ferromagnets

Ferromagnets are materials in which the magnetic moments of atoms tend to align parallel to each other due to the exchange interaction, leading to a large spontaneous magnetization below a characteristic temperature known as the Curie temperature. This parallel alignment arises from quantum mechanical interactions that favor parallel spins, and it can occur in a wide range of materials, including metallic elements such as iron, nickel, and cobalt, as well as certain alloys and permanent magnets.

In ferromagnets, the ordered moments are organized into magnetic domains, locally aligned regions separated by domain walls. In zero external field, the domains are oriented in different directions, so the net magnetization is often small. Application of a magnetic field tends to enlarge domains aligned with the field, increasing the net magnetization. The response forms a hysteresis loop when the field is cycled, and some magnetization remains when the field is removed (remanence). The Curie temperature varies among materials (iron about 1043 K, nickel about 627 K, cobalt about 1123 K): above these temperatures, thermal fluctuations destroy long-range order and the material becomes paramagnetic.

Ferromagnets underpin many technologies, including transformer cores, electric motors, magnetic storage media, and permanent magnets such as NdFeB and SmCo alloys. The phenomenon is distinct from ferrimagnetism and antiferromagnetism, which involve different patterns of moment alignment but can share similar microscopic origins in the exchange interaction.