About magnetic materials

Magnetization and magnetic domains are key concepts in understanding magnetic materials. Magnetization refers to the process in which the internal magnetic moments of a material tend to be arranged regularly under the action of an external magnetic field; while magnetic domains are small areas inside the material formed by spontaneous magnetization and with relatively uniform magnetic moment directions. When no external magnetic field is applied, the directions of magnetic moments of different magnetic domains cancel each other out, making the material as a whole non-magnetic to the outside.

Although magnetic materials can store a certain amount of energy, their energy storage density is lower than other forms of energy storage. The current theoretical understanding of the behavioral characteristics of magnetic materials is mainly based on the research system of “field and wave” in electromagnetism. David Cheng and other scholars have made important contributions in this field and laid the foundation for the development of magnetic material theory.

When analyzing the properties of magnetic materials, the magnetization curve (BH curve) is the core tool. This curve describes the relationship between magnetic flux density (B) and magnetic field strength (H). In the initial stage of magnetization, the material is in a linear response zone, that is, the BH curve shows a linear characteristic. At this time, the magnetic permeability remains constant and the magnetic flux density increases linearly with the magnetic field strength. However, when the magnetic field strength exceeds a certain critical value, the material gradually enters a saturation state, the magnetic permeability begins to decrease, and the growth rate of the magnetic flux density slows down significantly. The essence of this phenomenon is that as the degree of magnetization deepens, the number of magnetic moments that can be flipped inside the material gradually decreases, which eventually causes the magnetization process to stagnate.

In the design of inductors, saturation characteristics are parameters that must be considered. When the magnetic material enters the saturation state, its equivalent magnetic permeability approaches the vacuum magnetic permeability, and the inductance value will drop sharply, which may cause abnormal working conditions similar to short circuits in the circuit. Therefore, it is necessary to ensure that the operating point of the inductor is far away from the saturation zone by reasonably designing parameters such as the core size and number of turns. In addition, different magnetic materials have differentiated BH characteristic curves. For example, although Soft Magnetic alloy materials have hysteresis loss and eddy current loss, their low coercive force characteristics are suitable for high-frequency scenarios; and although ferrite materials have a low saturation flux density (about 0.3T), they have high resistivity and low high-frequency loss, which has advantages in specific frequency bands; metal magnetic materials such as pure iron are often used in high-current inductor design due to their high saturation flux density (about 1.8T) and good DC bias characteristics.

The saturation characteristics of magnetic materials have a decisive influence on the performance of inductors. During the circuit design stage, it is necessary to select appropriate magnetic materials according to the actual working conditions and establish an accurate equivalent circuit model. As an energy storage element, the energy storage capacity of the inductor depends on the product of the inductance value and the square of the current, and the inductance value is closely related to the magnetic permeability, cross-sectional area and number of coil turns of the core material. In high-frequency applications, the loss characteristics of magnetic materials (including hysteresis loss, eddy current loss and residual loss) will become a key factor restricting the efficiency of inductors, so it is necessary to optimize the material formula, improve the manufacturing process and other means for comprehensive control.