Basic Characteristics of Magnetic Materials

1. Magnetization Curve of Magnetic Materials
　　Magnetic materials are composed of ferromagnetic or ferrimagnetic substances. Under the influence of an external magnetic field H, they exhibit a corresponding magnetization M or magnetic flux density B. The curves describing how M and B vary with H are known as magnetization curves (M–H or B–H curves). In general, these curves are nonlinear and display two key features: magnetic saturation and magnetic hysteresis. Specifically, when the magnetic field strength H becomes sufficiently large, the magnetization M reaches a definite saturation value Ms; further increases in H do not change Ms. Moreover, once the material’s magnetization has reached saturation, reducing the external field H to zero does not restore M to zero; instead, M decreases along the Ms–Mr curve. The operating state of the material corresponds to a specific point on the M–H or B–H curve, which is commonly referred to as the operating point.
　　2. Soft Magnetic material Common magnetic property parameters of materials
　　Saturation magnetic flux density Bs: Its magnitude depends on the material’s composition, and the corresponding physical state is characterized by the orderly alignment of the material’s magnetization vector.
　　Residual magnetic induction Br: a characteristic parameter of the hysteresis loop, representing the value of B when the magnetic field strength H returns to zero.
　　Rectangular ratio: Br∕Bs
　　Coercivity Hc: a measure of the ease with which a material can be magnetized, dependent on its composition and defects (such as impurities and stress).
　　Permeability μ: the ratio of B to H at any point on the hysteresis loop, which is closely related to the device’s operating conditions.
　　Initial permeability μi, maximum permeability μm, differential permeability μd, amplitude permeability μa, effective permeability μe, and pulse permeability μp.
　　Curie temperature Tc: The magnetization of a ferromagnetic material decreases as temperature rises; at a certain temperature, spontaneous magnetization vanishes, and the material transitions to paramagnetism. This critical temperature is known as the Curie temperature, and it defines the upper operating limit for magnetic devices.
　　Losses P: hysteresis loss Ph and eddy current loss Pe. P = Ph + Pe = af + bf² + c. Pe ∝ f²t²/ρ. To reduce hysteresis loss Ph, decrease the coercivity Hc; to reduce eddy current loss Pe, thin the magnetic material (reduce thickness t) and increase its resistivity ρ. In free still air, the relationship between core losses and core temperature rise is: total power dissipation (mW) / surface area (cm²).
　　3. Soft Magnetic material Conversion between the magnetic parameters of the material and the electrical parameters of the device
　　When designing soft magnetic components, the first step is to determine the device’s voltage–current characteristics based on circuit requirements. These characteristics are closely linked to the core’s geometry and its magnetization state. Designers must be well-versed in the material’s magnetization process and master the relationships between the material’s magnetic parameters and the component’s electrical parameters. The design of soft magnetic components typically involves three steps: selecting the appropriate magnetic material; appropriately defining the core’s geometry and dimensions; and, in accordance with the desired magnetic parameters, simulating the core’s operating conditions to derive the corresponding electrical parameters.