20.1 Magnetic Materials

All materials have magnetic properties. These characteristic properties may be divided into five groups: diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic and ferrimagnetic. Only the ferromagnetic and ferrimagnetic materials have properties which are useful in practical applications.

Ferromagnetic properties are confined almost entirely to iron, nickel and cobalt in their alloys. The only exceptions are some alloys of manganese and some of the rare earth elements. Of these gadolinium has the highest Curie temperature, about 16 C (see the Appendix for definitions and conversion factors). Some of the magnetic properties of the ferromagnetic elements including the rare earths are given in Tables 20.1 and 20.2. Ferrimagnetism is the magnetism of the oxides of the ferromagnetic elements. These are variously called ferrites and garnets. The basic ferrite is magnetite (Fe ₃O ₄) which can be written as FeO Fe ₂O ₃. By substituting for the FeO with other divalent oxides a wide range of compounds with useful properties can be produced. The main advantage of these materials is that they have high electrical resistivity which minimises eddy currents when they are used at high frequencies.