Uses of ferromagnetic materials
(i) Permanent magnets
The ideal material for making permanent magnets should possess high retentivity (residual magnetism) and high coercivity so that the magnetisation lasts for a longer time. Examples of such substances are steel and alnico (an alloy of Al, Ni and Co).
The material used for making an electro-magnet has to undergo cyclic changes. Therefore. the ideal material for making an electromagnet has to be one which has the least hysteresis loss. Moreover. the material — should attain high values of magnetic induction B at low values of magnetising field H. Soft iron is preferred for making electromagnets as it has a thin hysteresis loop (Figure) [small area, therefore less hysteresis loss] and low retentivity. It attains high values of B at low values of magnetising field H.
(iii) The core of the transformer
A material used for making transformer core and choke is subjected to cyclic changes very rapidly. Also, the material must have a large value of magnetic induction B. Therefore, soft iron that has a thin and tall hysteresis loop is preferred. Some alloys with low hysteresis loss are radio-metals, pern-alloy and mumetal.
(iv) Magnetic tapes and memory store
The magnetization of a magnet depends not only on the magnetizing field but also on the cycle of magnetization it has undergone. Thus, the value of magnetization of the specimen is a record of the cycles of magnetization it has undergone. Therefore, such a system can act as a device for storing memory.
Ferro magnetic materials are used for coating magnetic tapes in a cassette player and for building a memory store in a modern computer.
Examples : Ferrites (Fe, Fe2O, MnFe2O4 etc.).