Basis of magnetization and hysteresis

The changes in magnetization due to an applied magnetic field can be either reversible or irreversible. In reversible magnetization, the material can return to the original magnetic state upon removal of the field. Irreversible magnetization is dissipative as external restoring forces such as large magnetic fields are needed to bring it to its original state. When a ferromagnetic material is cooled below its Curie temperature, the magnetic moments become ordered within magnetic domains. Each domain has all its moments aligned parallel producing a spontaneous magnetization M_s . In the absence of magnetic field, the direction of M_s varies from domain to domain producing almost zero bulk magnetization in the material.

In the presence of small magnetic field, domains get oriented favorably with respect to the field and grow at the expense of the remaining domains. The main magnetization mechanism at this phase is domain wall motion. With increase in magnetic field, entire domains rotate to orient with the easy axis [Figure 29.1]. This produces a burst region in the M − H and ε − H curve in which small field changes produce fairly large magnetization or strain changes.

Figure 29.1 Basis of magnetostriction and effect of prestressing