What is a magnetic domain

What is a magnetic domain?

According to the characteristics of the material in the external magnetic field, the material can be divided into five categories: paramagnetic material, diamagnetic material, ferromagnetic material, ferrimagnetic material and antiferromagnetic material. Ferromagnetic substance is a kind of substance that can maintain its magnetization state after being magnetized under the action of an external magnetic field, even if the external magnetic field disappears. The basic feature of ferromagnetic substance is the existence of spontaneous magnetization and magnetic domain structure inside the substance.
Magnetic domain theory is the basis of modern magnetization theory. Almost all magnetic applications use domains as the basic unit instead of an electron spin as the unit. The magnetic moment discussed in magnetism is based on domains; however, magnetic domains are It’s something that we can’t see with the naked eye, so it’s difficult to understand and recognize it. Today, I will show you what is a magnetic domain.
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Effect map of metallic magnetic domain based on scanning electron microscope photos

Formation of magnetic domains

We all know that ferromagnetic materials do not show magnetism to the outside before they are magnetized (magnetized). This is because below the Curie temperature, a large ferromagnetic crystal will form a magnetic domain structure, and the spontaneous magnetization inside each magnetic domain is Uniform, but the direction of spontaneous magnetization between different magnetic domains is different, the magnetic moments cancel each other, and the vector sum is zero, so the ferromagnet does not show magnetism in a macroscopic view.
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Magnetic domains are small magnetized regions with different directions in order to reduce the magnetostatic energy during the spontaneous magnetization of ferromagnetic materials. Each region contains a large number of atoms, and the magnetic moments of these atoms are as neat as small magnets. Arrangement, but the directions of the atomic magnetic moments are different between adjacent different regions. The interface between each magnetic domain is called a magnetic domain wall.
The formation of magnetic domains can be simply understood as reducing the magnetic dipole energy carried by the stray field full of external space, that is, the demagnetization energy. The following figure c-ⅰ is a single-domain magnet with a large stray field distribution area. In order to weaken this area, the magnet will spontaneously redistribute the magnetic moment and form magnetic domains. The most intuitive redistribution is to form the upper and lower domains as shown in Figure c-ii, and the stray field can be greatly weakened. If the upper and lower four domains are further formed, as shown in Figure c-iii, the stray field will be further weakened. However, this process will in turn increase the magnetostatic exchange energy, which can be roughly understood as domain wall energy. The final shape and size of magnetic domains are the products of competition between magnetic dipole energy and exchange energy. If the size and appearance of the ferromagnetic sample change, the morphology of the magnetic domain will have a rich expression, such as the formation of a domain structure like Figure c-ⅴ.
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Magnetic domain wall

The boundary between the magnetic domain and the magnetic domain is called the magnetic domain wall. The magnetic domain wall with the opposite magnetic moments of the adjacent magnetic domain atoms (the magnetic moment angle is 180°) is called the 180° domain wall; the adjacent magnetic domain atoms magnetic moment The magnetic domain walls perpendicular to each other are 90° domain walls. The magnetic domain wall has a thickness of several atoms, and the thickness of the magnetic domain wall is different for different materials.
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The magnetic domain wall is a transition zone with a certain thickness. The magnetization direction of the magnetic domain cannot suddenly turn a large angle at the domain wall, but gradually turns over a certain thickness of the domain wall, that is, the atomic magnetic moment in this transition zone gradually changes direction. The energy inside the domain wall is always higher than the energy inside the domain.

Technical magnetization process

In order to distinguish the spontaneous magnetization in the magnetic domains of ferromagnets or ferrimagnets, we call the magnetization of ferromagnets or ferrimagnets in a magnetic field as technical magnetization.
We all know that the technical magnetization curve (M~H curve) of ferromagnetic or ferrimagnetic material is nonlinear, the ordinate is the magnetization M, and the abscissa is the magnetic field H. Suppose the magnet has two magnetic domains:
When the magnetic field is zero, the atomic magnetic moments of the upper magnetic domain and the lower magnetic domain have the same number and opposite directions, the vector sum of the atomic magnetic moments is zero, and the magnetization of the substance is zero. As shown in figure (a):

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When the magnetic field H1 is applied along the positive direction of the abscissa, the angle between the magnetic moment M of the upper domain and the external magnetic field is less than 90 ° and the magnetostatic energy is relatively low and stable, while the angle between the magnetic moment M of the lower domain and the external magnetic field is more than 90 ° and the magnetostatic energy is relatively high and unstable. Therefore, under the action of the external magnetic field H1, the upper domain will expand and the lower domain will shrink, that is, the 180 ° domain wall will shift along the direction of the arrow as shown in figure (b), resulting in the increase of magnetization along the direction of the magnetic field, and the displacement speed of the 180 ° domain wall may be very fast, and the AB section of the M ~ H magnetization curve becomes very steep.

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When the external magnetic field increases HR, that is, at point C in the figure, the domain wall displacement has ended, and the 180 ° domain wall has been driven out of the magnet. The whole magnet is a single domain body, and the atomic magnetic moment still stays in the direction of the original magnetic moment of the upper domain. As shown in figure (c), from point a to point C, that is, the process of technical magnetization, is the process of domain wall displacement.

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When the external magnetic field gradually increases from HR point to HS point, that is, from C point to D point, the atomic magnetic moment gradually rotates in the same direction as the external magnetic field, as shown in figure (d). When the external magnetic field increases to HS point, the atomic magnetic moment has basically turned to the direction parallel to the direction of the external magnetic field. At this time, the magnet has reached the saturation state of technical magnetization, and the magnetization at this time is called saturation magnetization. The technological magnetization process is basically realized by domain wall displacement and magnetic moment rotation.

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If the external magnetic field is reduced to zero, the atomic magnetic moment will gradually move to the long axis direction, as shown in figure (E). This process is the process of magnetic moment rotation. It can be seen that the magnetization does not decrease to zero after removing the external magnetic field, and there is still Mr value in the positive direction of the magnetic field, which is called residual magnetization.

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Source: China Permanent Magnet Manufacturer – www.ymagnet.com



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