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How does an external magnetic field align domains in a ferromagnetic material?

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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 a ferromagnetic material, the magnetization process occurs through the interaction of an external magnetic field with the material's internal domain structure. A ferromagnetic solid is composed of many small regions called domains, each magnetized to saturation but randomly oriented, resulting in no net magnetism for the material as a whole.12

When an external magnetic field is applied, the alignment of these domains proceeds in three stages:

Low magnetizing field: Domains oriented favorably toward the external field grow at the expense of those that are unfavorably oriented through reversible movements of the domain walls.1
Medium magnetizing field: Larger, often irreversible movements of domain walls occur, significantly increasing the volume of favorably oriented domains.1
High magnetizing field: Domain walls are swept out of the specimen, and the magnetization directions of the remaining domains rotate until they are parallel to the external field, reaching a state of saturation.12

This process is hindered by factors such as hysteresis, material imperfections, and magnetic anisotropy, which influence how easily domains can move or rotate.1