All materials are defined as being magnetic in that they respond to the application of an applied magnetic field differently to that of air or vacuum. Only selected elements or alloys have useful magnetic properties in engineering applications. Magnetic materials which are easily magnetized and the magnetized are often termed soft. Conversely, those magnetic materials where any induced magnetism is difficult to remove are termed hard or permanent.
When a practical or highly permeable material is influenced by an external magnetic field it may acquire a large magnetization or magnetic induction. The level of the magnetization will be related to the individual intrinsic permeability of the material in question. The relationship between the external field in the induction is:
B = µH
where B is the magnetic induction and where µ is the permeability and where H is the external magnetic field.
In the S. I. system of units, B is defined in terms of the tesla (T) and the magneticfield H in ampere per meter (A/m).
When the external field, H and induction, B are identical (i.e. in a vacuum) the -7 permeability µ is exactly 4π x 10 in units of henry per meter for the equation to balance. It is termed µo . Hence:
B = µo H
In strongly magnetic materials the relationship is:
B = µo µr H
where µr is the relative permeability of the material.
B is defined as the total induction. It will comprise that from the externally applied field added to that of the strong internal magnetization. Hence the following relationship exists:
B = µo H + ȷ
where ȷ is the intrinsic magnetization of the material, also in tesla (T).
When magnetizing or measuring permanent magnets with high coercivity µo H as can be seen, ȷ has large significance. ȷ will define the ultimate stability of the magnet since it influences the coercivity Hcb and hence the linearity of a magnet. The temperature coefficient of the intrinsic coercivity Hcȷ is most often used with the temperature coefficient for remanence Br to describe the stability of a material.
In soft magnetic materials µo H is small compared to B (which is much higher than in permanent magnets). In the case of most engineering applications the hysteresis loop for ȷ vs H is practically identical to that for B vs H. In extremely high excitation fields ȷ and B are separated but this would not be expected in conventional electromagnetic devices.