Soft Magnetic and Amorphous Steels

soft magnetic alloys are materials that are amenable to easy magnetisation and demagnetization.
Soft magnetic materials therefore are typically characterized by an intrinsic coercivity below 1000 Am-1. They are used primarily to enhance and/or channel the flux produced by an electric current. The main parameter which often serves as a figure of merit for soft magnetic materials, is the relative permeability ( mr, where mr = B/moH), which is a measure of how readily the material responds to the applied magnetic field. The other main parameters of interest are the coercivity, the saturation magnetisation, magneto-resistivity eddy current structure and losses (as a function of sample dimensions), heat conductivity and the electrical conductivity.
The types of applications for soft magnetic materials fall into two main categories, of namely, alternating current (AC) and direct current (DC) applications.

In DC soft magnetic applications the material is magnetised in order to perform a specific task and then demagnetised at the end of that operation, such as switchable electromagnets. 

In AC applications the soft magnetic material will be continuously cycled, often at rather high frequencies, from being magnetised in one direction to the other, throughout the entire period of operation, e.g. a power supply transformer. A high permeability will be desirable for each type of application but the significance of the other properties varies.

For DC applications the main consideration for material selection is most likely to be the permeability. This would be the case, for example, in shielding applications where the flux must be channelled through the material. Where the material is used to generate a magnetic field or to create a force then the saturation magnetisation may also be significant.
For AC applications the important consideration is how much energy is lost in the system as the material is permanently and a very high frequencies cycled around its hysteresis loop. The energy loss can originate from three different sources: 

1. Hysteresis loss, which is related to the area contained within the hysteresis loop; 

2. Eddy current loss, which is related to the generation of electric currents in the magnetic material and the associated resistive losses and 

3. Anomalous loss, which is related to the movement of the magnetic domain walls (Bloch walls) within the material.

Hysteresis losses can be reduced by the reduction of the intrinsic coercivity, with a consequent reduction in the area contained within the hysteresis loop. Eddy current losses can be reduced by decreasing the electrical conductivity of the material and by laminating the material, which has an influence on overall conductivity and is important because of skin effects at higher frequency. Finally, the anomalous losses can be reduced by having a completely homogeneous material, within which there will be no hindrance to the motion of domain walls.