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Why do permanent magnets retain their magnetism after an external field is removed?

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Diagram showing a circular loop of wire with an electric current, surrounded by curved blue lines representing magnetic field lines that loop around the wire and pass through its center. — magnetism Some lines of the magnetic field B (blue) for an electric current I (gold) in a loop.

Permanent magnets retain their magnetism after an external field is removed due to a phenomenon known as remanence. This occurs because ferromagnetic materials are composed of small regions called magnetic domains, where atomic magnetic moments are aligned.12

When an external magnetic field is applied, these domains reorient themselves to align with the field. In materials suitable for permanent magnets, this reorientation involves irreversible movements of domain walls. These walls become "pinned" or obstructed by internal imperfections, such as impurities or crystalline discontinuities within the material. Because these obstacles prevent the domain walls from returning to their original, unmagnetized positions once the external field is removed, the material maintains a residual magnetization.12

The ability of a material to resist demagnetization is further influenced by magnetic anisotropy, which refers to preferred directions of magnetization within the crystal structure or shape of the material. A high coercive force—the strength of a reverse magnetic field required to reduce the magnetization to zero—is essential for a material to function as a stable, long-lasting permanent magnet.21