Ferromagnetic materials acquire their magnetic properties not only because their atoms carry magnetic moments, but also because the materials are composed of small regions called magnetic domains. In each domain, all atomic dipoles are connected together in a preferred direction. This alignment develops as the material develops its crystal structure during solidification from the molten state. A magnetic force microscope (MFM) can be used, and as shown below, an image of a structural domain can be constructed to detect magnetic domains.
A magnetic force microscope (MFM) image showing the magnetic domains of a heat-treated carbon steel.
During the solidification process, one trillion or more atoms are arranged in parallel at a moment, making the magnetic force in the domain strong in one direction. The ferromagnetic materials are characterized by “spontaneous magnetization” because they obtain the saturation magnetization in each domain without external magnetic field application. Even if the domains are magnetically saturated, loose materials may not show any signs of magnetism because the domains develop themselves and randomly oriented relative to each other.
When a ferromagnetic material is magnetized, the magnetic domains within the material are oriented. This can be done by placing the material in an external strong magnetic field or by passing current through the material. Part or all of some fields can be aligned. The multiple domains are aligned, the material in a strong magnetic field. When all the domains are aligned, the material is considered to be magnetically saturated. When the magnetic saturation of a material, no additional amount of external magnetizing force will result in an increase in the level of its internal magnetization.