The 30-Second Version
Ferrite guides. Nanocrystalline concentrates. Absorber eats. A ferrite sheet redirects magnetic flux around metal at kHz–MHz frequencies with almost no loss. A nanocrystalline sheet does the same job with ~100× the permeability in a fraction of the thickness. An EMI absorber sheet is lossy by design — it converts high-frequency noise (10 MHz–18 GHz) into heat. Pick by answering one question: what should happen to the magnetic energy?
If the energy must be guided somewhere useful — into a tag, across a charging gap — you need a low-loss material: ferrite, or nanocrystalline when space is tight. If the energy is noise that must disappear, you need loss: an absorber. Everything else in this guide is detail.
Why Three Black Sheets Do Three Different Jobs
All three materials are judged by the same two numbers — real permeability μ′ and magnetic loss μ″ — but they sit at opposite ends of the design space:
- Ferrite sheet (e.g. XAK-NFC, S-F): μ′ 20–150 in the NFC/WPT bands, μ″ ≈ 0. It is a flux director — the magnetic equivalent of a copper wire for current.
- Nanocrystalline sheet (e.g. XAK F-N): μ′ ≈ 13,000 at 100 kHz, still low loss. It is a flux concentrator — same job as ferrite, ~100× the permeability per millimeter.
- Absorber sheet (e.g. XAK S-A): μ′ 20–300 with deliberately high μ″ in its working band. It is an energy sink — the magnetic equivalent of a termination resistor.
Frequency is the other axis. Flux guiding matters at kHz–MHz (RFID, NFC, Qi charging). Dissipation matters from tens of MHz upward, where conductors reflect and resonances live. A material cannot be excellent at both jobs in the same band — high μ″ (needed to absorb) is exactly what you must avoid when guiding flux to a tag or coil.
The Decision Table
| Question | Ferrite Sheet | Nanocrystalline Sheet | EMI Absorber Sheet |
|---|---|---|---|
| What it does | Guides flux around metal | Concentrates flux in minimal thickness | Dissipates noise as heat |
| μ′ class | 20–150 @ 1 MHz | ≈ 13,000 @ 100 kHz | 20–300 @ 1 MHz (lossy) |
| Loss μ″ in its band | ≈ 0 (must be) | Low | High (by design) |
| Working frequency | kHz–MHz (13.56 MHz NFC, 87–360 kHz Qi) | kHz–hundreds of kHz (Qi/Qi2, FPC coils) | 10 MHz–18 GHz |
| Thickness | 0.05–1 mm plates or 0.05–0.3 mm sheets | 0.06–0.4 mm | 0.03–2 mm |
| Form | Sintered plate or flexible sheet | Flexible ribbon, rolls | Flexible polymer sheet |
| Relative material cost | Low | High | Mid |
| XAK series | XAK-NFC / S-F | F-N | S-A |
| Reach for it when… | NFC on metal, standard Qi TX/RX with room | Slim/curved RX, hot modules, tight Z-height | EMI noise, cavity resonance, harmonic suppression |
Five Real Scenarios, Walked Through
1. NFC payment antenna against a metal battery
Problem: read range collapses on metal. Energy must reach the tag, not be eaten. → Ferrite sheet, μ′ 100–150, 0.10–0.20 mm, fully covering the coil. Product: XAK-NFC.
2. Qi2 MPP receiver in a 0.4 mm stack
Problem: ferrite plate needs more Z-height than exists. → Nanocrystalline F-N at 0.10–0.20 mm reaches the coupling target where ferrite physically cannot fit. Full walk-through: Qi2 MPP guide.
3. Radiated-emissions failure at 600 MHz
Problem: shielding reflects noise into a cavity resonance. Energy must disappear. → Absorber S-A, starting from the μ′ 20–50 class for that band, die-cut over the noise source. Product: S-A series.
4. 125 kHz RFID reader next to a motor drive
Problem: interference field, not metal. → Absorber in the high-μ′ grades (S-A 25000/30000) between drive and reader, because at 125 kHz the lossy material isolates the antenna field instead of detuning it.
5. Slim wearable that charges hot and drops NFC range
Problem: two problems — this is a stack, not a sheet. → Nanocrystalline behind the RX coil (coupling, F-N) plus a ferrite sheet behind the NFC antenna (XAK-NFC), each layer doing one job.
The Three Crossover Traps
- Absorber behind an NFC antenna. The lossy sheet consumes the 13.56 MHz field you were trying to save. Symptom: read range gets worse after adding the sheet.
- Ferrite sheet against GHz noise. Low-loss material does almost nothing at 100 MHz–6 GHz; the EMC report stays red. You needed dissipation, not redirection.
- Nanocrystalline where plain ferrite suffices. It works, but you paid nanocrystalline prices to solve a ferrite problem. If Z-height and curvature are comfortable, ferrite is the honest answer — we supply both.
Still Between Two Materials?
Send us three things: the frequency (or band), what you want the material to do (guide / concentrate / absorb), and the thickness budget. We confirm the family and grade in writing before sampling — the "wrong family" conversation costs one email; the wrong-family sample cycle costs a month. Start on the product pages for NFC ferrite sheets, nanocrystalline sheets and EMI absorber sheets.