How do magnetic domains become aligned in an iron electromagnet?

In an iron electromagnet, magnetic domains—small regions where atomic magnetic moments are spontaneously aligned—become aligned through the application of an external magnetizing field.

In an unmagnetized state, these domains are oriented randomly, resulting in no net magnetization for the material as a whole. When an external field is applied, the alignment process occurs in three primary stages:123

• Low magnetizing field: Domains already oriented in the general direction of the field grow at the expense of those that are unfavorably oriented through the reversible movement of 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: Remaining domain walls are swept out of the specimen, and the magnetization within the domains rotates until it is parallel to the external field, reaching a state of saturation.14

Once the external field is removed, the material may retain some remanent magnetization depending on the material's properties and the extent to which domain walls were moved or "locked" by internal imperfections.13