A magnetic domain describes a region within a material which has uniform magnetization. Magnetization is defined as the quantity of Magnetic moment per unit volume 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. A domain wall is a term used in Physics which can have one of two distinct but similar meanings in either Magnetism or String theory.
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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 ferromagnet and he came up with the so called Weiss mean field. Ferromagnetism is the basic mechanism by which certain materials (such as Iron) form Permanent magnets and/or exhibit strong interactions with Magnets it 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 Ms 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. Magnetostriction is a property of Ferromagnetic materials that causes them to change their shape when subjected to a Magnetic field. 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
Magnetic domains can be observed with magnetic force microscopy and the magneto-optic Kerr effect. An Atomic force microscopy (AFM Unlike typical AFM magnetic materials are used for the sample and tip so that not only the atomic force but also the magnetic interaction are detected Magneto-optic Kerr effect (MOKE is one of the Magneto-optic effects It describes the changes of light reflected from Magnetized media
See also
- Weiss domains
- Magnetostatic energy
References
- Jiles, David (1998). Weiss domains are small areas in a Crystal structure of a ferromagnetic material with uniformly oriented magnetic momenta Introduction to magnetism and magnetic materials. London: Chapman & Hall. ISBN 0-412-79860-3.
External links
- Interactive Java tutorial on magnetic domains National High Magnetic Field Laboratory
- Magnetismus und Magnetooptik a german text about magnetism and magneto-optics
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