Magnetization
The process of becoming magnetized; also the property and in particular the extent of being magnetized. Magnetization has an effect on many of the physical properties of a substance. Among these are electrical resistance, specific heat, and elastic strain. See Magnetocaloric effect, Magnetoresistance, Magnetostriction
The magnetization M of a body is caused by circulating electric currents or by elementary atomic magnetic moments, and is defined as the magnetic moment per unit volume of such currents or moments. In the mks (SI) system of units, M is measured in webers per square meter. For M , 1 weber/m2 = 104/4&pgr; gauss.
The magnetic induction or magnetic flux density B is given by the equation
The topic of magnetization is generally restricted to materials exhibiting spontaneous magnetization, that is, magnetization in the absence of H . All such materials will be referred to as ferromagnets, including the special category of ferrimagnets. A ferromagnet is composed of an assemblage of spontaneously magnetized regions called domains. Within each domain, the elementary atomic magnetic moments are essentially aligned, that is, each domain may be envisioned as a small magnet. An unmagnetized ferromagnet is composed of numerous domains, oriented in some fashion.
The process of magnetization in an applied field H consists of growth of those domains oriented most nearly in the direction of H at the expense of others, followed by rotation of the direction of magnetization against anisotropy forces. See Ferromagnetism
On removal of the field H , some magnetization will remain, called the remanence M r.
Curves, sometimes called B-H curves, are used to describe magnetic materials. They are plotted with H as abscissa and with either M or B as ordinate. In the illustration, B r is the remanent induction ( B r = M r); H c is the coercive force, or reverse field required to bring the induction B back to zero; and M s is the saturation magnetization, or magnetization when all domains are aligned. The saturation magnetization is equal to the spontaneous magnetization of a single domain, except that it is possible to increase this magnetization slightly by application of an extremely large field. Saturation magnetization is temperature dependent, and disappears completely above the Curie temperature Tc where a ferromagnet changes into a paramagnet. See Curie temperature
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The irreversible nature of magnetization is shown most strikingly by the fact that the path of demagnetization does not retrace the path of magnetization—path 2 of the illustration does not retrace path 1. There is a tendency for the magnetization to show hysteresis, that is, to lag behind the applied field, and the loop of the illustration is called a hysteresis loop.
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