A material that can respond to a magnetic field in a certain way is called a magnetic material. According to the magnetic strength of the material in the external magnetic field, it can be divided into diamagnetic material, paramagnetic material, ferromagnetic material, antiferromagnetic material and ferrimagnetic material. Most materials are diamagnetic or paramagnetic, and they respond weakly to external magnetic fields. Ferromagnetic materials and ferrimagnetic materials are ferromagnetic materials. Generally speaking, magnetic materials refer to ferromagnetic materials. For magnetic materials, the magnetization curve and hysteresis loop are characteristic curves that reflect their basic magnetic properties. Ferromagnetic materials are generally Fe, Co, Ni elements and their alloys, rare earth elements and their alloys, and some Mn compounds. Magnetic materials are generally divided into soft magnetic materials and hard magnetic materials according to the degree of difficulty of their magnetization.

Introduce

Experiments show that any substance can be magnetized more or less in an external magnetic field, but the degree of magnetization is different. According to the characteristics of substances in an external magnetic field, substances can be divided into five categories: paramagnetic substances, diamagnetic substances, ferromagnetic substances, ferrimagnetic substances, and diamagnetic substances.

According to the molecular current hypothesis, matter should show roughly similar properties in a magnetic field, but this tells us that the properties of matter in an external magnetic field are very different. This reflects the limitations of the molecular current hypothesis. In fact, there are differences in the microstructure of various substances, and this difference in the structure of the substance is the cause of the difference in the magnetic properties of the substance.

We call paramagnetic materials and diamagnetic materials weakly magnetic materials, and ferromagnetic materials as strong magnetic materials.

Generally speaking, magnetic materials refer to ferromagnetic substances. Magnetic materials can be divided into soft magnetic materials and hard magnetic materials according to the difficulty of demagnetization after magnetization. The material that is easy to demagnetize after magnetization is called soft magnetic material, and the material that is not easy to demagnetize is called hard magnetic material. Generally speaking, the remanence of soft magnetic materials is small, and the remanence of hard magnetic materials is large.

Basic characteristics

1. The magnetization curve of magnetic materials

Magnetic materials are composed of ferromagnetic or ferrimagnetic materials. Under the action of an external magnetic field H, there must be a corresponding magnetization M or magnetic induction intensity B. Their change curve with the magnetic field intensity H is called the magnetization curve (M～H Or B～H curve). Generally speaking, the magnetization curve is nonlinear, with two characteristics: magnetic saturation and hysteresis. That is, when the magnetic field intensity H is large enough, the magnetization M reaches a certain saturation value Ms, and continues to increase H, and Ms remains unchanged; and when the M value of the material reaches saturation, the external magnetic field H decreases to zero, M and It does not return to zero, but changes along the MsMr curve. The working state of the material is equivalent to a certain point on the M～H curve or B～H curve, and this point is often called the working point.

2. Common magnetic performance parameters of soft magnetic materials

Saturation magnetic induction intensity Bs: Its size depends on the composition of the material, and its corresponding physical state is that the magnetization vector inside the material is neatly arranged.

Residual magnetic induction Br: is the characteristic parameter on the hysteresis loop, the value of B when H returns to 0.

Rectangular ratio: Br∕Bs

Coercivity Hc: It is a quantity that indicates the degree of difficulty of magnetization of a material, and it depends on the composition and defects of the material (impurities, stress, etc.).

Permeability μ: is the ratio of B to H corresponding to any point on the hysteresis loop, which is closely related to the working state of the device.

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 substance decreases with increasing temperature. When a certain temperature is reached, the spontaneous magnetization disappears and turns into paramagnetism. The critical temperature is the Curie temperature. It determines the upper limit temperature of the magnetic device.

Loss P: Hysteresis loss Ph and eddy current loss Pe P = Ph + Pe = af + bf2+ c Pe ∝ f2 t2 /, ρ is reduced, the method to reduce the hysteresis loss Ph is to reduce the coercive force Hc; the method to reduce the eddy current loss Pe It is to reduce the thickness t of the magnetic material and increase the resistivity ρ of the material. The relationship between the loss of the magnetic core and the temperature rise of the magnetic core in free still air is: total power dissipation (mW)/surface area (cm²)

3. Conversion between the magnetic parameters of soft magnetic materials and the electrical parameters of the device

When designing a soft magnetic device, first determine the voltage-current characteristics of the device according to the requirements of the circuit. The voltage-current characteristics of the device are closely related to the geometry and magnetization state of the magnetic core. The designer must be familiar with the magnetization process of the material and master the conversion relationship between the magnetic parameters of the material and the electrical parameters of the device. The design of soft magnetic devices usually includes three steps: correct selection of magnetic materials; reasonable determination of the geometric shape and size of the magnetic core; according to the requirements of magnetic parameters, simulate the working state of the magnetic core to obtain the corresponding electrical parameters.

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