Coercive magnetic field

In order to completely rid the material of its remnant magnetization, the polarity of the magnetizing field has to be reversed. The value of H at which M goes to zero is called the coercive magnetic field intensity (Fig. 15.8 ). 

Fig. 7. Thermomagnetic recording, (a) A focused laser beam generates a thermal profile in the magnetic layer, (b) The coercive force in the layer is reduced and its magnetisation can be reversed by a small magnetic field, here 30 kA/m. At room temperature, the coercive force is high and the written domains are...

If a very high field is appHed the magnetisation can reach its saturated state ia which all the magnetic dipoles are aligned ia the direction of the field. If the magnetic field is switched off, the remanent magnetisation M is left. If the M (or B) is then reduced to sero, a special field strength, the coercivity, is required. 

Strength H r is needed to remove the remanence. He, the magnetic field at which Js becomes zero, is called coercive force or coercivity. Each type of magnetism shown in Figure 6.8 is associated with characteristic features which are displayed by the different Fe oxides. 

FIGURE 11.9 Plot of the magnetic field needed to demagnetise ferromagnetic particles (coercivity, //c) as a function of particle size. The particle becomes single domain at radius Dc. 

Table 36 shows some quality requirements for the most important applications of magnetic pigments. Column 4 gives the coercive field strength (Hc) required for information storage materials. The coercive field is the magnetic field required to demagnetize the sample. 

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