The ferromagnetic material is a kind of material with special performance and wide use. For example, aerospace, communications, automated instrumentation, and control are all ferromagnetic materials(iron, cobalt, nickel, steel, and iron-containing oxides are ferromagnetic substances). Therefore, the study of magnetization properties of ferromagnetic materials is of great significance both in theory and in practical applications. In this experiment, the magnetization curve and hysteresis loop of two ferromagnetic materials with different magnetic properties are measured under the action of alternating magnetic field.
1, magnetization and magnetic conductivity of ferromagnetic materials
The magnetization process of ferromagnetic material is complex, which is mainly due to its hysteresis characteristics. Generally, the magnetic law of magnetic field is studied by measuring the relationship between magnetic field strength H and magnetic induction intensity B.
When there is no magnetization field in the ferromagnetic material, both H and B are zero, that is, the coordinate origin 0 of the B-H curve in Figure 20-1. As the magnetization field H increases, B also increases, but the relationship between the two is not linear. When H is increased to a certain value, B no longer increases(or increases very slowly), indicating that the magnetization of the substance has reached saturation. The magnetic field strength and magnetic induction strength of Hm and Bm at saturation are respectively. If H is gradually retreated to zero, B will gradually decrease at the same time. However, the curve trajectory corresponding to H and B does not return along the original curve track a0, but descends to Br along the other curve AB, indicating that when H falls to zero, a certain amount of magnetism is still retained in the ferromagnetic material. This phenomenon is called hysteresis. Br is called residual magnetism. The magnetization field is reversed and its strength is gradually increased until H =-Hc, and the magnetic induction intensity disappears. This means that to eliminate the residual magnetism, a reverse magnetic field Hc must be applied. Hc is called recalcitrance. Its size reflects the ability of ferromagnetic materials to maintain their magnetic state. Figure 20-1 shows that when the magnetic field changes in the order of Hm → 0 →-Hc →-Hm → 0 → Hc → Hm, The corresponding change experienced by B is Bm → Br → 0 →-Bm →-Br → 0 → Bm. A closed B-H curve is then obtained, called a hysteresis loop. Therefore, when the ferromagnetic material is in a alternating magnetic field(such as the iron center in the transformer), it will be magnetized repeatedly along the hysteresis loop → magnetization → reverse magnetization → reverse magnetization. In this process, additional energy is consumed and released from ferromagnetic materials in the form of heat. This loss is called hysteresis loss. It can be proved that the hysteresis loss is proportional to the area enclosed by the hysteresis loop.
It should be stated that for ferromagnetic materials with initial States of H = 0 and B = 0, during the magnetization of the alternating magnetic field strength from weak to strong, a cluster of magnetic hysteresis lines with an area expanding from small to large can be obtained., As shown in Figure 20-2. The connection of these hysteresis loop vertices is called the basic magnetization curve of ferromagnetic materials. This can be approximated to determine its permeability μ = B/H. Since B and H are non-linear, the μ of the ferromagnetic material is not constant, but changes with H, as shown in Figure 20-3. In practical applications, relative permeability μr = μ / μ0 is often used. μ0 is the permeability in vacuum. The relative permeability of ferromagnetic materials can be as high as thousands or even tens of thousands. This feature is one of the main reasons for its wide use.