Physics of Magnetism

80 CHAPTER 8. SOME BASIC CONCEPTS AND UNITS

Générale des Poids et Mesures. These units form a coherent system and are based on seven
basic units: meter, kilogram, second, ampere, kelvin, mole, and candela. The use of the
SI units has been recommended by the International Union of Pure and Applied Physics
(Cohen and Giacomo, 1987). In order to make easy contact with the enormous amount of
magnetic data published in the scientific literature during the years, many scientists still
use the older cgs–emu units even at present. For this reason, we have listed in Table 8.2
relationships and conversion factors between the SI and the older units.
The flux density B is not always a good measure to characterize a magnetic material
since we have seen that it may include contributions from external magnetic sources. The

zation M (in J

between

polarization

intrinsic properties of a given material are therefore always characterized by the magneti-

or the magnetic polarization (in T). The following relation exists

and

The flux density and field strength are related by the equation

Ferromagnetic materials are characterized by the presence of hysteresis loops. Examples
of such loops are shown in Fig. 8.2. In so-called soft-magnetic materials, the loops are very
narrow; in hard-magnetic materials the loops can be extremely broad. We will return to
these points in Chapters 12–14. Here, we will restrict ourselves to a comparison of different
types of representations of hysteresis loops.
Plots of B versus H and J versus H for a given ferromagnetic material are compared
in Fig. 8.2. Both quantities B and J become zero at sufficiently high negative fields, which
defines the corresponding coercive fields, indicated by and respectively. The
field where the magnetic polarization or the magnetization vanishes is often referred
to as the intrinsic coercivity. Many authors plot the magnetization M measured versus the
corresponding magnetic field strength by using the symbol B of the flux density for the
latter. In these particular cases, the flux density B is considered to represent an external
quantity not related to the material under investigation. It is obtained by applying Eq. (8.29)
to an empty measuring coil (J = 0) and leads to the relation where can
now be given in units of tesla. The advantage of this procedure is that field strengths given