# Magnetic domains

A magnetic domain describes a region within a material which has uniform magnetization. This means that the individual moments of the atoms are aligned with one another. In most materials, domains do not naturally exist. The materials have to be exposed to a magnetic field, which will cause the individual moments to try and align with the field, which will eventually nucleate domains. The regions separating magnetic domains are called domain walls where the magnetisation rotates coherently from the direction in one domain to that in the next domain.

Development of Domain Theory

Magnetic domain theory was developed by Weiss who suggested their existence in ferromagnets. He suggested that large number of atomic magnetic moments (typically 1012-1018) were aligned parallel. The direction of alignment varies from domain to domain in a more or less random manner although certain crystallographic axis may be preferred by the magnetic moments, namely easy axes. Weiss still had to explain the reason for the alignment of atomic moments within a ferromagnetic and he came up with the so called Weiss mean field. This was essentially an interatomic interaction that caused neighbouring moments to align parallel since it was more energetically favourable.

In the original Weiss theory the mean field was proportional to the bulk magnetisation M, so that

$H_e = alpha M$

where $alpha$ is the mean field constant. However this is not applicable to ferromagnets due to the variation of magnetisation from domain to domain. In this case, the interaction field is

$H_e = alpha M_s$

Where $M_s$ is the saturation magnetisation at 0K.

Energy Considerations

The existence of magnetic domains is a result of energy minimisation.Landau and Lifshitz [1] proposed theoretical domain structures based on a minimum energy concept, which forms the basis for modern domain theory. The primary reason for the existence of domains within a crystal is that their formation reduces the magnetic free energy. In the simplest case for such a crystal, the energy, E, is the sum of several free energy terms:

E = (Eex+Ek)+Eλ+ED+EH (3) where Eex is the exchange energy, Ek is the magnetocrystalline anisotropy energy, Eλ is the magnetoelastic energy, ED is the magneto-static energy, and EH is the energy of the domains in the presence of an applied field. There is also a wall energy Ew which is examined in detail in section 1.5.4. However, since Ew comprises Eex and Ek, it is not necessary to include Ew as a separate term in equation 3.

1. Cited in Carey R., Isaac E.D., Magnetic domains and techniques for their observation, The English University Press Ltd, London, (1966).

Magnetostatic Energy

This is essentially the energy associated with sources of internal or external fields.

Magnetostrictive Energy

This energy is based on the effect of magnetostriction. The magnet establishes a preferred axis when pressed in order to decrease the pressure.

Anisotropy Energy

The favourability for moments to align along certain axes.

Zeeman Energy

Energy resulting from an externally applied field.

Domain Observation

There are many ways to observe magnetic domains. Each method has a different application because not all domains are the same. In condensed matter, domains can be circular, square, irregular, elongated, and striped, all of which have varied sizes and dimensions. Large domains, within the range of 25-100 micrometers can be easily seen by Kerr microscopy, which applies a physical phenomenon called the magneto-optic Kerr effect. Other domains, such as domains within the range of a few nanometers can be documented by the use of magnetic force microscopy.

ee also

*Weiss domains
*Magnetostatic energy

References

*cite book |author=Jiles, David |title=Introduction to magnetism and magnetic materials |publisher=Chapman & Hall |location=London |year=1998 |isbn=0-412-79860-3

* [http://www.magnet.fsu.edu/education/tutorials/java/domains/index.html Interactive Java tutorial on magnetic domains] National High Magnetic Field Laboratory
* [http://schulzeundschultze.anphy.uni-duesseldorf.de/veroff/Diplome/Volker.Solinus/node3.phtml Magnetismus und Magnetooptik] a German text about magnetism and magneto-optics

Wikimedia Foundation. 2010.

### Look at other dictionaries:

• Magnetic reconnection — is a physical process in highly conducting plasmas in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle acceleration. Magnetic reconnection occurs on timescales intermediate …   Wikipedia

• Magnetic hyperthermia — is the name given to an experimental cancer treatment. It is based on the fact that magnetic nanoparticles, when subjected to an alternating magnetic field, produce heat. As a consequence, if magnetic nanoparticles are put inside a tumor and the… …   Wikipedia

• Magnetic-particle inspection — (MPI) is a non destructive testing (NDT) process for detecting surface and subsurface discontinuities in ferroelectric materials such as iron, nickel, cobalt, and some of their alloys. The process puts a magnetic field into the part. The piece… …   Wikipedia

• magnetic recording — magnetic recorder. the process of recording sound or other data on magnetic tape, wire, etc. [1940 45] * * * ▪ electronics Introduction       method of preserving sounds, pictures, and data in the form of electrical signals through the selective… …   Universalium

• Magnetic domain — Several grains of NdFeB with magnetic domains made visible via contrast with a Kerr microscope. A magnetic domain describes a region within a magnetic material which has uniform magnetization. This means that the individual magnetic moments of… …   Wikipedia

• Magnetic refrigeration — Gadolinium alloy heats up inside the magnetic field and loses thermal energy to the environment, so it exits the field cooler than when it entered. Magnetic refrigeration is a cooling technology based on the magnetocaloric effect. This technique… …   Wikipedia

• Magnetic core — A magnetic core is a piece of magnetic material with a high permeability used to confine and guide magnetic fields in electrical, electromechanical and magnetic devices such as electromagnets, transformers, electric motors, inductors and magnetic …   Wikipedia

• Magnetic lock — A magnetic lock is a simple locking device that consists of an electromagnet and armature plate. By attaching the electromagnet to the door frame and the armature plate to the door, a current passing through the electromagnet attracts the… …   Wikipedia

• magnetic ceramics — Introduction       oxide materials that exhibit a certain type of permanent magnetization called ferrimagnetism. Commercially prepared magnetic ceramics are used in a variety of permanent magnet, transformer, telecommunications, and information… …   Universalium

• Magnetic detector — 1902 1918: Marconi s Wireless Magnetic Detector (London) The magnetic detector was one of the first practical devices able to make radio signals audible through a pair of headphones. It was not as sensitive as some detectors but despite that, it… …   Wikipedia