page_title: Ferromagnetism
section_title: Magnetic domains
hierarchical_section_title: Ferromagnetism / Explanation / Magnetic domains
caption_attribution_description: Deutsch: Weiss-Bezirke im Gefügeschliffbild English: Magnetic domains (Weiss domains) in a piece of ferromagnetic material, revealed in a Kerr micrograph. The metal is composed of microcrystalline grains. The magnetic domains are the red and green stripes within each grain. Due to its magnetic anisotropy, the crystal lattice of each grain has an "easy" preferential direction of magnetization, so the domains within each grain are oriented parallel to this easy axis. The magnetization of red and green domains is in opposite directions parallel to the long axis of the domain.
context_page_description: Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished. Ferromagnetism is the strongest type and is responsible for the common phenomenon of magnetism in magnets encountered in everyday life. Substances respond weakly to magnetic fields with three other types of magnetism—paramagnetism, diamagnetism, and antiferromagnetism—but the forces are usually so weak that they can be detected only by sensitive instruments in a laboratory. An everyday example of ferromagnetism is a refrigerator magnet used to hold notes on a refrigerator door. The attraction between a magnet and ferromagnetic material is "the quality of magnetism first apparent to the ancient world, and to us today".
  Permanent magnets are either ferromagnetic or ferrimagnetic, as are the materials that are noticeably attracted to them. Only a few substances are ferromagnetic. The common ones are iron, cobalt, nickel and most of their alloys, and some compounds of rare earth metals.
context_section_description: The above would seem to suggest that every piece of ferromagnetic material should have a strong magnetic field, since all the spins are aligned, yet iron and other ferromagnets are often found in an "unmagnetized" state.  The reason for this is that a bulk piece of ferromagnetic material is divided into tiny regions called magnetic domains (also known as Weiss domains).  Within each domain, the spins are aligned, but (if the bulk material is in its lowest energy configuration; i.e. unmagnetized), the spins of separate domains point in different directions and their magnetic fields cancel out, so the object has no net large scale magnetic field.
  Ferromagnetic materials spontaneously divide into magnetic domains because the exchange interaction is a short-range force, so over long distances of many atoms the tendency of the magnetic dipoles to reduce their energy by orienting in opposite directions wins out. If all the dipoles in a piece of ferromagnetic material are aligned parallel, it creates a large magnetic field extending into the space around it.  This contains a lot of magnetostatic energy.  The material can reduce this energy by splitting into many domains pointing in different directions, so the magnetic field is confined to small local fields in the material, reducing the volume of the field. The domains are separated by thin domain walls a number of molecules thick, in which the direction of magnetization of the dipoles rotates smoothly from one domain's direction to the other.
caption: Kerr micrograph of metal surface showing magnetic domains, with red and green stripes denoting opposite magnetization directions.
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