High Magnetic Permeability Sheets For EMI

A specification that says thirty decibels of shielding sounds like a complete answer, and it rarely is. A material that achieves that figure in one band can do almost nothing in another, and the sheet that is chosen from a headline number is often the wrong one. The band is the first variable, and the material family follows from it.

This article looks at how the frequency range decides the material, how shielding effectiveness is used at low frequency, and how permeability behaves as the frequency rises. The examples are drawn from electric vehicle and industrial equipment applications where low frequency fields are the problem.

Why The Frequency Band Comes First

The mechanism that suppresses a field depends on the frequency. Below about thirty megahertz the field is largely magnetic, and a material with a high magnetic permeability diverts the flux away from the protected volume. Above a few hundred megahertz the material becomes a lossy absorber, and the useful parameter is the imaginary part of the permeability together with the impedance match to free space.

Above a few gigahertz the permeability of most magnetic materials falls toward one, and the absorber relies on dielectric loss, on a resistive film or on a structured surface rather than on magnetic loss. Choosing a material from the wrong family therefore produces a sheet that is well made, correctly specified and ineffective in the application.

The reason the low frequency range needs its own approach is that a conductive shield alone is inefficient there. A thin metal sheet reflects a high frequency field well, but a magnetic field at a few hundred kilohertz passes through it with little attenuation unless the metal is very thick. A high permeability sheet presents a low reluctance path that carries the flux around the protected volume, so a thin layer can achieve the attenuation that a thick conductor cannot.

The choice also interacts with the mechanical design, because a material with a higher permeability is often supplied as a thicker ribbon, and the space available inside the enclosure sets a practical limit on the thickness that can be used.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/Flexible-Substrates-in-4-layer-rigid-flex-PCB.jpg" alt="Magnetic absorber sheet applied to the inside of an enclosure wall” />

Shielding Effectiveness At Low Frequency

Shielding effectiveness is the ratio of the field without the barrier to the field with it, expressed in decibels, and it is the relevant figure in the low frequency range. A thin sheet of a high permeability material can achieve figures in the region of eighty decibels over a narrow low frequency band, which is why these materials appear in the suppression of motor drive and power supply fields.

The figure is a function of the permeability, the thickness and the conductivity of the sheet, and it is measured in a defined fixture. As with any such number, the value applies to the fixture and the frequency at which it was measured, and the shielding effectiveness at the frequency of interest is what matters. A curve is more informative than a single figure, and the thickness at which the curve was obtained should be recorded with it.

Magnetic Permeability And Its Limits

Permeability is expressed as a complex quantity, with a real part that describes how strongly the material concentrates flux and an imaginary part that describes the loss. A high initial permeability means the material responds strongly at low frequency, and it is the real part that governs the shielding in that range. A real part of a few hundred at one megahertz represents a high permeability grade.

The permeability falls as the frequency rises because the domains cannot follow the field any more, and the loss part rises at the same time. The transition is what limits the useful band of a given material, and it is why a grade with a very high permeability at low frequency has a narrower band than one with a moderate permeability. The selection is therefore a compromise between the value at the low end and the width of the band.

Nanocrystalline And Amorphous Materials

Nanocrystalline alloys are produced by rapidly solidifying a molten alloy into a thin ribbon and then crystallising it under controlled conditions, which produces grains of a few tens of nanometres. The small grain size raises the permeability and lowers the loss compared with a conventional crystalline alloy, and the ribbon can be laminated into a flexible sheet.

Amorphous alloys take a similar route without the crystallisation step, and they are characterised by a high permeability and a low loss over a wide band. Both types are more expensive than a ferrite tile, and both are used where the thickness budget is tight, because a high permeability allows the required attenuation with a thinner sheet. Impedance matching is the other consideration: a material whose impedance is far from that of the surrounding medium reflects rather than absorbs, which reduces the attenuation in a cavity.

Reliability Testing For Automotive Use

An automotive qualification adds environmental requirements to the electrical ones. A temperature cycling test between the extremes of the operating range, repeated many times, checks that the magnetic properties do not drift beyond a defined percentage; a damp heat test with a bias applied checks the insulation and the stability of the sheet; and a vibration test checks that the adhesive holds.

The drift allowance is the figure to watch. A material that changes its permeability by a few percent after a thousand cycles still performs, while one that changes by tens of percent has effectively been replaced by a different material. The adhesion is the other failure mode, because a sheet that lifts from the surface no longer diverts the flux, and the loss of adhesion usually begins at the edges where the moisture reaches first.

Selecting A Sheet For A Product

The process starts with a measurement of the field, at the frequency and in the location where the problem occurs, because a sheet placed in the wrong position does nothing regardless of its properties. The band and the allowed thickness then determine the material family, and the attenuation requirement within the band determines the grade.

The mechanical format is the last filter: a flexible laminated sheet for a curved or a complex surface, a rigid tile for a flat panel, and an adhesive backing where the sheet has to stay in place for the life of the product. Verifying the result with the same measurement that showed the problem is the only way to confirm that the choice was right, and the general approach to reducing the emission at its source, described for EMI suppression and for switching regulator layout, should be applied first. Where the assembly also needs environmental protection, the same thinking as for conformal coating applies to the edges of the sheet.

FAQ

Is a higher permeability always better? Not always. A very high permeability at low frequency usually comes with a narrower useful band, so the choice depends on the range that has to be covered.

Why does the same sheet that works at low frequency do little at high frequency? The permeability falls as the frequency rises and the loss mechanism changes. Above a few gigahertz most magnetic materials behave almost like free space.

What does an impedance mismatch do to an absorber? It causes the energy to be reflected rather than absorbed, which can raise the level inside a cavity. The impedance of the material relative to the surrounding medium is part of the selection.