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How does hysteresis explain why a ferromagnet does not return to its original magnetization?

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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.

In physics, hysteresis describes the lagging of a material's magnetic flux density ( $B$ ) behind the applied magnetizing field ( $H$ ). A ferromagnet does not return to its original, unmagnetized state when the external field is removed because the magnetization process involves irreversible changes to the material's internal structure.123

Ferromagnetic materials are composed of small regions called domains, which are magnetized to saturation in specific directions. When an external field is applied, these domains reorient or their boundaries (domain walls) move to align with the field. In many cases, these domain wall movements are hindered by internal imperfections, impurities, or crystalline structures that act as obstacles.32

Once the magnetizing field drives a domain wall past such an obstacle, the wall cannot return to its original position even after the field is removed. This results in remanence—a residual, permanent magnetization. To return the material to a zero-magnetization state, a reverse magnetic field must be applied to provide the energy necessary to drive the domain walls back across these obstacles, a value known as the coercive force.231