The rotation of magnets with different poles creates an alternating magnetic field. In aluminum, this creates an eddy current that heats it up. This is similar to the operation of an induction cooker, only there an alternating field is created electronically with a high frequency of 20-100 kHz. By the way, the magnets are kept cold!
https://en.wikipedia.org/wiki/Eddy_cu...
https://en.wikipedia.org/wiki/Inducti...
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