Table of Contents
What Is Magnetic Permeability?
Magnetic permeability (μ) is a material property that describes how well it supports the formation of a magnetic field. It is the ratio of magnetic flux density (B) to magnetic field strength (H). Materials with high permeability concentrate magnetic field lines, making them essential for transformer cores, inductors, and magnetic shielding.
The relative permeability (μr) compares a material's permeability to that of free space (μ0 = 4π × 10-7 H/m). Diamagnetic materials have μr slightly less than 1, paramagnetic materials slightly greater than 1, and ferromagnetic materials can have μr from hundreds to hundreds of thousands, depending on the field strength and the specific alloy.
Permeability Relations
Material Permeabilities
| Material | μr | Type |
|---|---|---|
| Vacuum | 1.000000 | Reference |
| Copper | 0.999994 | Diamagnetic |
| Aluminum | 1.000022 | Paramagnetic |
| Nickel | 100-600 | Ferromagnetic |
| Silicon Steel | 4,000-7,000 | Ferromagnetic |
| Mu-metal | 50,000-100,000 | Ferromagnetic |
Frequently Asked Questions
Why does permeability vary with field strength?
In ferromagnetic materials, permeability is not constant. At low fields, domain walls move easily and permeability increases. At moderate fields, most domains have aligned and permeability reaches a maximum. At high fields, the material saturates as all domains are fully aligned, and the incremental permeability drops toward μ0. This non-linear B-H relationship forms the hysteresis loop.
What is magnetic susceptibility?
Magnetic susceptibility (χ) measures how much a material magnetizes in response to an applied field: M = χH. It relates to permeability by μr = 1 + χ. Diamagnetic materials have small negative χ, paramagnetic have small positive χ, and ferromagnetic have large positive χ that depends on the field and temperature.
What is mu-metal used for?
Mu-metal (a nickel-iron alloy with μr up to 100,000) is used for magnetic shielding. It provides a low-reluctance path that diverts magnetic field lines around the shielded volume. Applications include shielding CRT monitors, sensitive electronics, MRI rooms, and scientific instruments from stray magnetic fields.