magnetization retained as magnetic minerals cool through blocking temperatures in an ambient field. Subsequent alteration, reheating and remagnetization can change the record.
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Thermoremanent magnetization (TRM) is a type of magnetization that occurs in certain materials, particularly ferromagnetic and ferrimagnetic minerals, as they cool through their Curie temperature (the temperature above which a material loses its permanent magnetic properties). When these materials are heated to a high temperature and then cooled down in the presence of an external magnetic field, they acquire a permanent magnetization that is aligned with that field.