What Ferrite Is
Ferrite is a ceramic magnetic material made by sintering iron oxide together with metal oxides such as manganese, zinc and nickel at high temperature. Its defining property is simple: it barely conducts electricity, yet it conducts magnetism.
Metallic magnetic materials have good magnetic properties but conduct electricity, so at high frequency eddy currents flow inside them, causing loss and heat. They also risk shorting if placed directly on a circuit. Ferrite has very high resistivity and avoids both problems. It is effectively the only class of magnetic material that can sit right beside — or directly on top of — a high-frequency circuit without any risk of a short, which is why it is so widely used in electronic components.
Ferrite performance is expressed by two values. μ′ (the real part) is the ability to draw in and carry magnetic flux; μ″ (the imaginary part) is the ability to convert magnetic energy into heat and dissipate it. The balance between the two is set by material design, and it is what decides the application: guiding flux for shielding needs a high μ′, while absorbing and removing noise needs a high μ″.
As a sintered ceramic it offers good corrosion resistance at relatively low cost, but it is brittle under impact and loses its magnetic properties above the Curie temperature (roughly 100-300°C depending on the material). Both points belong in the design review.
Soft Ferrite and Hard Ferrite
Ferrite splits into two families by magnetisation behaviour. Everything used for noise suppression and for tapes and sheets is soft ferrite.
| Item | Soft ferrite | Hard ferrite |
|---|---|---|
| Magnetisation | Magnetises under a field, releases when removed (low coercivity) | Retains magnetisation once magnetised — permanent magnet (high coercivity) |
| Typical composition | Mn-Zn, Ni-Zn | Barium (Ba), strontium (Sr) |
| Main uses | Inductor and transformer cores, EMI suppression, tapes and sheets | Motors, speakers, magnet holders and other permanent magnets |
Mn-Zn or Ni-Zn: Frequency Decides the Material
Soft ferrite divides further into Mn-Zn and Ni-Zn. Which one to use is decided by the target frequency.
| Item | Mn-Zn | Ni-Zn |
|---|---|---|
| Effective range | Low frequency, up to a few MHz | High frequency, a few MHz to GHz |
| Permeability μ′ | High | Comparatively low |
| Resistivity | Comparatively low; surface insulation may be required | Very high; favourable for surface insulation |
| Examples | SMPS transformers, wireless charging, NFC sheets | EMI beads and clamp cores above 100 MHz |
Below roughly a few tens of MHz, choose Mn-Zn; for high-frequency noise above 100 MHz, choose Ni-Zn. A mismatch between frequency and material is the most common reason a countermeasure does not work. If a part has been fitted and nothing improved, check the material choice before you reconsider the position.
The Structure of Ferrite Tape
Ferrite tape (magnetic sheet) is ferrite formed into a thin sheet and laminated with an adhesive. It is typically built from the layers below, with a total thickness designed in roughly the 0.05-0.5 mm range.
- Protective film (PET)
- Ferrite layer (sintered or polymer composite)
- Adhesive (PSA)
- Release liner
It can be die-cut to the shape a design requires, and because it is an insulator it can be applied directly over a circuit without any risk of a short.
Sintered Type and Polymer Type
| Item | Sintered sheet | Polymer sheet |
|---|---|---|
| Construction | Sintered ferrite thinned, crack-treated, then film-laminated | Ferrite powder compounded into a rubber or resin binder |
| Permeability | High — the performance advantage | Comparatively low |
| Flexibility | Limited — bending increases internal cracking and can shift properties | Good — suited to curved surfaces |
| Cost | Comparatively high | Comparatively low |
Two Mechanisms: Guiding and Absorbing
Ferrite tapes that look alike can have opposite purposes. The difference is whether μ′ or μ″ was designed to be large.
-
Flux guiding (shielding) — using μ′
High permeability creates a magnetic path so the field travels through the sheet, keeping flux away from the metal behind it (battery, chassis) and preventing eddy-current loss and heating. -
Noise absorption — using μ″
Magnetic loss rises at high frequency, converting noise energy into heat and dissipating it. This is what suppresses radiated noise from ICs and FPCs.
Choosing the wrong type gives you no benefit, or introduces loss you did not want. Fitting a guiding type where noise must be removed, or an absorbing type where charging efficiency must be preserved, are the classic mistakes. The wider difference between reflection and absorption is covered in EMI Absorber vs. Shielding.
What Each Application Requires
| Application | Frequency | Key property | How it is used |
|---|---|---|---|
| Wireless charging (WPC/Qi) | around 100-200 kHz | μ′ (guiding) | Applied between the charging coil and the battery to raise efficiency and prevent battery heating |
| NFC / RFID | 13.56 MHz | μ′ (guiding) | Prevents loss of read range caused by metal behind the antenna |
| EMI noise control | tens of MHz to GHz | μ″ (absorbing) | Applied over ICs, FPCs and cables to suppress radiated noise |
| Wave absorption | GHz to mmWave | μ″ (absorbing) | Suppresses resonance in radar and high-speed communication components |
Points to Watch When Applying
- Keep it in close contact. Apply it tight against the noise source, coil or antenna, and cover enough area that flux does not leak past it.
- Match the frequency first. Check the material's property curve at the frequency of the application before selecting.
- Use the polymer type on curves. Bending a sintered sheet increases internal cracking and can shift its properties.
- Thickness is a trade-off. Thicker performs better but conflicts with device thickness limits, so design for the thinnest sheet that meets the requirement.
Our EMI material line-up, including ferrite tape for cable noise suppression, is on the EMI Shielding Solution page.
FAQ
- Are a ferrite sheet and an EMI absorber sheet different products?
- The categories overlap. Many absorber sheets that work by magnetic loss are ferrite-based, and a ferrite sheet designed for high μ″ is used as an absorber. Ferrite sheets for wireless charging and NFC are different in nature, because their purpose is to guide flux rather than absorb it. Rather than the product name, check whether the sheet was designed for μ′ or for μ″.
- I applied ferrite tape but the noise did not drop.
- The most common cause is a mismatch between frequency and material: a low-frequency grade does almost nothing against a problem above 100 MHz. Next, check whether the applied area is too small so flux escapes around it, and whether the tape sits too far from the noise source. On a power line carrying high current, core saturation may also have collapsed the effect.
- Is thicker always better?
- On performance alone, thicker is better, but it conflicts with the device's thickness budget. In practice the goal is the thinnest sheet that still meets the requirement. Tell us the space constraint and the target characteristic and we can review it with you.
- Can it be applied directly over a circuit?
- Yes. Ferrite is a high-resistivity insulator, so direct application over a circuit carries no risk of a short. This is what sets ferrite apart from metallic magnetic materials. The adhesive's heat resistance and removability specifications should still be checked against your process.
Reviewing flux management around a wireless charging or NFC antenna, or a noise countermeasure?
Tell us the location and target frequency and we will review a suitable material construction with you.
