HARD AND SOFT FERROMAGNETIC SUBSTANCES

The substances with large retentivity are called hard ferromagnetic substances which are used in producing permanent magnets. The hysteresis cycle for such substances is broad. Alnico ( an alloy of Al, Ni, Co and Cu ) is a hard ferromagnetic material. Hence permanent magnets are made using Alnico. The substances with small retentivity, i.e., with narrow hysteresis cycle, are called soft ferromagnetic substances, e.g., soft iron. Such materials are used for making electromagnets.

Permanent Magnets and Electromagnets: -
The ferromagnetic substances which retain magnetism for a long time at room temperature are called permanent magnets. 400 years back, iron rods fixed in north-south direction were tapped using a hammer in order to prepare a magnet. Another way is to continuously run one end of a magnet on a fixed steel rod always in one direction. The steel rod then acquires permanent magnetism. When current is passed through a solenoid containing a steel rod, then the rod acquires permanent magnetism. Steel or hard iron or hard alloys or iron like alnico are considered best for this purpose. Soft iron has large permeability  and small retentivity and hence is suitable for making electromagnets. When current is passed through a solenoid containing a rod of soft iron, magnetic field in it increases many times making it an electromagnet. On switching off the current, magnetic field more or less vanishes due to small retentivity of soft iron. Thus, the soft iron rod attains magnetism as long as there is current in the solenoid. Electromagnets are used in electric bells, loud-speakers and telephone receivers. Huge  electromagnets are used in cranes to lift heavy things made of iron.