Demagnetisation is the process of removing magnetic properties of a magnet.
Methods of demagnetisation :
1) All magnets have a temperature called the curie temperature at which they lose their magnetism. Thus, the magnetic property of a magnet can be removed by heating it on its curie point.
2) Some magnets can be quickly demagnetised by placing them in a coil of wire connected to an alternating electric current source.
3) Magnets can also be demagnetised by hammering or dropping the magnets (from height) many times on the floor.