A material is classified as ferromagnetic primarily because it contains atoms with permanent magnetic moments due to unpaired electron spins in partially filled electron shells. These atomic magnetic moments arise mainly from the quantum mechanical spin of electrons and, to a lesser extent, from their orbital motion around the nucleus
. The key factors causing ferromagnetism are:
- Presence of unpaired electrons: Atoms must have partially filled electron shells with unpaired spins, which produce a net magnetic dipole moment. Fully filled shells have paired electrons whose opposite spins cancel out magnetic moments, so such atoms are not ferromagnetic
- Strong quantum mechanical exchange interaction: Neighboring atomic magnetic moments interact via quantum mechanical effects that favor parallel alignment of spins. This lowers the system’s energy compared to antiparallel spin arrangements, allowing spontaneous alignment of atomic moments even without an external magnetic field
- Formation of magnetic domains: In ferromagnetic materials, billions of atomic moments align parallel within small regions called magnetic domains. Although domains may be randomly oriented in an unmagnetized state, applying a magnetic field or other magnetizing processes aligns these domains, resulting in a strong net magnetization and the ability to form permanent magnets
- High magnetic permeability and coercivity: Ferromagnetic materials exhibit a large positive magnetic susceptibility and permeability, meaning they strongly amplify and retain magnetic fields. This allows them to be strongly attracted to magnets and to maintain magnetization after removal of an external field
In summary, a material is ferromagnetic because its atoms have permanent magnetic moments from unpaired electron spins, and these moments strongly interact to align parallel spontaneously, creating magnetic domains that produce a large, stable macroscopic magnetization
. Common ferromagnetic materials include iron, cobalt, nickel, and their alloys
