The NDT Resource Center describes exposing a magnet to a reversing and decreasing magnetic field as one way to remove its magnetic properties. This is not the most effective method of demagnetizing a magnet, but it is far simpler to perform than the most effective method, which involves heating the magnetic material to a high temperature. Exposing a magnet to a reversing magnetic field demagnetizes it almost completely.
In order to demagnetize a magnet without the struggle of heating it to 770 C or greater, the NDS Resource Center recommends pulling the magnet out and away from a coil through which alternating current is passing. After this process has been completed, a field meter can be used to check that the magnet is no longer emitting magnetic flux. If the flux is below 3 gauss, most industry standards consider the magnet to be demagnetized.
According to Goudsmit Magnetic Systems, the effectiveness of the process of demagnetizing a magnet using a magnetic field is dependent on several factors. One of these is magnetic field strength. The harder and stronger the magnet, the stronger the current (and magnetic field) required to demagnetize it. The placement of a magnet in a basket that carries it through the coil also affects the demagnetization process. If the basket shields parts of the magnet, the magnet is not always thoroughly demagnetized.