When a magnetic field acts on a demagnetized ferromagnetic body, the component of the macroscopic magnetization along the field direction increases non-linearly. In general, the increase takes place with a low initial slope, a drastic increase of this slope at intermediate fields, and a new decrease at higher fields up to the technical saturation (total saturation requires 0 K or applied fields with strength tending to infinity). When the field is removed, the macroscopic magnetization decreases along a curve located well above the initial magnetization curve. For zero field a noticeable magnetization generally remains that is called remanence or remanent magnetization, M _r. After reversal of the field, the magnetization drops and vanishes for a reversal field known as coercive field, H _c. Further increase of the field leads to the saturation. When the field is cyclically applied, the magnetization curve is a hysteresis loop as that shown in Fig. 1. If the maximum applied field is not high enough to get saturation, the loop is called a minor loop.

Figure 1: (a) Hysteresis loop and initial magnetization curve showing some characteristic regions: the Rayleigh region, minor loops at the irreversible region and the approach to saturation region. (b) Magnetic energy stored in a ferromagnetic material after being magnetized up to M _f.

The coercive field permits the scaling of the applied fields as weak, lower than H _c, intermediate, those of the order of H _c, and high fields, stronger than H