Asked by: Barney Collins, by email
So-called permanent magnets are constructed from materials made up of magnetic domains, in which atoms have electrons whose spins are aligned with each other. This alignment is damaged over time, principally as the result of heat and stray electromagnetic fields, and this weakens the level of magnetism. The process is very slow, however: a modern samarium-cobalt magnet takes around 700 years to lose half its strength.
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