Information about Magnetic Domain
A magnetic domain describes a region within a material which has uniform magnetisation. This means that the individual moments of the atoms are aligned with one another. 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.
In the original Weiss theory the mean field was proportional to the bulk magnetisation M, so that
where
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
Where
is the saturation magnetisation at 0K.
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).
Development of Domain Theory
Magnetic domain theory was developed by Weiss who suggested their existence in ferromagnets. He suggested that large number of, typically 1012-1018, atomic magnetic moments were aligned parallel. The direction of alignment varies from domain to domain in a more or less random manner although certain crystallographic axes 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 ferromagnet 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
where
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
Where
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 fieldsMagnetostrictive Energy
This energy is based on the effect of magnetostriction. The magnet establishes an easy axe when being pressed on in order to decrease the pressure.Anisotropy Energy
The favourability for moments to align along certain axesZeeman Energy
Energy resulting from an externally applied fieldDomain Wall Observation
See Also
- Weiss domains
- Magnetostatic energy
External links
- Magnetismus und Magnetooptik a german text about magnetism and magneto-optics
Magnetization is a property of some materials (e.g. magnets) that describes to what extent they are affected by magnetic fields, and also determines the magnetic field that the material itself creates. Magnetization is defined as the amount of magnetic moment per unit volume.
..... Click the link for more information.
..... Click the link for more information.
Ferromagnetism is the "normal" form of magnetism with which most people are familiar, as exhibited in horseshoe magnets and refrigerator magnets, for instance. It is responsible for most of the magnetic behavior encountered in everyday life.
..... Click the link for more information.
..... Click the link for more information.
Magnetostriction is a property of ferromagnetic materials that causes them to change their shape when subjected to a magnetic field. The effect was first identified in 1842 by James Joule when observing a sample of nickel.
..... Click the link for more information.
..... Click the link for more information.
Weiss domains are small areas in a crystal structure of a ferromagnetic material with uniformly oriented magnetic momenta. They were named after the French physicist Pierre-Ernest Weiss (1865–1940).
..... Click the link for more information.
..... Click the link for more information.
This article is copied from an article on Wikipedia.org - the free encyclopedia created and edited by online user community. The text was not checked or edited by anyone on our staff. Although the vast majority of the wikipedia encyclopedia articles provide accurate and timely information please do not assume the accuracy of any particular article. This article is distributed under the terms of GNU Free Documentation License.
Herod_Archelaus
