What is Magnetic Permeability?
You might think that magnetic permeability is a property unique to “magnetic materials,” but in fact, all substances (including vacuum, air, copper, water, etc.) have magnetic permeability. It is a physical quantity that measures the degree of response of any medium to an applied magnetic field. Magnetic permeability determines how strong the magnetic flux density can be maintained inside the material under an external magnetic field. This means that the easier a material is to magnetize, the higher its magnetic permeability.
Classification of Magnetic Permeability Materials
1.Diamagnetic Materials
The relative permeability of diamagnetic materials is constant and slightly less than 1, so the magnetic flux density inside them is slightly lower than in a vacuum. Diamagnetic materials experience a weak repulsive force in an external magnetic field. Bismuth, copper, silver, and water are typical diamagnetic materials.
2.Paramagnetic Materials
Paramagnetic materials have a constant relative permeability, slightly greater than 1. Aluminum, platinum, and oxygen are all paramagnetic materials.
3.Ferromagnetic Materials
The permeability of ferromagnetic materials is dynamic. The same material can have extremely high permeability, ranging from thousands to hundreds of thousands.
The magnetization process of ferromagnetic materials is as follows: as the magnetic field strengthens, the permeability initially increases rapidly to its peak value, then decreases as the material gradually reaches magnetic saturation. Ferromagnetic materials are the strongest of all magnetic materials. Iron, cobalt, nickel, and their alloys are common ferromagnetic materials.
4.Ferrimagnetic Materials
This is a very important class of engineering materials whose macroscopic magnetic properties are similar to those of ferromagnetic materials, but with much lower relative permeability, typically ranging from tens to thousands. The most common example is ferrite, which is widely used in high-frequency electronic fields.
Calculation Formula for Magnetic Permeability
B represents magnetic flux density, which refers to the total strength of the magnetic field actually present inside the material. It represents the ultimate effect of the magnetic field, with units of tesla.
H represents magnetic field strength, which refers to the original force driving the magnetic field produced by external currents; it is independent of the material itself. Units are amperes per meter.
Vacuum permeability (μ₀): This is a physical constant with a value of 4π×10⁻⁷ H/m. It represents the basic properties of the magnetic field in a vacuum without any substance present.
The formula for absolute permeability is: μ = B / H. The international unit for magnetic permeability is henry per meter (H/m) or newton per ampere squared (N·A⁻²), describing the material’s overall response to the magnetic field.
You might have a question: when people commonly refer to “magnetic permeability,” do they mean absolute permeability or relative permeability? The answer here is relative permeability, because it is easier to compare different materials. Below is the formula for relative permeability:
Relative permeability (μᵣ): Formula: μᵣ = μ / μ₀. If a material’s relative permeability is 1000, it means its magnetic conduction ability is 1000 times that of a vacuum.
However, it is worth noting that relative permeability is not a fixed constant; it varies. For example, the permeability of a piece of pure iron is roughly between 200 and 5000, depending on the external magnetic field and the type of material.
Factors Affecting Magnetic Permeability
1.Chemical Composition and Crystal Structure
This is the most fundamental factor. Pure iron, silicon steel, and Permalloy all contain iron, but due to differences in composition and structure, their magnetic permeabilities vary greatly.
2.Temperature
For ferromagnetic materials, there is a critical temperature, namely the Curie temperature. When the temperature exceeds the Curie point, ferromagnetism disappears, and the permeability drops sharply to near 1.
3.Frequency of the Exciting Magnetic Field
At high frequencies, effects such as eddy current losses and magnetic domain flipping relaxation become significant, leading to a decrease in permeability. This must be considered when selecting magnetic core materials for high-frequency transformers.
4.Applied Magnetic Field Strength
The permeability of ferromagnetic materials changes dramatically with the strength of the magnetic field, usually increasing first and then decreasing, with a maximum value.
5.Others
Mechanical stress and processing techniques can also affect the magnetic permeability by altering the material’s microstructure.
What Does High Magnetic Permeability Mean?
High magnetic permeability materials can “attract” and “confine” magnetic field lines within themselves, just as copper wire is a good pathway for electric current; it serves as an excellent pathway for magnetic flux. In transformers, a high-permeability iron core can produce the strongest magnetic field with minimal energy, thereby reducing energy waste. High magnetic permeability materials can conduct and concentrate magnetic fields with extremely high efficiency, making them ideal media for achieving efficient electromagnetic energy conversion.
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