Magnetic domain

A region within a magnetic material in which many atomic magnetic moments align in roughly the same direction. Domains form to reduce the material’s total magnetic energy.

Exchange interaction: A quantum-mechanical interaction that favors particular relative orientations of neighboring spins. It aligns neighboring moments, helping create a uniformly magnetized region.
Magnetic moment: A vector quantity describing the strength and direction of a magnetic source. The aligned moments within a domain combine into its net magnetization.
Magnetic recording: The storage of information through controlled changes in magnetic states. Recorded bits are represented by magnetization patterns in magnetic media.
Single-domain particle: A sufficiently small magnetic particle that remains uniformly magnetized rather than dividing into domains. It lacks the internal domain walls found in larger particles.
Magnetostatic energy: The energy associated with magnetic fields produced by a magnetized body. Splitting into domains can reduce the stray-field energy of a uniformly magnetized specimen.
Spin (physics): An intrinsic form of angular momentum carried by elementary particles. Electron spin is a major source of the atomic moments that order within domains.
Hard disk drive: A data-storage device that records information magnetically on rotating disks. Its data-bearing regions rely on stable magnetic states in thin films.
Superparamagnetism: A nanoscale magnetic state in which thermal fluctuations rapidly reverse a particle’s net moment. Its moment can fluctuate as a whole instead of changing through stable domain-wall motion.
Magnetic anisotropy: Dependence of a material’s magnetic energy on the direction of its magnetization. It favors particular domain orientations, called easy axes.
Ferromagnetism: Magnetic ordering in which a material can sustain spontaneous magnetization. Ferromagnetic order supports domains separated by walls.

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