How to Magnetize and Demagnetize a Permanent Magnet, respectively?
Due to random orientation for micro magnetic domains, a permanent magnet usually does not provide any magnetic flux when it is produced. It needs to be magnetized to saturation for use.
So how to magnetize a permanent magnet? The basic principle is using a coil, i.e. an electromagnet, to generate a magnetic field. The generated magnetic field increases as the charging current increases, it drives micro magnetic domains of a permanent magnet rotate to the magnetization direction. When all the micro magnetic domains aligned along the same direction, the permanent magnet is magnetized to the saturation state.
As the above magnetizing process has been acknowledged in view of micro magnetic domains, it is easy to understand the opposite process if the permanent magnet needs to be demagnetized. Just alternate the charging current direction through the coil to generate a reverse magnetic field. The reverse magnetic field or demagnetizing field increases as the charging current increases, and again more and more micro magnetic domains rotate to the demagnetization direction. When the reverse magnetic field reaches the intrinsic coercivity (Hcj) point, the permanent magnet is thoroughly demagnetized.
Besides the former field demagnetization, there is another way to demagnetize a permanent magnetic material by increasing temperature which is called thermal demagnetization. In the latter way, the aligned micro magnetic domains become unstable as the temperature increases. When the temperature excesses its Curie temperature (Tc), the whole micro magnetic domains turn to a random orientation state so that the macro magnetic moment is zero, in other words, the permanent magnetic material is thoroughly demagnetized.