ferromagnetic

Ferromagnetism is a form of magnetism in which certain materials exhibit spontaneous alignment of magnetic moments, producing a strong, enduring magnetization even without an external field. It arises from quantum mechanical exchange interactions that favor parallel alignment of electron spins on neighboring atoms, creating a cooperative state that can extend across regions called magnetic domains. Within a domain, the moments add to a net magnetization, and the overall magnetization depends on how domains polarize and how domain walls respond to external influences.

The behavior is temperature dependent. Below a characteristic Curie temperature, ferromagnetism is stable and materials can retain magnetization after the external field is removed. Above Tc thermal agitation disrupts long-range order, and the material becomes paramagnetic. The phenomenon is often described by models such as the Weiss molecular field. When a magnetic field is applied, domain walls move and domains reorient, producing a magnetization curve that exhibits hysteresis, with remanent magnetization and coercivity.

Common ferromagnets include iron, nickel, cobalt, and various alloys and compounds. Some materials have high Curie temperatures and strong magnetic anisotropy, making them suitable for permanent magnets, such as NdFeB and SmCo alloys. Others, like gadolinium, are ferromagnetic only near room temperature or below. Key properties include saturation magnetization, magnetic anisotropy, and coercivity, which determine suitability for different applications.

Applications span permanent magnets for electric motors and generators, transformers, magnetic recording media, sensors, and other electronic components. Ferromagnetism combines quantum exchange physics with material science to explain and exploit persistent magnetization.