Permeability

Content Overview

Measurement by Eddy Current Testing | Definition | Classifications | Hysteresis | Applications

Permeability Measurements by Eddy Current

Thin films with magnetic material content such as Cobalt, Nickel or Iron are used in various industries for various reasons.

High Frequency Eddy Current Systems are very sensitive to magnetic properties. This effect is used for permeability characterization of thin-film. In many case the permeability is enhancing the measurement effect, which can be used to derive other parameters such as material thickness from those films. 

Please feel free to consult with the SURAGUS for the ability to characterize your magnetic films.

Definition

In electromagnetism permeability is the degree of magnetization of a material in response to a magnetic field. It is defined as the property of the material to allow the magnetic line of force to pass through the object. The magnetic line of force is directly proportional to the conductivity of the material.

Magnetic permeability is usually described by µ. The unit of magnetic permeability is H/m (henries per meter) or N*A-2 (Newtons per Ampere squared). The permeability of free space, when a magnetic field is forming in vacuum, is called permeability constant µ0. Until 2019 µ0 was defined as magnetic constant with the value of μ0 = 4π × 10−7 H/m or 12.57×10−7 H/m.

Since then an revision of the International System of Units came into effect. µ0 turned in to a value that needs to be assessed experimentally, by 4π × 1.00000000082(20)×10−7 H⋅m−1

Permeability is depending from a variety of parameters, such as position in the medium, applied frequency of the magnetic field, humidity, temperature etc..

The magnetic permeability of a material equals the ratio of field intensity B to flux density H of a material.

Relative magnetic permeability  of any material is the comparison of permeability concerning air or vauum:

Classifications

Magnetic materials can be classified according to their permeablity µr:

  • Diamagnetic materials 0 ≤ µr < 1, such as Cu, N or Water
  • Paramagnetic materials µr > 1, such as O2 or atmosphere
  • Ferromagnetic materials µm >> 1, such as Fe, Co, Ni

The permeability of diamagnetic materials is slightly smaller than permeability in vacuum. These materials or elements tend to push the magnetic field out of their body. They magnetize themselves against the external magnetic field. Paramagnetic elements increase the magnetic field by aligning their atomic magnetic moments according to the external magnetic field. Ferromagnetic materials and elements tend to have very high permeability. These materials are historically being applied in electronics, such as coils, motors and transformer.

Hysteresis

The permeability in ferromagnetic materials is not linearly dependent from the external magnetic field. The behavior of ferromagnetic materials is often described by the hysteresis curve. Depending on the application different definitions of permeability are being used. Beside the typical calculation as mentioned above the differential permeability, as progression of the hysteresis curve. The permeability µ equals the ratio of the progression of

Due to magnetic saturation and magnetic remanence the permeability is not constant.

Applications

  • Chemical industry, e.g. for  characterization of particle and powder materials
  • Metallurgy, e.g. non-destructive quality control
  • Microwave absorbing materials, e.g. by increasing magnetic permeability by modifying the intrinsic material parameters
  • Microwave and RF electronics
  • III-V Semiconductor
  • Photonic devices, e.g. waveguide optical modulator

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