A permanent magnet can lose some or all of its magnetic field under certain circumstances. When a permanent magnet is magnetized, its magnetic domains are aligned. Certain conditions can cause the domains to lose this alignment; thereby aligning randomly. This will cause the magnet to either have lost some or all of its magnetic field.
The primary method for deliberately de-magnetizing magnets is by heating them. Every magnetic material has a characteristic temperature known as the Curie Temperature. At this temperature the thermal agitations apply more force than the resistance of the magnetic domains to movement and the domains of the magnet randomize. After the material reaches the Curie temperature throughout its bulk, it will show virtually no net magnetization, and can be treated as virgin material.
In the case of samarium-cobalt permanent magnets, there is an additional factor to consider. The Curie temperature of most samarium-cobalt magnets is on the order of 700-800 °C (1300-1500 °F). At these temperatures, the material itself will tend to break down magnetically. After this process, the material's performance will be significantly degraded. Therefore, it is generally considered impractical to de-magnetize samarium-cobalt magnets.
Subjecting a magnet to a strong alternating magnetic field, which can be produced by using alternating current to power an electromagnet, will usually demagnetise it.
Hard impact can also demagnetise something, for example hitting it with a hammer.