How do magnetic domains make a piece of iron become magnetized?

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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.

A piece of iron is composed of many small regions called ferromagnetic domains, each magnetized to saturation. In an unmagnetized state, the directions of these individual domains are distributed at random, causing the material as a whole to appear unmagnetized.1

When an external magnetic field is applied, the iron becomes magnetized through three primary stages involving the movement of domain walls:

Low field: Domains oriented in the direction of the applied field grow at the expense of those unfavorably oriented. These wall movements are reversible.1
Medium field: Larger, often irreversible movements of domain walls occur, significantly increasing the volume of favorably oriented domains. This results in remanent magnetization once the field is removed.1
High field: Domain walls are swept out of the specimen, and the magnetization of the remaining domains rotates until it is parallel to the applied field, reaching saturation.1

The ease of this process depends on material purity; imperfections act as obstacles to domain wall movement, which can increase the material's coercive force and remanence.1