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. Thermal demagnetization 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. Source: