Magnetics  (Page 3/6)

where χ = susceptibility, M = variation of magnetization, and H = applied field.

Magnetic permeability

Magnetic permeability is the ability of a material to conduct an electric field. In the same way that materials conduct or resist electricity, materials also conduct or resist a magnetic flux or the flow of magnetic lines of force ( [link] ).

Ferromagnetic materials are usually highly permeable to magnetic fields. Just as electrical conductivity is defined as the ratio of the current density to the electric field strength, so the magnetic permeability, μ, of a particular material is defined as the ratio of flux density to magnetic field strength. However unlike in electrical conductivity magnetic permeability is nonlinear.

μ = B/H

Permeability, where μ is written without a subscript, is known as absolute permeability. Instead a variant is used called relative permeability.

μ = μ _o x μ _r

Absolute permeability is a variation upon 'straight' or absolute permeability, μ, but is more useful as it makes clearer how the presence of a particular material affects the relationship between flux density and field strength. The term 'relative' arises because this permeability is defined in relation to the permeability of a vacuum, μ ₀ .

μ _r = μ/μ _o

For example, if you use a material for which μ _r = 3 then you know that the flux density will be three times as great as it would be if we just applied the same field strength to a vacuum.

Initial permeability

Initial permeability describes the relative permeability of a material at low values of B (below 0.1 T). The maximum value for μ in a material is frequently a factor of between 2 and 5 or more above its initial value.

Low flux has the advantage that every ferrite can be measured at that density without risk of saturation. This consistency means that comparison between different ferrites is easy. Also, if you measure the inductance with a normal component bridge then you are doing so with respect to the initial permeability.

Permeability of a vacuum in the si

The permeability of a vacuum has a finite value - about 1.257 × 10 ^-6 H m ^-1 - and is denoted by the symbol μ ₀ . Note that this value is constant with field strength and temperature. Contrast this with the situation in ferromagnetic materials where μ is strongly dependent upon both. Also, for practical purposes, most non-ferromagnetic substances (such as wood, plastic, glass, bone, copper aluminum, air and water) have permeability almost equal to μ ₀ ; that is, their relative permeability is 1.0.

The permeability, μ, the variation of magnetic induction,

with applied field,

μ = B/H

Background contributions

A single measurement of a sample's magnetization is relatively easy to obtain, especially with modern technology. Often it is simply a case of loading the sample into the magnetometer in the correct manner and performing a single measurement. This value is, however, the sum total of the sample, any substrate or backing and the sample mount. A sample substrate can produce a substantial contribution to the sample total.