Magnetic permeability is a crucial concept in the world of magnetism, and it refers to the extent to which a magnetic material responds to an applied magnetic field. This article aims to provide a clear explanation of magnetic permeability, its importance, and its relevance in various materials.
Magnetic Permeability Simplified
If a material’s internal dipoles are easily oriented by an applied magnetic field, it is considered a high-permeability material. Conversely, if the material’s internal dipoles do not align easily with the applied magnetic field, it is regarded as a low-permeability material.
In simpler terms, magnetic permeability is the ability of a material to form an internal magnetic field under the influence of an external magnetic field. The more easily a material becomes magnetized, the higher its permeability. However, it’s crucial to understand that “ease of magnetization” differs from “strength of magnetization,” as most materials that are easily magnetized are not strong magnets.
“Magnetic permeability is the ability of a material to form an internal magnetic field under the influence of an external magnetic field.”
Permeability of Different Materials
When discussing permeability, it’s essential to consider the practicality of the material for magnetic purposes. For example, the magnetic permeability of air or vacuum can be measured, but these substances have very low permeability due to their low mass. In contrast, hard (permanent) magnetic materials also exhibit low permeability despite their higher mass.
Permeability of Soft Magnetic Materials
Materials like silicon steel are considered highly permeable because they are easily magnetized. Many electromagnets and solenoids have cores made of iron or silicon steel due to their high permeability.
Here are examples of soft magnetic materials:
- Iron-Silicon Alloys
- Amorphous & Nano-Crystalline Alloys
- Nickel-Iron Alloys
- Soft Ferrites
But that’s not the whole story. The permeability equation is as follows:
µ = B/H
Where:
µ = permeability
B = Flux Density, measured in Teslas or Gauss
H = Magnetizing Force, measured in Oersteds
As we can see, permeability is calculated by dividing the flux density by the magnetizing force. This means that when the flux density is high and the magnetizing force is low, the permeability is low. Conversely, if the flux density is low and the magnetizing force is high, the permeability is high.
While discussing this topic, it’s worth mentioning that iron and silicon steels have low remanence and low coercivity, which are additional factors that make them excellent core materials. Their ability to be magnetized easily (low coercivity) and demagnetized easily (low remanence) is partly due to their high permeability. Moreover, the addition of silicon to silicon steels helps reduce eddy currents and enhances the steel’s long-term stability, ensuring that its characteristics remain consistent over an extended period.
Permeability of Hard Magnetic Materials
For instance, NdFeB rare earth magnets exhibit very low permeability due to their strong magnetic dipoles that resist external magnetic fields. This means that these magnets do not easily re-orient their domains when influenced by an external magnetic field.
Additionally, NdFeB magnets possess a high coercive force (coercivity), making them difficult to demagnetize.
Typically, low-permeability materials are used as permanent magnets, while high-permeability materials are utilized as soft magnetic materials. Figure 1 (below) illustrates the common hysteresis curves for both hard and soft magnetic materials.
Examples of hard or permanent magnetic materials include:
- NdFeB (Neodymium)
- SmCo (Samarium Cobalt)
- Hard Ferrite (Ceramic)
- Alnico (Aluminum, Nickel, Cobalt Alloy)
Hysteresis Loop and Permeability
The hysteresis loop provides valuable insights into a material’s permeability. A tall, narrow hysteresis loop indicates high permeability, while a wider loop represents low permeability. Several factors influence the hysteresis loop, which determines the material’s magnetic properties.
Materials with a wider hysteresis loop generally exhibit higher remanence (residual magnetism), lower permeability, higher retentivity, higher coercive force (coercivity), and higher reluctance. On the other hand, materials with a thinner hysteresis loop have lower remanence (residual magnetism), higher permeability, lower retentivity, lower coercive force (coercivity), and lower reluctance.
The Importance of Magnetic Permeability
Understanding magnetic permeability is crucial because it determines how a material interacts with external magnetic fields. Non-permeable materials like air or vacuum allow magnetic field lines to pass through them without storing energy or affecting their surroundings. In contrast, hard magnetic permeable materials prevent external field lines from entering their domain.
Permeability of Selected Materials
Comparing the relative permeability values of various materials reveals interesting observations. For instance, highly purified iron has the highest permeability on the chart, 50 times greater than electrical steel. Neodymium magnets, on the other hand, have a relative permeability of 1.05, which is strikingly similar to air’s relative permeability of 1.00.
In conclusion, understanding magnetic permeability is vital for comprehending how different materials interact with magnetic fields. It plays a significant role in the practical application of various magnetic materials and their effectiveness in specific scenarios.
