When a magnetic material such as magnetite or iron is placed in a magnetic field, it is magnetized, as shown in Figure 13.l.l, with the change in the applied magnetic field strength. Bulk material or powder is not magnetized originally unless it is exposed to a magnetic field, since the original directions of the magnetic dipoles in magnetic domains of bulk material or in powder are random. The magnetic moment of each domain results from the spontaneous alignment of electron spins due to superexchange interactions between electrons. Thus, the magnetization of magnetic solid is brought about by the orientation of magnetic dipoles. As the magnetic field increases, the degree of orientation of the dipole increases and saturation magnetization is attained at the perfect orientation of the dipoles. Once the magnetic material is magnetized, even though the magnetic field has decreased to zero, the material will have a certain magnetization, the so-called residual or remanent magnetization designated by Mr. A magnetic field of opposite direction is required to bring the magnetization to zero. This magnetic field is called coercive force, frequently desig-

H

Fig. 13.1.1 Magnetization curve.