimum performance is achieved along this preferred axis, but the characteristics quantifying the material's properties have to be measured outside the magnet. The internal parameter M cannot in fact be measured directly, and although the intrinsic curve is the more fundamental characteristic of a magnet, it must be deduced from an external measurement of the normal B versus H loop.

This example illustrates a problem, which is commonly encountered by users of permanent magnets. Whereas they design a device to operate in a certain manner, it later appears that the real magnet does not meet these expectations. Later in this Chapter, we discuss various techniques that are used for measuring magnetic parameters - their application to the properties of magnets themselves will provide the basis for quality control. Unfortunately, the magnitude of applied field that is required to saturate a particular magnet is a somewhat empirical quantity.