Magnetic Domains - MagLab
In the experiment below, the magnetic domains are indicated by the arrows in the metal material. You can think of magnetic domains as miniature magnets. Relationship between microstructure and magnetic domain structure of Nd-Fe-B melt-spun ribbon magnets. Authors; Authors and affiliations. Magnetic domain definition, a portion of a ferromagnetic material where the magnetic moments are aligned with one another because of interactions between.
Unlike poles attract and will stick together. Magnets attract iron rich materials and like poles and the repulsion between like poles can be reduced if a strip of iron is placed between them. The Domain Theory of Magnetism How can we explain these intriguing properties? The domain theory states that inside a magnet there are small regions in which the magnetic direction of all the atoms are aligned in the same directions.
These regions are known as domains. Within a domain, the aligment of the magnetic direction is the same. In the next domain it may be in a completely different direction.
On average over the many domains in the magnet there there is no preferential direction for the magnetic force. However, using an external magnetic field from another magnet say, the direction of the magnetic direction in each domain can be made to align with the magnetic field the net magnetic field can be increased.
Why do Magnetic Domains Form? Consider a bar magnet which has been been magnetised such that the entire magnet forms a single magnetic domain. Surface charges will appear at either end of the crystal. Associated with the surface charges is a secondary magnetic field called the demagnetising field which acts to reduce the magnetic field.
The energy of the surface charges is called the magnetostatic energy. Domain Formation in a Magnet The magnetostatic energy can be reduced if the crystal forms a second domain, magnetised in the opposite direction.
In this way, the separation of positive and negative surface charges are reduced decreasing the spatial extent of the demagnetising field. Naturally, one might ask, if the magnetostatic energy is reduced by the formation of domains, can they carry on forming indefinitely? To which the answer is no. The reason being that energy is required to produce and maintain the region of transition from one domain to another, the domain wall. Equilibrium will be reached when the magnetostatic energy is equal to the energy required to maintain the domain walls.
However, domains are much larger than the individual molecules within the magnet.
Magnetic domain - Wikipedia
There are only 4 ferromagnetic elements at room temperature. Of these, iron Fenickel Niand cobalt Co are shown above. The fourth is gadolinium Gd. The pictures below show the formation made visible with the use of magnetic colloidal suspensions which concentrate along the domain boundaries.
The domain boundaries can be imaged by polarized light, and also with the use of electron diffraction. Observation of domain boundary movement under the influence of applied magnetic fields has aided in the development of theoretical treatments.
It has been demonstrated that the formation of domains minimizes the magnetic contribution to the free energy. If a magnetic field is applied to the crystal, the domains that align with the magentic field will grow as the expense of the domains that are pointing in other directions. Atomic Theory of Magnetism We are familiar with the model of the atom with a nucleus that contains the protons and neutrons and electron orbit the nucleus.
Within the atom, the electrons behave as if they are magnets. The domain structure of actual magnetic materials does not usually form by the process of large domains splitting into smaller ones as described here. When a sample is cooled below the Curie temperature, for example, the equilibrium domain configuration simply appears. But domains can split, and the description of domains splitting is often used to reveal the energy tradeoffs in domain formation.
Size of domains[ edit ] As explained above, a domain which is too big is unstable, and will divide into smaller domains.
But a small enough domain will be stable and will not split, and this determines the size of the domains created in a material. This size depends on the balance of several energies within the material. The exchange interaction which creates the magnetization is a force which tends to align nearby dipoles so they point in the same direction. Forcing adjacent dipoles to point in different directions requires energy. Therefore, a domain wall requires extra energy, called the domain wall energywhich is proportional to the area of the wall.
Thus the net amount that the energy is reduced when a domain splits is equal to the difference between the magnetic field energy saved, and the additional energy required to create the domain wall. The field energy is proportional to the cube of the domain size, while the domain wall energy is proportional to the square of the domain size.
So as the domains get smaller, the net energy saved by splitting decreases. The domains keep dividing into smaller domains until the energy cost of creating an additional domain wall is just equal to the field energy saved.
Then the domains of this size are stable. Magnetic anisotropy[ edit ] Micrograph of surface of ferromagnetic material, showing the crystal grains, each divided into several domains parallel to its "easy" axis of magnetization, with the magnetization in alternating directions red and green areas.
Animation showing how magnetostriction works. A changing external magnetic field causes the magnetic dipoles to rotate, changing the dimensions of the crystal lattice. An additional way for the material to further reduce its magnetostatic energy is to form domains with magnetization at right angles to the other domains diagram c, rightinstead of just in opposing parallel directions.
However, forming these domains incurs two additional energy costs. First, the crystal lattice of most magnetic materials has magnetic anisotropywhich means it has an "easy" direction of magnetization, parallel to one of the crystal axes. Changing the magnetization of the material to any other direction takes additional energy, called the " magnetocrystalline anisotropy energy ".
Magnetostriction[ edit ] The other energy cost to creating domains with magnetization at an angle to the "easy" direction is caused by the phenomenon called magnetostriction. The change in magnetic field causes the magnetic dipole molecules to change shape slightly, making the crystal lattice longer in one dimension and shorter in other dimensions.
However, since the magnetic domain is "squished in" with its boundaries held rigid by the surrounding material, it cannot actually change shape. So instead, changing the direction of the magnetization induces tiny mechanical stresses in the material, requiring more energy to create the domain. This is called " magnetoelastic anisotropy energy". To form these closure domains with "sideways" magnetization requires additional energy due to the aforementioned two factors.