According to the difference in electrical conductivity, magnetic materials can be divided into two major categories: metal and ferrite magnetic materials. Magnetic Ceramics is also an extremely important category in advanced ceramic materials. It can be divided into ferrites containing iron (Ferrite ) Ceramics and magnetic ceramics that do not contain iron. They are mostly semiconductor materials, with a resistivity of (10~97)x10Ω·m, which is an indispensable material in modern electronic technology. Using them to replace low-resistivity metals and alloy magnetic materials can greatly reduce eddy current loss. For high frequency occasions. The high-frequency magnetic permeability of magnetic ceramics (Magnetic Permeability) is also high. This is unmatched by other magnetic materials. The biggest weakness of ferrite is that the saturation magnetization is relatively low, about 1/3 to 1/5 of pure iron, and the Curie Temperature is not high. It is not suitable to work under high temperature or low frequency and high power conditions.
In this way, they have been widely used in modern radio electronics, automatic control, microwave technology, electronic computers, information storage, laser modulation, etc.
As far as the magnetic properties of materials are concerned, they can be classified according to their magnetic susceptibility, which can be divided into three categories, namely paramagnetic, diamagnetic and ferromagnetic (strong magnetic materials).
1. Paramagnetic and diamagnetic
In atoms, molecules, or ions with unpaired electrons, there is a magnetic moment (Magnetic Moment) due to the unpaired electrons. This magnetic moment is provided by the orbital motion and spin motion of the unpaired electrons. Such substances are called paramagnetism. When a magnetic field is added to the paramagnet, the magnetic moments are arranged in the direction of the magnetic field. This phenomenon is called magnetization. The intensity of magnetization is proportional to the intensity of the magnetic field. If the magnetic field is removed, the magnetization becomes zero.
2. Ferromagnets and antiferromagnets
In a paramagnet, the direction of the magnetic moment is messy, so it is not magnetized when no magnetic field is applied. For a ferromagnet (Ferrogmagnet), when there is no external magnetic field, the magnetic moment is also neatly arranged in the same direction. At this time, it is magnetized or produced sponaneous Magnetization (Sponaneous Magnetization) was born, which is simply a so-called "magnet". In ferromagnets, the energy required to align the magnetic moments neatly is greater than the energy of thermal motion that disturbs the alignment. When the ferromagnetic body is heated for a time, the thermal motion energy is increased due to the increase in temperature, which causes the arrangement of the magnetic moments to be disturbed, so it becomes a paramagnetic body. People call the temperature at which the paramagnetic body and the ferromagnetic body transform into each other as the Curie Temperature. Or Curie point.
3. Magnetic domains and magnetic domain walls
In the absence of an external magnetic field, the electron spin magnetic moments in ferromagnetic materials can be arranged "spontaneously" in a small range to form small "spontaneous magnetization regions" called magnetic domains. When there is no external magnetic field, , The orientation of molecular magnetic moments in each magnetic domain is different. This arrangement of magnetic domains keeps the magnet in a state of minimum energy. Therefore, the sum of the loss of each magnetic domain of the ferromagnetic substance without magnetization cancels each other out and does not show magnetism to the outside. The interface between the magnetic domains is called the magnetic domain wall.
4. Magnetization and hysteresis
Before magnetization, the magnetic domains have different magnetization directions, and the vector sum of each magnetic domain cancels each other out (M=0). If you add
The external magnetic field, at this time, some of the magnetic domains that are close to the direction of the external magnetic field grow up, and the magnetic domains in the other magnetic field direction become smaller. Increasing the strength of the external magnetic field, the magnetic domain will grow further, and M is larger at this time; but the magnetization direction and the direction of the external magnetic field are not exactly the same, even if it becomes a single domain magnetic region.
5. Magnetostriction constant and magnetocrystalline anisotropy constant
When an external magnetic field is magnetized parallel to the axis of a rod-shaped sample, on the one hand, the magnetic field overcomes the anisotropy and orients the magnetic moment in the direction of the external magnetic field; on the other hand, the length of the rod will also change. The magnetostriction constant λs is one of the intrinsic characteristic parameters of ferromagnetic materials.
When the rod is elongated, λs>0; when the rod is shortened, λs<0.
Most ferromagnetic materials λs<0, and the order of magnitude is 10-5~10-6.
The magnetocrystalline anisotropy constant K1 is one of the intrinsic characteristic parameters of ferromagnetic materials. The degree of magnetization of the magnetic crystal in all directions is inconsistent, there are easy magnetization directions and difficult magnetization directions, usually the phenomenon of magnetization difficulty and crystal symmetry is called magnetocrystalline anisotropy. The crystal symmetry is different, and the K1 value is different, such as spinel, K1<0.