Ferromagnetism

Ferromagnetism is magnetic order in which neighboring atomic moments align, producing spontaneous magnetization that can persist after an external field is removed.

Exchange interaction: A quantum-mechanical interaction that favors particular relative orientations of neighboring spins. Its energy favors parallel neighboring spins in common ferromagnets.
Electron spin: An intrinsic quantum property of electrons that contributes to angular momentum and magnetic moment. Electron spin supplies much of the moment that orders in ferromagnets.
Antiferromagnetism: Magnetic order in which neighboring moments align oppositely, usually canceling the net magnetization. It has ordered spins but typically no spontaneous bulk magnetization.
Permanent magnet: An object made from material that maintains a persistent magnetic field without continuous electrical power. Hard ferromagnets retain strong magnetization after the magnetizing field is removed.
Wilhelm Eduard Weber: A German physicist whose work included early theories of molecular magnetism. His molecular-current model was an early attempt to explain persistent magnetism.
Magnetic domain: A region in a magnetic material where magnetic moments share a common orientation. Ferromagnets divide into domains to reduce magnetic energy.
Magnetic moment: A vector quantity that measures the strength and orientation of a source of magnetism. Ferromagnetic order aligns many microscopic moments into a macroscopic magnetization.
Ferrimagnetism: Magnetic order with oppositely aligned moments of unequal magnitude, leaving a net magnetization. It retains net magnetization through a different pattern of opposing moments.
Magnetic recording: A method of storing information by controlling magnetization in a recording medium. Ferromagnetic regions encode data through stable magnetic orientations.
Pierre-Ernest Weiss: A French physicist who developed the molecular-field theory of ferromagnetism. His theory introduced an effective internal field and magnetic domains.

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