methods of demagnetisation of a magnet?

Answer 1

Heat the magnet, and strike it with something. It rearranges the domains in the magnet, causingt them to be jumbled

Answer 2

Demagnetisation Methods

Answer 3

One way is to have the low voltage current flow through a solenoid. Place the magnet in the solenoid.

You can do it in at least three ways:

heating
hammering
as suggested in the pervious answer, using an AC (alternating current) field
All of the above act to randomize the unit magnets or magnetic dipoles in the magnet.

Magnetism is a result of the alignment of magnetic dipoles in material into a general direction. (refer to the domain theory). In general, a piece of iron, for example, would not be magnetic unless on average we find that the magnetic dipoles or unit magnets are aligned in some direction. Hence, to demagnetize a magnet what you need to do is to cause disorder to the unit magnets, randomizing them.

In the case of heating, energy is provide to the magnetic dipoles causing them to free themselves from the initial order, thus destroying or reducing the ordering of the magnetic dipoles. This causes the magnetic poles to point in different direction in space. So on average along any direction you have very little magnetic poles alignment. Since magnetism arise from such alignment, in this case we have very little or no magnetization. Similarly when we hammer or drop the magnet many times on the floor, the vibration induced on the magnet causes the magnetic dipoles to randomize.

The last method uses a magnetic field generated by a current flowing through a wire to change the ordering of the magnetic dipole. An AC current is used to produce magnetic field of changing direction near the magnet. The (electro-)magnetic field caused the magnetic dipoles to switch direction many times. Each time the field point to one direction some magnetic dipoles will try to align along that direction. If the field is big enough, many will align along that direction. When the field is reduced and reversed not all these magnetic poles will reverse. Hence, by repeating this process many time (that's why an AC current is needed) and also by reducing the magnitude of the current as we go, to reduce the strength of the (electro-)magnetic field, the magnetism of the magnet can be reduced to almost zero or perhaps for most practical purpose zero.

Answer 4

dropping it on the ground