What are Magnetic Domains?
According to the properties of substances in an external magnetic field, substances can be divided into five categories: paramagnetic substances, diamagnetic substances, ferromagnetic substances, ferrimagnetic substances and antiferromagnetic substances. A ferromagnetic substance is a substance that is magnetized under the action of an external magnetic field and can maintain its magnetized state even if the external magnetic field disappears. The basic characteristics of a ferromagnetic substance are the spontaneous magnetization and magnetic domain structure inside the substance.
Magnetic domain theory is the basis of modern magnetization theory. Almost all magnetic applications use domain as the basic unit instead of an electron spin. The discussion of magnetic moments in magnetism is based on domains; but magnetic domains are It is a kind of thing that is invisible to our naked eyes, so it is difficult to understand and recognize it. Today, we will take you to understand what a magnetic domain is.
1. Formation of magnetic domains
We all know that ferromagnetic substances do not show magnetism to the outside before they are magnetized (magnetized). This is because below the Curie temperature, a magnetic domain structure will be formed in a large ferromagnetic crystal, and the spontaneous magnetization inside each magnetic domain is Uniform and consistent, but the spontaneous magnetization direction is different between different magnetic domains, the magnetic moments cancel each other, and the vector sum is zero, so the ferromagnet does not show magnetism macroscopically.
A magnetic domain is a ferromagnetic material in the process of spontaneous magnetization, in order to reduce the magnetostatic energy and produce small magnetized regions with different directions, each region contains a large number of atoms, and the magnetic moments of these atoms are as neat as small magnets Arrangement, but the orientation of atomic magnetic moment arrangement is different between different adjacent regions. The interface between individual magnetic domains 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 fields filling the external space, that is, the demagnetization energy. Figure c-i below is a single-domain magnet with a large stray field distribution area. In order to weaken this area, the magnetic moment will spontaneously redistribute inside the magnet to form a magnetic domain. The most intuitive redistribution is the formation of upper and lower domains as shown in Figure c-ii, and the stray field can be greatly weakened. If the upper and lower 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 the magnetic domain is the product of the competition between the magnetic dipole energy and the exchange energy. If the size and appearance of the ferromagnetic sample change, the morphology of the magnetic domains will be richly expressed, such as the formation of domain structures such as Figure c-v.
2. Magnetic domain walls
The boundary between the magnetic domain and the magnetic domain is called the magnetic domain wall, and the magnetic domain wall with the opposite magnetic moment of the adjacent magnetic domain atoms (the angle between the magnetic moments is 180°) is called the 180° domain wall; the magnetic moment of the adjacent magnetic domain atoms 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.
The magnetic domain wall is a transition region 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 changes direction gradually in this transition region. The energy inside the domain wall is always higher than the energy inside the domain.
3. Technical magnetization process
In order to distinguish the spontaneous magnetization in the magnetic domain of ferromagnet or ferrimagnet, we call the magnetization of ferromagnet or ferrimagnet in the magnetic field as technical magnetization.
We all know that the technical magnetization curve (M~H curve) of ferromagnetic or ferrimagnetic substances is nonlinear, the ordinate is the magnetization M, and the abscissa is the magnetic field strength H. Suppose a magnet has two magnetic domains:
When the magnetic field is zero, the number of atomic magnetic moments in the upper magnetic domain and the lower magnetic domain is equal, and the directions are opposite. The vector sum of the atomic magnetic moments is zero, and the magnetization of the substance is zero. Figure (a)
When the magnetic field H1 is applied along the positive direction of the abscissa, the angle θ between the magnetic moment M of the upper magnetic domain and the external magnetic field is <90°, and the magnetostatic energy is low and relatively stable; while the angle between the magnetic moment M of the lower magnetic domain and the external magnetic field is When the angle θ>90°, the magnetostatic energy is high and relatively unstable. Therefore, under the action of the external magnetic field H1, the upper magnetic domain will expand, and the lower magnetic domain will shrink, that is, the 180° domain wall is displaced along the direction of the arrow as shown in figure (b), resulting in an increase in magnetization along the magnetic field direction, and 180° °The domain wall displacement speed may be very fast, and the ab segment of the M~H magnetization curve becomes very steep.
When the external magnetic field increases to a large HR, that is, point c in the figure, the domain wall displacement has ended, and the 180° domain wall has been driven out of the magnet, and the whole magnet is a single domain body, and the atomic magnetic moment still stays at the original magnetic domain In the direction of the magnetic moment, as shown in figure (c), from point a to point c is the technical magnetization process, which is the process of domain wall displacement.
When the external magnetic field gradually increases from the HR point to the HS point, that is, from point c to point d, 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 the HS point, the atomic magnetic moments have basically rotated to the direction parallel to the direction of the external magnetic field. At this time, the magnet has reached the technical magnetization saturation state, and the magnetization at this time is called saturation magnetization. The technical magnetization process is basically realized by domain wall displacement and magnetic moment rotation.
If the external magnetic field is reduced to zero, the atomic magnetic moment will gradually move to the direction of the long axis, 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. In the positive direction of the magnetic field, the magnetization still retains the Mr value, which is called the residual magnetization.