Two distinct characteristics of ferromagnetic materials are (1) their spontaneous magnetization and (2) the existence of magnetic ordering temperature. Spontaneous magnetization is the net magnetization that exists inside a uniformly magnetized microscopic volume even in the absence of external magnetic field. The magnitude of this magnetization at absolute temperature depends on the spin magnetic moments of electrons. Although electronic exchange forces in FMs are very large, thermal energy eventually overcomes the exchange energy and produces a randomizing effect leading to paramagnetism. This occurs at a particular temperature called the Curie temperature (T_C). Below T_C, the moments are ordered (ferromagnetic) and above it, disordered (paramagnetic). The saturation magnetization goes close to zero above T_C.

Figure 1.4: Magnetic hysteresis loop of a ferromagnetic material

Magnetic Hysteresis behavior:

All FM materials exhibit magnetic hysteresis behavior, i.e., the formation of loop under the application of magnetic field (M - H loop) (see Figure 1.4). A great deal of information about the magnetic properties of a FM material can be learned by studying its hysteresis loop. The loop is generated by measuring the magnetic flux of a FM material while the magnetizing force is changed continuously. FM materials that have never been exposed to magnetic field or have been thoroughly demagnetized follow the dashed line as applied field is increased and reach the point 'a' where almost all of the magnetic domains are aligned and an additional increase in the magnetizing field produces a very little or no increase in magnetic flux. The material has almost reached the point of magnetic saturation. When the field is reduced to zero, the curve moves from point 'a ' to 'b '.