Soft magnetic absorbing materials: from "reflection" to "absorption" - how high magnetic permeability reshapes the engineering logic of EMI suppression

1、 Electromagnetic waves trapped by metal reflections

In the EMC rectification of electronic devices, engineers often encounter a dilemma: metal shielding covers have been installed inside the equipment, but radiation emissions in certain frequency bands still exceed the standard. The problem lies in the reflection - although the metal shielding cover blocks the electromagnetic waves from radiating outward, inside the enclosed cavity, the electromagnetic waves reflect back and forth between the metal walls, forming standing waves and cavity resonance, generating abnormally high field strength at specific frequencies, which actually exacerbates the interference on sensitive circuits.

The solution to this problem is not to continue 'containment', but to 'diversion' - introducing a material that can absorb electromagnetic waves instead of reflecting them. Soft magnetic absorbing materials are born for this demand.

2、 Why can soft magnetic materials' absorb 'electromagnetic waves

The absorption capacity of soft magnetic absorbing materials comes from their unique magnetic loss mechanism. Soft magnetic materials have typical characteristics of narrow hysteresis loops, low remanence, and low coercivity, which enable them to quickly respond to magnetic field changes in alternating magnetic fields. When electromagnetic waves are incident on the surface of soft magnetic absorbing materials, the magnetic moment inside the material repeatedly flips with the alternating magnetic field. Mechanisms such as hysteresis loss, natural resonance, and domain wall resonance convert the energy of electromagnetic waves into thermal energy, thereby achieving effective attenuation of electromagnetic waves.

The loss mechanism of nickel zinc ferrite is mainly natural resonance and magnetic domain resonance, and its resonance angular frequency is closely related to the saturation magnetization and magnetic crystal anisotropy field. Soft magnetic alloys have rich magnetic loss mechanisms, including hysteresis loss, eddy current loss, and natural resonance, showing great potential for applications in the gigahertz frequency band.

3、 Impedance matching: the "first gate" that determines absorption efficiency

The core engineering problem faced by soft magnetic absorbing materials is impedance matching. When electromagnetic waves are incident on the surface of a material from air, if the electromagnetic impedance of the material differs too much from free space (about 377 Ω), most of the electromagnetic waves will be reflected back on the surface and cannot enter the interior of the material for absorption.

The dielectric constant and magnetic permeability of materials are key parameters that affect impedance matching. High dielectric constant and high magnetic permeability usually mean higher impedance, which may lead to electromagnetic wave reflection. Therefore, it is necessary to adjust these parameters to achieve matching with free space. When the dielectric constant of soft magnetic alloy powder is too high due to high conductivity, coating it with high resistance ferrite can significantly reduce the dielectric constant value, achieve impedance matching, and reflect the good electromagnetic wave absorption characteristics of the system.

Advanced Institute of TechnologyThe research on platinum brand soft magnetic absorbing materials adopts a multi-layer composite structure of alternating high permeability soft magnetic layers and high dielectric constant dielectric layers. By matching impedance layer by layer, electromagnetic waves are effectively guided into the material and attenuated layer by layer, reducing surface reflection.

4、 Material System: Performance Comparison of Ferrite, Soft Magnetic Alloy, and Nanocrystals

The performance of soft magnetic absorbing materials is rooted in their absorbent system. Different materials have their own trade-offs in terms of magnetic permeability, saturation magnetization, applicable frequency range, and cost.

Data source: Industry public information andAdvanced Institute of Science and Technology Laboratory

Ferrite absorbing materials have the characteristics of high absorption frequency band, high absorption rate, and thin matching thickness. They have good performance in suppressing current eddy currents in the mid to high frequency range and are widely used in mid to high frequency application scenarios. The saturation magnetic induction intensity of iron-based nanocrystalline soft magnetic alloy materials is 1.2-1.5T, which is higher than the 0.2-0.5T of ferrite; the Curie temperature is 570-620 ℃, which is much higher than below 200 ℃ of ferrite. More importantly, the temperature characteristics of iron-based nanocrystals are more stable compared to ferrites - when the ambient temperature is above 40 ℃ or below -20 ℃, ferrite materials often cannot function properly due to the large fluctuation of magnetic permeability, while iron-based nanocrystalline soft magnetic alloy materials can still provide their excellent soft magnetic properties stably.

5、 Key Performance Parameters and Selection Points

When evaluating soft magnetic absorbing materials, the following indicators are the most critical:

Real part of magnetic permeability (μ ')Determine the ability of materials to guide magnetic flux. The higher the μ ', the easier it is for electromagnetic waves to be introduced into the interior of the material. The magnetic permeability range of absorbing materials is usually between 40-250 (13.56MHz frequency band), which can be selected according to the specific application frequency band.

Magnetic loss tangent (tan δ μ)Determine the efficiency of materials in converting electromagnetic energy into thermal energy. The higher the magnetic loss tangent, the higher the loss efficiency. Soft magnetic alloy composite materials have both dielectric loss (ε ″=12.19-15.46) and magnetic loss (μ ″=0.41-1.04), and this dual loss synergistic mechanism enables efficient attenuation of low-frequency electromagnetic waves at thin thicknesses.

Effective absorption bandwidth (EAB)Refers to the frequency range where the reflection loss is below -10dB (i.e. absorption rate exceeds 90%). By using compositional control methods such as La Nd doping, the effective absorption bandwidth of FeCo soft magnetic alloys can be extended from 1.51GHz to 7.56GHz.

Thickness is a key constraint for soft magnetic absorbing materials in spatially constrained scenarios. The thickness of the material affects impedance matching and absorption efficiency - thicker materials may increase the propagation path of electromagnetic waves, but may also increase reflection, requiring finding the optimal thickness between impedance matching and efficient absorption. The thickness range of the current mainstream soft magnetic wave plate covers 0.025mm to 0.5mm.

6、 Application scenarios: From wireless charging to EMI suppression

Wireless charging with magnetic isolation and shielding.This is the field with the fastest growth in the usage of soft magnetic absorbing materials. In wireless charging systems, soft magnetic separators are placed between the receiving coil and the metal backplate to guide magnetic field lines to the effective area of the coil and reduce leakage to the metal backplate. Iron based nanocrystalline soft magnetic alloy materials have high magnetic permeability, high saturation magnetic induction intensity, and high Curie temperature, which can effectively reduce the weight and volume of electromagnetic shielding composite layered structures, making charging modules thinner and lighter. In high-power wireless charging scenarios for automobiles, iron-based nanocrystalline separators with the same volume can provide greater magnetic flux compared to ferrite separators, ensuring no magnetic saturation abnormalities and reducing heat generation.

EMI suppression and cavity resonance elimination.Inside the shielding cover of devices such as smartphones and tablets, soft magnetic absorbing materials are attached to the inside of the shielding cover to reduce electromagnetic interference and signal crosstalk, and eliminate the back and forth reflection of electromagnetic waves inside the shielding cavity. Absorbing materials can suppress multiple reflections of electromagnetic waves, absorb chaotic echoes, and eliminate the interference and influence of microwaves on various devices.

RFID/NFC anti metal interference.When RFID/NFC tags are tightly attached to metal surfaces, metal eddy currents severely weaken the antenna magnetic field. The soft magnetic isolation sheet provides a low magnetic resistance path for magnetic field lines through high magnetic permeability, concentrating the magnetic flux in the tag antenna area, effectively improving the read/write distance and recognition rate.

7、 Selection considerations: Engineering decisions from frequency to impedance matching

When engineers choose soft magnetic absorbing materials, the following dimensions deserve special attention:

• Target interference frequency matching.The frequency response of different soft magnetic material systems varies significantly. Ferrite is suitable for the mid to high frequency range above 50kHz, with a maximum of 100MHz or higher; Iron based nanocrystals have the advantage of high magnetic permeability in the low frequency range of 50Hz-50kHz; Soft magnetic alloy powder exhibits high saturation magnetic induction intensity in the range of 1kHz to 100kHz.
• Impedance matching design.Impedance matching is a key factor affecting the absorption performance. When selecting, attention should be paid to whether the ratio of the dielectric constant to the magnetic permeability of the material is close to the free space impedance, to ensure that electromagnetic waves can effectively enter the interior of the material and be absorbed, rather than being reflected on the surface.
• Thickness and installation space.In devices with extremely limited space such as smartphones, it is recommended to prioritize ultra-thin specifications (which can be as thin as 0.025mm); In scenarios where industrial equipment is not sensitive to thickness, thicker products can be chosen to achieve higher absorption efficiency.
• Working temperature range.Ferrite exhibits significant fluctuations in magnetic permeability when the ambient temperature is above 40 ℃ or below -20 ℃, while iron-based nanocrystals remain stable over a wide temperature range, making them suitable for scenarios such as automotive electronics that require high temperature stability.

8、 Conclusion

The essence of soft magnetic absorbing materials is to use magnetic loss mechanisms to "absorb" electromagnetic wave energy rather than "reflect" it as a functional material. It introduces electromagnetic waves into the material through high magnetic permeability, converts them into thermal energy through hysteresis loss, natural resonance, and eddy current loss, and eliminates electromagnetic interference from the source.

The evolution of materials from ferrite to nanocrystals reflects the upgrading of EMI suppression demand from "medium high frequency applicability" to "wideband stability". Understanding the decisive role of impedance matching in absorption efficiency, the frequency response differences of different material systems, and the engineering trade-off between thickness and absorption performance - these are the key to upgrading soft magnetic absorbing materials from a "functional material" to an "engineering decision".