One can demagnetize a magnet in several different ways. One would be to hit both ends of the magnet using a hammer.

This causes the ordering in the magnet to be destroyed. Another is to rub one magnet with another one. This should demagnetize one of the magnets.

The following are some methods to demagnetize a magnet :

• Heating a magnet: Due to heat energy the kinetic energy of molecules increases, hence magnetism is lost.
• Keep the magnet in a coil: Pass alternating current through the coil. Pull the magnet far away out of the coil in the east–west direction. When the alternating current rapidly changes the direction in the copper coil, the polarity set up in the coil also rapidly changes which in turn acts inductively on the bar magnet, whose molecular magnets rapidly try to align themselves with changing magnetic polarity. Thus, molecular magnets form closed chains and hence bar magnet get demagnetized.
• By Induction: When a given magnet is placed in contact with another similar magnet such that their similar poles are facing each other then both the magnets get demagnetized in a couple of days. It is because both magnets will induce opposite polarity on each other. In doing so the molecular magnets in each magnet form closed molecular chains and hence get demagnetized.
• By Self–Induction: A single bar magnet has a tendency to lose its magnetism. The molecular magnets (dipoles) are arranged in straight line chains. However, two or more parallel chains have their north and south poles facing each other. Thus dipoles act inductively on each other and hence turn to form closed molecular chains. Thus, single bar magnet gets demagnetized.

MORE INFO