Which material becomes strongly magnetized in an external field and commonly forms permanent magnets?

A material that becomes strongly magnetized in an external field and can form a permanent magnet is a ferromagnetic material.

Ferromagnetism arises when magnetic moments in a material interact so that many of them align in the same direction. Regions called magnetic domains can become aligned, producing a strong net magnetization. Iron, cobalt, nickel, and some alloys are familiar ferromagnetic materials.

A ferromagnetic object does not always appear permanently magnetized because its domains may point in different directions. Applying a field can realign them. After the field is removed, some materials retain substantial magnetization; this property is called remanence and is useful in permanent magnets and data storage.

Ferromagnetism changes with temperature. Above a material’s Curie temperature, thermal motion disrupts long-range alignment and the material becomes paramagnetic. This is distinct from diamagnetism, a much weaker response found to some degree in all materials.