Abstract
The rapid commercialization of wide-bandgap silicon carbide (SiC) and gallium nitride (GaN) devices is pushing power electronics into the megahertz regime. Conventional ferrites have become a critical bottleneck because of their low saturation magnetization and sharply increasing high-frequency loss, whereas soft magnetic composites (SMCs) with high magnetic induction and low loss have therefore attracted increasing attention. The macroscopic magnetic performance of SMCs is governed by the microstructure and interfacial characteristics of the insulating coating on magnetic powders. This review summarizes recent progress in insulating coating technologies for SMCs intended for high-frequency power electronics. It first discusses the origin of high-frequency eddy-current loss and the key design requirements for insulating layers. It then traces the development of coating strategies from organic coatings to inorganic, composite, and nanoscale coatings, with emphasis on the process-structure-property relationships of phosphate, oxide, glass/ceramic, and atomic-layer-deposited (ALD) oxide systems. From the perspective of interface engineering, the review further examines how core-shell architectures suppress eddy currents while preserving acceptable permeability, highlighting the trade-offs among coating thickness, uniformity, adhesion, and magnetic dilution. Finally, the performance of representative coating routes is compared in terms of high-frequency loss reduction, thermal stability, and manufacturing feasibility, and future challenges and opportunities are discussed. This review provides a systematic framework for the rational design of next-generation high-performance SMCs for high-frequency power electronics.
1 Introduction
Power electronics is undergoing a paradigm shift from silicon (Si) devices to wide-bandgap semiconductor devices based on SiC and GaN. These devices can increase switching frequencies from the conventional tens-of-kilohertz range to several megahertz, substantially reducing the volume of passive components and increasing power density. This transition is central to electric vehicles, renewable-energy grid integration, data-center power supplies, and aerospace power systems (). The accompanying frequency increase, however, imposes stringent requirements on magnetic components. At megahertz frequencies, magnetic materials must simultaneously provide high saturation magnetization (), sufficient permeability (), and extremely low power loss (). Conventional MnZn power ferrites have high electrical resistivity and relatively low high-frequency loss, but their Ms is only approximately 0.5 T, which is insufficient for high-current, high-power-density transformers and inductors. Metallic soft magnetic materials, including Fe, FeSi, FeSiAl, FeNi, amorphous alloys, and nanocrystalline alloys, offer much higher Ms, but their very low resistivity leads to severe skin effects and eddy-current loss in bulk form at high frequency (Wang et al., 2021; ; Wang et al., 2025).
Soft magnetic composites (SMCs) address this limitation by coating magnetic micro- or nanopowders with an insulating layer and consolidating them into compacted components. This architecture divides a conductive bulk metal into numerous electrically isolated magnetic units and thereby suppresses macroscopic eddy-current circulation. The electrical resistivity of SMCs can be several orders of magnitude higher than that of the corresponding bulk metals, while the saturation magnetization remains close to that of the original powder. SMCs are therefore promising candidates for SiC/GaN-based high-frequency power electronics (Shokrollahi and Janghorban, 2007). Several commercial SMC products, such as Somaloy® powders and FeSiAl powder cores, are already available. Nevertheless, their performance ceiling remains apparent: organic coatings lack thermal stability, conventional inorganic coatings either provide insufficient insulation or markedly reduce permeability, and customized designs for operation above 2 MHz remain underdeveloped (; ).
The insulating coating fundamentally determines the performance bottleneck of SMCs. The choice of coating material, microstructure, thickness, and bonding state at the powder-coating interface jointly determines the electrical insulation, packing density, permeability, loss mechanisms, and thermal stability of each core-shell unit. Recent research has moved from organic coatings to inorganic coatings, including phosphates, oxides, and glass-based systems, and further toward composite and nanoscale coatings. In particular, atomic layer deposition (ALD) enables precise nanoscale control of inorganic coatings, driving the insulating layer toward thinner, more uniform, and more thermally robust architectures (). At the same time, interface engineering has emerged as a third design dimension distinct from powder selection and coating chemistry. By regulating interfacial reactions, diffusion, and stress, it becomes possible to optimize eddy-current suppression and permeability retention synergistically.
This review focuses on insulating coating technologies for SMCs. Starting from the physics of magnetic loss, it summarizes the evolution of organic, inorganic, composite, and nanoscale coating strategies, analyzes recent advances in core-shell interface engineering, compares the performance of representative coatings under high-frequency and high-temperature conditions, and discusses the major challenges and future directions of the field.
2 High-frequency loss mechanisms and design criteria for insulating coatings
The total core loss of an SMC () can be divided into hysteresis loss (Ph), eddy-current loss (), and residual loss (). In the megahertz regime, the contribution of eddy-current loss increases rapidly and often becomes dominant (). According to classical eddy-current theory, the eddy-current loss inside an individual spherical conductive particle is proportional to the square of both particle diameter and frequency and inversely proportional to electrical resistivity (Vesa et al., 2023).
To quantitatively analyze these contributions, the total core loss is generally described by the classical loss separation model (; ):where is the hysteresis loss, is the eddy-current loss, and is the residual (excess) loss. For soft magnetic composites (SMCs) operating at moderate frequencies, is often negligible compared to the other two components. The hysteresis loss per unit volume is proportional to the area of the quasi-static B-H loop and can be expressed as:where is the frequency, is the maximum flux density, is the hysteresis loss coefficient, and is the Steinmetz exponent (typically between 1.5 and 2.5). Hysteresis loss is governed by intrinsic magnetic properties (e.g., coercivity, anisotropy) and is largely insensitive to coating type, although interfacial stress from coating can indirectly increase .
In contrast, eddy-current loss in SMCs has two distinct origins:
Intraparticle eddy-current loss, which arises from circulating currents within each conductive magnetic particle. For an isolated spherical particle, this loss is given by:
Interparticle eddy-current loss, which originates from macroscopic circulating currents that flow across particle boundaries. This component is effectively suppressed by the insulating coating, which breaks the electrical connectivity between particles. The interparticle resistance must be raised to Ω·cm or higher to reduce this loss to acceptable levels at MHz frequencies.
where is the characteristic size (e.g., particle diameter), is the electrical resistivity, and is a geometric constant. This component is controlled by particle size and core material composition, not directly by the coating.
The residual (excess) loss is associated with domain-wall displacement dynamics, microeddy currents around moving domain walls, and local magnetic inhomogeneities. In SMCs, excess loss becomes increasingly important above 1 MHz, particularly when coating non-uniformity or interfacial defects create localized flux pinning sites. Recent studies have shown that by combining high-resistivity coatings (e.g., Fe2O3 or Fe3O4) with optimized compaction, the total eddy-current loss (intraparticle + interparticle) can be reduced by over 80% at ∼100 kHz, while excess loss can be minimized through uniform nanoscale coatings and stress-relief annealing (; ).
This physical picture defines several stringent requirements for the insulating layer (Shokrollahi and Janghorban, 2007). First, the coating must provide sufficiently high electrical resistivity and dielectric breakdown strength so that interparticle resistance remains on the order of Ω·cm or higher during compaction and service. Second, because the coating is a nonmagnetic phase, it inevitably dilutes the magnetic volume fraction and lowers the effective permeability and saturation magnetization of the composite. The coating should therefore be as thin as possible while still ensuring reliable insulation; in many high-frequency designs, the ideal thickness is below 100 nm and preferably within the tens-of-nanometers range. Third, for SiC and GaN power modules that may operate at 150 °C–250 °C and for post-compaction annealing at 500 °C–800 °C to remove internal stress, the coating must resist decomposition, softening, and harmful interfacial reactions with the magnetic substrate. Fourth, robust adhesion between the coating and metallic powder is essential. Weakly bonded interfaces tend to crack or delaminate during high-density compaction, typically above 7 g cm-3, creating conductive pathways that negate the insulation design. Finally, the coating should exhibit chemical stability and process compatibility during impregnation, encapsulation, and long-term operation, while also being suitable for scalable manufacturing. Although traditional organic coatings perform well in insulation and processability, their decomposition temperatures are generally below 300 °C. This limitation prevents high-temperature stress-relief annealing and high-temperature service, providing the fundamental motivation for the transition from organic to inorganic coating systems.
By using insulating coatings to divide the bulk metal into electrically isolated units, SMCs interrupt the pathways for interparticle circulating currents. Under these conditions, total eddy-current loss is mainly determined by microscopic eddy currents within individual particles and dielectric losses arising from capacitive coupling between particles.
This physical picture defines several stringent requirements for the insulating layer (Shokrollahi and Janghorban, 2007). First, the coating must provide sufficiently high electrical resistivity and dielectric breakdown strength so that interparticle resistance remains on the order of 104 Ω cm or higher during compaction and service. Second, because the coating is a nonmagnetic phase, it inevitably dilutes the magnetic volume fraction and lowers the effective permeability and saturation magnetization of the composite. The coating should therefore be as thin as possible while still ensuring reliable insulation; in many high-frequency designs, the ideal thickness is below 100 nm and preferably within the tens-of-nanometers range (Xu et al., 2024). Third, for SiC and GaN power modules that may operate at 150 °C–250 °C and for post-compaction annealing at 500 °C–800 °C to remove internal stress, the coating must resist decomposition, softening, and harmful interfacial reactions with the magnetic substrate. Fourth, robust adhesion between the coating and metallic powder is essential. Weakly bonded interfaces tend to crack or delaminate during high-density compaction, typically above 7 g cm-3, creating conductive pathways that negate the insulation design. Finally, the coating should exhibit chemical stability and process compatibility during impregnation, encapsulation, and long-term operation, while also being suitable for scalable manufacturing (). Although traditional organic coatings perform well in insulation and processability, their decomposition temperatures are generally below 300 °C. This limitation prevents high-temperature stress-relief annealing and high-temperature service, providing the fundamental motivation for the transition from organic to inorganic coating systems ().
3 Evolution of insulating coating technologies
3.1 Organic coatings: foundation and limitations
The earliest mature coating strategy for SMCs involved the use of organic polymers, such as epoxy resin, phenolic resin, silicone, and polyimide, to form insulating films on magnetic powders. These materials provide excellent insulation, good flexibility, effective tolerance to compaction deformation, simple processing, and relatively low cost (; ; ; ). For example, early Somaloy-based materials used organic coatings and could deliver suitable loss characteristics below 100 kHz. However, most organic materials decompose below 300 °C. As a result, high-temperature annealing cannot be applied after compaction to remove residual internal stress, leaving hysteresis loss relatively high. In high-temperature service, the resistivity and mechanical integrity of organic coatings also deteriorate substantially, raising concerns about long-term reliability. These intrinsic deficiencies have shifted research toward inorganic coating systems.
3.2 Inorganic coatings: thermal robustness and material diversity
Inorganic coatings exhibit high thermal stability and can generally withstand annealing at 550 °C–800 °C, which helps relieve compaction-induced stress, reduce hysteresis loss, and enable high-temperature operation of SMCs. According to chemical composition, inorganic coatings can be broadly classified into phosphate, oxide, and glass/ceramic systems.
Phosphate coatings are the most widely used inorganic coatings. A typical process treats Fe-based powders in phosphoric acid or phosphate solutions, forming amorphous or crystalline phosphate layers such as FePO4, AlPO4, and MnPO4 through surface reactions (; ). Phosphate layers adhere strongly to Fe substrates, usually have thicknesses of 50–200 nm, and can tolerate annealing temperatures up to approximately 600 °C. Appropriate phosphating can increase the resistivity of pure-iron SMCs by three to four orders of magnitude and reduce loss at 100 kHz/0.1 T from above 1000 kW m-3 to below 300 kW m-3. Phosphate-coated FeSiAl and FeSi materials have also been commercialized. Nevertheless, single phosphate layers have limited thermal stability; above 600 °C, they may decompose or react excessively with the substrate. Their insulation can also be insufficient in the megahertz range, where local breakdown may reduce interparticle resistance ().
Oxide coatings are designed to provide higher insulation and thermal stability. Common oxide materials include SiO2, Al2O3, MgO, TiO2, and ZrO2. Preparation methods include sol-gel processing, chemical precipitation, mechanical mixing, and thermal oxidation (Zhang et al., 2024; Xie et al., 2023; ). In the case of SiO2, hydrolysis of tetraethyl orthosilicate (TEOS) can deposit a uniform SiO2 layer of approximately 100–300 nm on Fe powders, providing much higher resistivity than phosphate layers. Al2O3 has very high hardness and chemical stability, and dense layers can be formed on FeSiAl surfaces by sol-gel routes or heterogeneous nucleation, markedly reducing loss at 10 MHz in compacted cores. However, oxide coatings commonly face three challenges: sufficient insulation often requires relatively large thickness, which lowers effective permeability; rigid oxide layers can crack under high compaction pressure and generate conductive paths; and thermal expansion mismatch between some oxides, such as SiO2, and metallic substrates can induce microcracking during thermal cycling. Thus, single-oxide systems often require difficult compromises between insulation and magnetic performance.
Glass and ceramic coatings use low-melting glass powders, such as borosilicate glass, or ceramic precursors to form continuous coating layers on magnetic powders through mixing, compaction, and sintering. These materials have extremely high insulation resistance and can remain stable above 800 °C (Wang et al., 2024). For instance, borosilicate-glass-coated FeSiAl powders can yield SMCs with very high resistivity and suitability for applications above 10 MHz after sintering. However, glass coatings are usually thick, often in the micrometer range, and therefore substantially dilute magnetic performance. Liquid-phase flow during sintering may also cause local powder agglomeration and property inhomogeneity. In addition, controlling interfacial wetting between glass and metal remains a processing challenge. These inherent limitations of single-material coatings—whether phosphate, oxide, or glass—have motivated the exploration of more sophisticated composite and nanoscale architectures.
3.3 Composite and nanoscale coatings: a new paradigm for synergistic performance
Single-material coatings rarely satisfy the combined requirements of thinness, uniformity, insulation, adhesion, and thermal stability. This limitation has motivated the development of composite and nanoscale coatings.
Organic-inorganic hybrid coatings seek to combine the advantages of both categories. For example, a thin inorganic layer, such as phosphate or SiO2, can first be formed on the powder surface to provide thermal stability and baseline insulation, followed by an ultrathin organic layer, such as silicone resin, to fill cracks, improve flexibility, and increase breakdown strength. Such bilayer or multilayer designs can provide high insulation and good formability at relatively low total thickness (; ; Wan et al., 2024; Zheng et al., 2024). Similar approaches incorporate inorganic nanoparticles, such as nano-SiO2 or nano-Al2O3, into organic resins to form hybrid coatings with improved thermal stability and resistivity. Nevertheless, the presence of organic components still limits the maximum annealing temperature and prevents full exploitation of stress-relief annealing.
Fully inorganic multilayer or composite coatings represent a more advanced direction. For example, depositing SiO2 or Al2O3 on a phosphate underlayer combines strong interfacial bonding from the bottom layer with high resistivity and environmental isolation from the top layer. Multilayer designs can reduce total coating thickness by 20%–40% while maintaining comparable insulation, thereby alleviating magnetic dilution (). In addition, insulating nanoparticles, such as Al2O3 or TiO2, can be embedded on powder surfaces or incorporated into inorganic matrices by high-energy mixing to form nanocomposite coatings. The high surface area and dispersion-strengthening effect of nanoparticles can improve coating density and resistance to crack propagation.
Atomic layer deposition pushes coating precision to the atomic scale. ALD relies on alternating vapor-phase precursors that undergo self-limiting surface reactions, enabling subnanometer thickness control and large-area, pinhole-free, conformal coatings (Wang J. et al., 2022; ). Typical ALD oxides, including Al2O3 from trimethylaluminum and water, TiO2, and HfO2, can provide very high resistivity and breakdown strength at thicknesses of only 10–50 nm. After 50 nm Al2O3 ALD coating on Fe-based amorphous powders, the resistivity of SMCs can increase to the 106 Ω cm range, and loss at 5 MHz/0.1 T can decrease by more than one order of magnitude compared with uncoated or conventionally phosphate-coated powders. Because the coating is extremely thin, permeability decreases by only about 10%, achieving a favorable balance between eddy-current suppression and high permeability. ALD also operates at relatively low temperatures, typically 100 °C–250 °C, making it compatible with some temperature-sensitive powders, while coating uniformity is largely independent of powder shape. The main barriers to large-scale ALD application remain cost and throughput, although the rapid development of fluidized-bed powder ALD is making industrial implementation increasingly plausible.
Despite its exceptional precision, ALD is not without practical limitations. The technique relies on sequential, self-limiting surface reactions that are inherently slow; typical deposition rates are on the order of 0.1–0.2 nm per cycle, and achieving a 50 nm coating may require hundreds of cycles, resulting in processing times of several hours per batch. This low throughput translates into high capital and operational costs, making conventional ALD currently suitable primarily for high-value-added applications rather than mass-produced SMC components. Furthermore, while laboratory-scale fluidized-bed reactors have demonstrated promising results for powder coating, scale-up to industrial tonnage quantities remains challenging in terms of precursor consumption, waste management, and system reliability. In comparison, phosphate and sol–gel oxide coatings are far more cost-effective and easily integrated into existing compaction production lines, albeit with inferior thickness control and high-frequency performance. Therefore, the choice between ALD and conventional coatings involves a clear trade-off: ALD offers superior uniformity and minimal magnetic dilution at the expense of cost and throughput, whereas conventional methods provide economic feasibility at the cost of performance ceilings. The practical adoption of ALD in SMC manufacturing will likely depend on continued advances in spatial ALD, rotary-bed reactors, and hybrid vapor-phase–wet-chemistry approaches that can bridge the gap between laboratory excellence and industrial reality (Wang J. et al., 2022; ).
4 Core-shell interface engineering: balancing eddy-current suppression and permeability retention
The objective of coating technology is not merely to maximize resistivity, but rather to achieve the minimum insulation required for a target application at the lowest possible cost in magnetic dilution. This fundamentally transforms the problem into one of interface engineering at the core-shell unit level.
4.1 Effects of coating thickness and uniformity
According to effective-medium theory, the effective permeability μe of an SMC is closely related to the volume fraction v of the magnetic core. For nonmagnetic coatings, μe decreases monotonically as coating thickness increases (; Zhu et al., 2023; Zhou et al., 2024). Therefore, coating thickness should be minimized while ensuring reliable interparticle electrical isolation. Conventional wet-chemical methods struggle to control coating thickness below 50 nm with high reproducibility, and coating uniformity is strongly affected by powder morphology, leaving local weak-insulation sites. The essential advantage of ALD lies in its mechanism of self-limiting surface reactions, which yields pinhole-free, highly conformal coatings. This morphological uniformity is critical because it eliminates “weak spots” that are common in conventional wet-chemical coatings (e.g., phosphate), where non-uniform coverage leads to localized direct particle contacts during compaction. From a mechanism-based perspective, the superior performance of ALD is therefore not merely a result of its thinness, but a consequence of its ability to create a continuous and homogeneous insulating barrier that maximizes interparticle resistance with minimal non-magnetic volume fraction. In contrast, while phosphate coatings offer strong chemical adhesion (Fe-O-P bonds) that enhances mechanical integrity, their thickness non-uniformity introduces a statistical distribution of insulation reliability, making them less effective at MHz frequencies where local breakdown can initiate premature loss escalation. Studies have shown that only 15 nm of ALD Al2O3 can increase interparticle resistance in Fe-based SMCs by two orders of magnitude compared with conventional phosphate coatings of approximately 100 nm, while maintaining more than 30% higher permeability (Wang J. et al., 2022; Zhou et al., 2024; ; ; ; ).
4.2 Interfacial bonding and stress management
The bonding state between the coating and magnetic core directly affects stress sensitivity of permeability and high-frequency loss. Weakly bonded interfaces are prone to debonding during compaction, forming air gaps that reduce permeability and increase local demagnetizing fields and loss. Strong chemical bonding, such as Fe-O-P bonds formed between phosphate layers and Fe substrates, generally improves mechanical integrity and thermal stability. Excessively strong bonding, however, may introduce interfacial stress. During high-temperature annealing and subsequent cooling, mismatch in thermal expansion coefficients can generate additional magnetic anisotropy in the magnetic core and increase hysteresis loss (Xu et al., 2024; ; Zhang et al., 2025). One objective of interface engineering is therefore to identify an appropriate balance between adhesion and stress buffering. Composite and gradient coatings are particularly useful in this respect; a compliant underlayer or nanoparticle buffer layer can release stress, while a rigid outer layer provides electrical insulation (Zhou et al., 2026).
4.3 Electrical-magnetic coupling and high-frequency characteristics
When the operating frequency rises above several megahertz, intraparticle microscopic eddy currents and dielectric displacement currents begin to affect loss in addition to interparticle macroscopic eddy currents. The dielectric constant of the coating and capacitive coupling between particles become important. An excessively thin coating, especially one with a high dielectric constant, can produce substantial parasitic capacitance between particles, providing displacement-current pathways at high frequency and thereby increasing loss (Shi et al., 2026; ). Consequently, core-shell design must consider both resistance and capacitance. Proposed strategies include replacing Al2O3 (κ ≈ 9) with lower-dielectric-constant materials such as SiO2 (κ ≈ 3.9), or designing nanoporous coatings to reduce the effective dielectric constant and improve high-frequency performance. Interface engineering is therefore evolving from the pursuit of high resistivity alone toward multidimensional optimization of dielectric behavior, insulation reliability, and magnetic properties ().
In summary, the S-M-P relationship in SMC coatings can be distilled into a hierarchical chain: Processing → Morphology (thickness, uniformity) → Interface Structure (bonding, stress, dielectric state) → Macroscopic Properties. For instance, ALD produces a uniform, ultrathin Al2O3 morphology that minimizes magnetic dilution while maintaining high resistivity; its well-defined interface reduces stress-induced anisotropy but requires careful dielectric-constant management at high frequencies. In contrast, phosphate coatings offer superior adhesion through chemical reaction but suffer from thickness non-uniformity and moderate thermal stability.
5 Technology comparison and suitability for high-frequency power electronics
From the viewpoint of high-frequency power electronics, an ideal material should simultaneously achieve low loss, high Bs, and appropriate permeability in the 2–10 MHz range, while operating reliably at 150 °C–200 °C. Different insulating coating technologies exhibit substantial differences across these key performance metrics. However, the numerical data in Table 1 should be interpreted with caution for direct quantitative comparison. The reported core-loss and resistivity values are drawn from a wide range of independent studies that employ different raw materials, particle sizes, compaction pressures, annealing conditions, and testing protocols (e.g., excitation amplitude and frequency). Consequently, the values listed are best viewed as representative orders of magnitude or performance ranges that illustrate the general evolutionary trends—from organic to inorganic, and from single-layer to nanoscale coatings—rather than as absolute metrics for direct cross-study benchmarking. Figure 1 summarizes the complete theoretical and processing framework from high-frequency loss mechanisms to different coating routes and next-generation power-electronics applications.
TABLE 1
| Coating strategy | Thickness | Resistivity (Ω·cm) | Tmax (°C) | Core loss (1 MHz) | Performance characteristics | Industrial scalability | Relative cost |
|---|---|---|---|---|---|---|---|
| Organic resin (; Zhu et al., 2023; ) | 0.5–5 μm | 105–106 | <250 | >1000 | Mature technology; excellent formability; Bs reduction 8%–15%; μe reduction 10%–20%; low cost; poor thermal stability | High (established) | Low |
| Phosphate (; ; ; ; ; ) | 50–200 nm | 102–103 | 500–600 | 600–800 | Thin coating; excellent scalability; Bs reduction 3%–6%; μe reduction 5%–10%; moderate insulation | High (established) | Low–Medium |
| Sol-gel oxide (Xie et al., 2023; ) | 100–300 nm | ≈105 | 600–800 | 400–600 | Dense oxide layer; good thermal resistance; Bs reduction 5%–8%; μe reduction 8%–12%; multistep synthesis | Medium | Medium |
| Glass/ceramic (; ) | 1–5 μm | >107 | >800 | 200–300 | Outstanding insulation; Bs reduction 15%–30%; μe reduction 20%–35%; a thick coating reduces magnetic induction | Medium–Low | Medium–High |
| Organic-inorganic hybrid (; Wan et al., 2024; Zheng et al., 2024) | 100–500 nm | ≈106 | 300–400 | 300–400 | Balanced thermal stability and insulation; Bs reduction 4%–7%; μe reduction 6%–10% | Medium | Medium |
| Multilayer inorganic composite (Xu et al., 2024; ; Zhou et al., 2026; Wang et al., 2022b) | 30–150 nm | 106–107 | 600–800 | 150–200 | Thin multilayer coating; Bs reduction 2%–5%; μe reduction 3%–8%; excellent high-frequency performance | Low–Medium | High |
| ALD nanocoating (Wang et al., 2022a; Zhou et al., 2024; ; Wang et al., 2023) | 10–50 nm | >108 | >800 | <100 | Atomic-scale conformality; Bs reduction <2%; μe reduction <5%; ultralow eddy-current loss; high fabrication cost | Low (developing) | Very High |
Comparison of representative insulating coating strategies for soft magnetic composites (SMCs).
Bs, saturation magnetic flux density; μe, effective permeability; Tmax, maximum practical service or annealing temperature reported for representative systems. Core-loss values are representative literature values; excitation amplitude, temperature, specimen density, and test protocol vary considerably among the cited studies. Percentage reductions are approximate values relative to the corresponding uncoated or less heavily coated compact within each individual study and are intended to illustrate general trends rather than to serve as absolute benchmarks for direct cross-study comparison.
FIGURE 1
To move beyond empirical comparison, the performance trends summarized in
Table 1can be rationalized through four primary morphological descriptors:
Coating Thickness vs. Magnetic Dilution: Thicker coatings (e.g., glass/ceramic, >1 μm) provide higher resistivity but severely reduce effective permeability (μe) and saturation flux density (Bs) by introducing nonmagnetic volume. Conversely, nanoscale coatings (e.g., ALD, 10–50 nm) minimize dilution (ΔBs<2%), preserving μe while still ensuring interparticle insulation.
Coating Uniformity vs. Insulation Reliability: Non-uniform coatings (common in sol-gel or mechanical mixing) create “weak spots” where interparticle contact occurs during compaction, leading to localized conductive paths and premature breakdown. Conformal coatings (e.g., ALD, phosphate reaction layers) ensure homogeneous electrical barriers, maximizing breakdown strength at minimal thickness.
Interfacial Adhesion vs. Stress Management: Strong chemical bonding (e.g., Fe–O–P in phosphate) enhances mechanical integrity but may introduce thermal-mismatch stress during annealing, increasing hysteresis loss. Compliant or gradient interfaces (e.g., organic–inorganic hybrids) buffer this stress, reducing the coercivity increase.
Coating Dielectric Constant vs. High-Frequency Loss: At >5 MHz, displacement currents through the coating capacitance become non-negligible. High-κ coatings (e.g., Al2O3, κ≈9) exacerbate parasitic capacitive coupling, while low-κ materials (e.g., SiO2, κ≈3.9) or nanoporous structures mitigate this effect, suppressing residual loss at ultrahigh frequencies.
This S-M-P framework, integrated with the processing routes shown in Figure 1, provides a mechanistic basis for selecting or designing coating systems for specific frequency and temperature targets.
In terms of specific performance indices, conventional organic and inorganic coatings and emerging composite/nanoscale coatings are compared quantitatively and qualitatively in Table 1. The assessment draws on representative studies of resin and phosphate systems (; ; ; ; Zhu et al., 2023; ; ; ; ) and of multilayer core-shell and ALD technologies (Wang J. et al., 2022; Zhou et al., 2024; Zhou et al., 2026; Wang S. et al., 2022; ; ). As shown in Table 1, phosphate-coated FeSiAl systems have been industrially established for applications typically below 3 MHz, although their performance ceilings become increasingly apparent at higher frequencies and elevated temperatures (), as reflected by the representative loss values listed in Table 1. Glass and ceramic coatings can provide low loss after high-temperature treatment, but their relatively large thickness may reduce permeability and constrain their application range (; ). In contrast, inorganic multilayer/composite coatings (; Zhou et al., 2026; Wang S. et al., 2022) and ALD nanocoatings (Wan et al., 2024; ) approach the ideal design target. ALD coatings can achieve substantially higher resistivities with minimal magnetic dilution—often approaching or exceeding 108 Ω cm with only 10–50 nm thickness—and therefore offer an attractive balance between eddy-current suppression and permeability retention compared with conventional coatings (; ).
To further contextualize the performance of the reviewed coating technologies, it is instructive to benchmark them against recently reported high-performance SMCs that do not rely solely on advanced coatings but also employ microstructure optimization. For instance, Arif et al. () achieved an effective permeability () of ∼73 and a remarkably low core loss () of ∼275 kW/m3 (at 100 kHz, 0.05 T) in amorphous FeSiBCCr SMCs by optimizing hot-compaction parameters. Similarly, by filling interparticle voids with Fe2O3 nanoparticles, they reported an ultra-low of 16.13 kW/m3 (at 100 kHz, 10 mT) while maintaining good frequency stability.
Comparing these values with those of coated SMCs (Table 1) reveals a critical trade-off. While ALD nanocoating provides superior resistivity (>108 Ω cm) and minimal magnetic dilution at ultrahigh frequencies (>5 MHz), its current complex processing leads to lower permeability compared to the optimally compacted hot-pressed samples. Conversely, the hot-pressed FeSi-based composites achieve exceptional permeability and low loss at moderate frequencies (∼100 kHz) primarily through densification and stress relief, rather than through extreme interparticle insulation. This comparison underscores that the selection of a coating strategy is highly application-dependent: for ultrahigh-frequency power electronics where eddy-current suppression is paramount, advanced nanoscale coatings like ALD are indispensable; however, for medium-frequency, high-power density applications, optimizing the compaction process in conjunction with simpler coatings (e.g., phosphate or oxide) may offer a more balanced and cost-effective solution.
It should also be noted that ALD, while delivering the best overall high-frequency performance, currently suffers from low throughput and high cost, as indicated in Table 1. Its industrial deployment is still in its infancy, and for many medium-frequency applications (<2 MHz), phosphate or hybrid coatings may offer a more cost-effective balance. Thus, the selection of a coating strategy should be application-driven: ALD is most advantageous for ultrahigh-frequency (>5 MHz) and high-reliability scenarios where performance outweighs cost, whereas conventional coatings remain viable for broader commercial use where thermal and frequency demands are moderate.
6 Conclusions and outlook
By using insulating coatings to divide metallic magnetic powders into core-shell magnetic units, SMCs intrinsically suppress high-frequency eddy-current loss and have become indispensable magnetic materials for SiC/GaN high-frequency power electronics. This review has summarized the evolution of SMC insulating coating technologies from organic coatings to inorganic coatings and further to composite and nanoscale coatings, and has analyzed the central role of core-shell interface engineering in coordinating eddy-current suppression with permeability retention (; ; ; Wang et al., 2024; ). This trajectory shows that organic coatings can no longer satisfy the thermal demands of high-temperature and high-frequency operation; inorganic phosphate, oxide, and glass systems each have distinct advantages, but single-material coatings cannot simultaneously provide the ideal combination of thinness, density, adhesion, and thermal stability. The emergence of composite and multilayer coatings, particularly ALD-based nanocoatings, enables atomic-level thickness control and highly conformal coverage, producing a synergistic breakthrough in eddy-current suppression and magnetic-performance retention. These advances point toward the next-generation of high-frequency SMCs. At the same time, interface engineering has evolved from a simple pursuit of high resistance into multidimensional optimization of coating thickness, uniformity, adhesion, dielectric properties, and stress state, marking an important shift from empirical trial-and-error toward science-guided design ().
Looking forward, several directions deserve sustained attention from both academia and industry. Reducing the cost of advanced nanocoatings will determine whether they can move beyond the laboratory. High-throughput fluidized-bed ALD for powders, molecular layer deposition (MLD), and hybrid processes that combine vapor-phase deposition with wet chemistry are making this goal increasingly attainable (Wang J. et al., 2022; ; ). Intelligent interface design may introduce self-healing functionality, tunable dielectric constants, or magnetostriction-matched interfacial layers to further reduce residual loss in the megahertz range and shift core-shell design from passive insulation toward active regulation. The synergistic optimization of new magnetic powders and coating layers is also highly promising: combining the low magnetocrystalline anisotropy and low magnetostriction of amorphous or nanocrystalline powders with ultrathin high-resistance coatings may generate a new class of ultralow-loss SMCs (; ; Wan et al., 2024; Zheng et al., 2024). In addition, most current studies focus on static or quasi-static material properties, whereas coatings in real devices must operate for long periods under coupled thermal, electrical, magnetic, and mechanical stresses. Establishing degradation models for coating layers under multistress conditions will be essential for reliability design and lifetime prediction (). Finally, materials-genome approaches and artificial-intelligence-assisted high-throughput experimentation and machine learning offer a new paradigm for rapidly screening coating materials and processing parameters across vast design spaces, potentially shortening the path from fundamental research to engineering application (Zhichao et al., 2024).
Statements
Author contributions
JP: Writing – original draft, Writing – review and editing. XZ: Writing – original draft, Writing – review and editing. BX: Writing – original draft, Writing – review and editing. JL: Writing – original draft, Writing – review and editing. CD: Writing – original draft, Writing – review and editing.
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References
1
ArifM.HanD.ShinW.ChaS.PakC.KimY.et al (2025a). Ultra-low core loss and high-frequency permeability stability in hot-press sintered FeSi soft magnetic composites by Fe2O3 nanoparticles air gap filling. Materials18, 2013. 10.3390/ma18092013
2
ArifM.HanD.ChaS.PakC.KimY.-K.KimS. W.et al (2025b). Enhancement of soft magnetic properties in surface-oxidized Fe/Fe3O4 composites via Co-nanoparticle doping and hot pressing. Curr. Appl. Phys.80, 256–264. 10.1016/j.cap.2025.09.028
3
ArifM.HanD.ShinW. C.ChaS.KimY.-K.KimS. W.et al (2025c). Extremely low core-loss and enhanced permeability stability in hot press sintered FeSi soft magnetic composites by TiO2 and Fe nanopowders air gap filling. Curr. Appl. Phys.80, 242–249. 10.1016/j.cap.2025.09.024
4
ArifM.SonJ. Y.RhyeeJ.-S. (2026). Recent advances in hot-press sintered FeSi-based soft magnetic composites. J. Mater. Res. Technol.42, 5880–5895. 10.1016/j.jmrt.2026.04.174
5
BaiG.SunJ.ZhangZ.LiuX.BandaruS.LiuW.et al (2024). Vortex-based soft magnetic composite with ultrastable permeability up to gigahertz frequencies. Nat. Commun.15, 2348. 10.1038/s41467-024-46650-9
6
BaiR.ShaoL.DingH.LiX.ZhouJ.XueZ.et al (2025). Optimizing the high-frequency magnetic properties of soft magnetic composites via surface nanoengineering. J. Mater. Sci. Technol.211, 82–88. 10.1016/j.jmst.2024.04.081
7
GuptaB.HossainMd. A.RiazA.SharmaA.ZhangD.TanH. H.et al (2022). Recent advances in materials design using atomic layer deposition for energy applications. Adv. Funct. Mater.32, 2109105. 10.1002/adfm.202109105
8
HansonA. (2022). Opportunities in magnetic materials for high-frequency power conversion. MRS Commun.12, 521–530. 10.1557/s43579-022-00225-1
9
HansonA. J.BelkJ. A.LimS.SullivanC. R.PerreaultD. J. (2016). Measurements and performance factor comparisons of magnetic materials at high frequency. IEEE Trans. Power Electron.31, 7909–7925. 10.1109/TPEL.2015.2514084
10
HeY.LiG.YangS.TianM.XuJ.GennadiiG.et al (2023). Improvements of magnetic performances for carbonyl iron soft magnetic composites (CI-SMCs) induced by Mn-based phosphating coating. J. Alloys Compd.958, 170498. 10.1016/j.jallcom.2023.170498
11
HsiangH.-I.FanL.-F.HungJ.-J. (2018). Phosphoric acid addition effect on the microstructure and magnetic properties of iron-based soft magnetic composites. J. Magnetism Magnetic Mater.447, 1–8. 10.1016/j.jmmm.2017.08.096
12
JussilaT.PhilipA.TripathiT.NielschK.KarppinenM. (2023). Atomic layer deposition of magnetic thin films: basic processes, engineering efforts, and road forward. Appl. Phys. Rev.10, 041313. 10.1063/5.0172732
13
KangS.LeeS. (2025). Optimizing the manufacturing process control of Si-Based soft magnetic composites. Materials18, 2321. 10.3390/ma18102321
14
KeB.LiuJ.HuangJ.HuH. (2026). Interface physics and engineering of soft magnetic composites for high-frequency power electronics. J. Mater. Sci. Mater. Electron.37, 1308. 10.1007/s10854-026-17719-6
15
LaiJ.XiaoL.XiongZ.FangL.ZhuW.KuangF.et al (2023). Enhanced soft magnetic properties and high-frequency stability of FeNiMo powder cores by coating SiO2 insulation layer. RSC Adv.13, 15892–15900. 10.1039/d3ra01523h
16
LeeS.ChoiS.YouJ.-H.LeeJ.-W.YooS.-I.ParkC. (2024). Reduction in core loss of soft magnetic composites with TiO2 coated Fe powder. J. Compos. Sci.8, 521. 10.3390/jcs8120521
17
LiW.WangZ.YingY.YuJ.ZhengJ.QiaoL.et al (2019). In-situ formation of Fe3O4 and ZrO2 coated Fe-based soft magnetic composites by hydrothermal method. Ceram. Int.45, 3864–3870. 10.1016/j.ceramint.2018.11.058
18
LiW.PuY.YingY.KangY.YuJ.ZhengJ.et al (2020). Magnetic properties and related mechanisms of iron-based soft magnetic composites with high thermal stability in situ composite-ferrite coating. J. Alloys Compd.829, 154533. 10.1016/j.jallcom.2020.154533
19
LiX.YaoH.WanY.SunY.LiuX. (2024a). High-performance FeSiAl soft magnetic composites with polyethyleneimine/silicone resin double organic insulation layers for 50 kHz–1 MHz frequency power applications. J. Electron. Mater.53, 6901–6913. 10.1007/s11664-024-11339-8
20
LiW.YangW.YingY.YuJ.ZhengJ.QiaoL.et al (2024b). Potassium phosphate modulation of the microstructure and its influence on the magnetic properties of FeSiAl soft magnetic composites. J. Alloys Compd.993, 174552. 10.1016/j.jallcom.2024.174552
21
LiW.XiangW.KangY.ZouT.HanX.YingY.et al (2025). Soft magnetism enhancement and eddy current suppression in bioinspired Iron‐based nanocrystalline soft magnetic composites with nacre‐like structure. J. Mater. Sci. and Technol.206, 202–210. 10.1016/j.jmst.2024.04.002
22
LongH.WuX.LuY.ZhangH.HaoJ. (2022). Effect of polyimide-phosphating double coating and annealing on the magnetic properties of fe-si-cr SMCs. Materials15, 3350. 10.3390/ma15093350
23
LuoP.YuH.WangC.YuanH.LiuZ.WangY.et al (2023). Properties optimization of soft magnetic composites based on the amorphous powders with double layer inorganic coating by phosphating and sodium silicate treatment. Metals13, 560. 10.3390/met13030560
24
MazaleyratF. (2024). Statistical modeling of soft magnetic composites’ permeability. J. Magnetism Magnetic Mater.612, 172646. 10.1016/j.jmmm.2024.172646
25
MuñozÁ. S.BurgosN.ZhukovaV.TalaatA.IpatovM.GonzálezJ.et al (2024). Study of an iron phosphate soft magnetic composite and its stability at high temperatures. Ceram. Int.50, 22468–22478. 10.1016/j.ceramint.2024.03.349
26
NabihA.JinF.GadelrabR.LeeF. C.LiQ. (2023). Characterization and mitigation of dimensional effects on core loss in high-power high-frequency converters. IEEE Trans. Power Electron.38, 14017–14036. 10.1109/TPEL.2023.3285633
27
PengJ.OuC.PanQ.HeB.ZhangB.ZengS. (2025). Influence of phosphorization process and heat treatment on the magnetic properties of pure iron soft magnetic composite materials. J. Materi Eng Perform34, 17641–17652. 10.1007/s11665-024-10425-6
28
PérigoE. A.WeidenfellerB.KollárP.FüzerJ. (2018). Past, present, and future of soft magnetic composites. Appl. Phys. Rev.5, 031301. 10.1063/1.5027045
29
PiechullaP. M.ChenM.GoulasA.PuurunenR. L.van OmmenJ. R. (2025). Atomic Layer Deposition on Particulate Materials from 1988 Through 2023: A Quantitative Review of Technologies, Materials and Applications. 10.48550/arxiv.2506.17725
30
PoškovićE.FranchiniF.FerrarisL.FracchiaE.BidulskaJ.CarosioF.et al (2021a). Recent advances in multi-functional coatings for soft magnetic composites. Materials14, 6844. 10.3390/ma14226844
31
PoškovićE.FranchiniF.FerrarisL.CarosioF.Actis GrandeM. (2021b). Rapid characterization method for SMC materials for a preliminary selection. Appl. Sci.11, 12133. 10.3390/app112412133
32
RafinS.AhmedR.HaqueMd.HossainMd.HaqueMd.MohammedO. (2023). Power electronics revolutionized: a comprehensive analysis of emerging wide and ultrawide bandgap devices. Micromachines14, 2045. 10.3390/mi14112045
33
Rahman KiranM.Rabiul IslamM.MuttaqiK. M.SutantoD.RaadR. (2024). A comprehensive review of advanced core materials-based high-frequency magnetic links used in emerging power converter applications. IEEE Access12, 107769–107799. 10.1109/ACCESS.2024.3436562
34
Salman SarfrazM.HongH.Su KimS. (2023). Lightweight thermoplastic coated soft magnetic composites (SMC) and application for high-efficiency brushless DC motors. Compos. Struct.320, 117195. 10.1016/j.compstruct.2023.117195
35
ShiK.ZhangD.ZhaoX.RenZ. (2026). Design of LLC magnetically integrated transformer based on nanocrystalline soft magnetic composite leakage core. CES Trans. Electr. Mach. Syst.9, 454. 10.30941/CESTEMS.2025.00057
36
ShokrollahiH.JanghorbanK. (2007). Soft magnetic composite materials (SMCs). J. Mater. Process. Technol.189, 1–12. 10.1016/j.jmatprotec.2007.02.034
37
VesaJ.HyvärinenL.RasiloP. (2023). Eddy-current loss model for soft magnetic composite materials considering particle size distribution. IEEE Trans. Magn.59, 1–8. 10.1109/TMAG.2023.3276217
38
WanL.SunX.LiJ.WuS. (2024). Preparation and magnetic properties of low-loss soft magnetic composites using MgO-Phenolic resin coating. Materials17, 4039. 10.3390/ma17164039
39
WangN.YingY.ZhengJ.LiW.QiaoL.CaiW.et al (2021). Development of Mn-Zn power ferrite with low losses over a broad temperature range for applications in the high frequency region of 0.5-3 MHz. Ceram. Int.47, 21425–21432. 10.1016/j.ceramint.2021.04.152
40
WangJ.LiuX.ZhengZ.QiuZ.LiK.XuJ.et al (2022a). Reduction of core loss for FeSi soft magnetic composites prepared using atomic layer deposition-based coating and high-temperature annealing. J. Alloys Compd.909, 164655. 10.1016/j.jallcom.2022.164655
41
WangS.ZhengJ.ZhengD.QiaoL.YingY.TangY.et al (2022b). Low core losses of Fe-Based soft magnetic composites with an Zn-O-Si insulating layer obtained by coupling synergistic photodecomposition. Materials15, 8660. 10.3390/ma15238660
42
WangJ.LiG.HeY.XuJ. L.QiuZ.ZhengZ.et al (2023). Improvement in core losses for FeSiAl soft magnetic composites induced by powder annealing treatment. J. Mater. Res. Technol.24, 2500–2513. 10.1016/j.jmrt.2023.03.168
43
WangR.LiK.WuX.ZhouZ.WuZ.LiJ. (2024). Optimisation of the magnetic properties of soft-magnetic composites through an interfacial solid-phase reaction based on hot-pressing sintering and secondary-magnetic-phase addition. Ceram. Int.50, 41066–41075. 10.1016/j.ceramint.2024.07.418
44
WangW.FanJ.LiC.YuY.WangA.LiS.et al (2025). Low-loss soft magnetic materials and their application in power conversion: progress and perspective. Energies18, 482. 10.3390/en18030482
45
XieQ.YuH.YuanH.HanG.ChenX.LiuZ. (2023). Enhanced magnetic properties and thermal conductivity of FeSiCr soft magnetic composite with Al2O3 insulation layer prepared by sol-gel process. Metals13, 813. 10.3390/met13040813
46
XuJ.ChenH.ShiQ.HeJ.NieM.WangC.et al (2024). Interdiffusion mechanism of hybrid interfacial layers for enhanced electrical resistivity and ultralow loss in Fe-based nanocrystalline soft magnetic composites. Appl. Surf. Sci.670, 160662. 10.1016/j.apsusc.2024.160662
47
ZhangJ.LuH.DongY.JiaX.LiY.YangL.et al (2024). Soft magnetic properties, microstructure, and growth mechanism of FeSiAl soft magnetic powder cores fabricated via hexafluozirconic acid passivation. J. Mater Sci. Mater Electron35, 747. 10.1007/s10854-024-12481-z
48
ZhangJ.QinY.LiuX.ZhaoY.DangW.FanX.et al (2025). Stress-induced magnetic anisotropy in Fe-Based Amorphous/Nanocrystalline alloys: mechanisms, advances and challenges. Materials18, 1499. 10.3390/ma18071499
49
ZhengW.ZhouZ.ZouR.YangM. (2024). Enhancing magnetic performance of FeNi50 soft magnetic composites with double-layer insulating coating for high-frequency applications. Magnetochemistry10, 45. 10.3390/magnetochemistry10070045
50
ZhichaoL.DongM.XiongjunL.LuZ. (2024). High-throughput and data-driven machine learning techniques for discovering high-entropy alloys. Commun. Mater5, 76. 10.1038/s43246-024-00487-3
51
ZhouZ.WenM.LiQ.ZhouR.WangX. (2024). Correlating the coating cycles, integrity and magnetic properties of Fe–Ni–Mo soft magnetic composites by a powder atomic layer deposition method. Ceram. Int.50, 29377–29387. 10.1016/j.ceramint.2024.05.231
52
ZhouB.ZhouY.XuS. (2026). Boosting electromagnetic properties of nanocrystalline soft magnetic composites by constructing a heterogeneous insulating multilayer. ACS Appl. Mater. and Interfaces18, 26633. 10.1021/acsami.6c05668
53
ZhuZ.LiuJ.ZhaoH.PangJ.WangP.ZhangJ. (2023). Study of the soft magnetic properties of FeSiAl magnetic powder cores by compounding with different content of epoxy resin. Materials16, 1270. 10.3390/ma16031270
Summary
Keywords
atomic layer deposition, core-shell structure, high-frequency loss, insulating coating, power electronics, soft magnetic composites
Citation
Peng J, Zhang X, Xing B, Liu J and Duan C (2026) High-performance soft magnetic composites for high-frequency power electronics: a review of insulating coating technologies. Front. Mater. 13:1953879. doi: 10.3389/fmats.2026.1953879
Received
30 July 2026
Revised
17 August 2026
Accepted
19 August 2026
Published
09 September 2026
Volume
13 - 2026
Edited by
Mohammad Islam, Hemlock Semiconductor Operations, United States
Reviewed by
Muhammad Arif, Global Campus, Republic of Korea
Updates
Copyright
© 2026 Peng, Zhang, Xing, Liu and Duan.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Jianing Peng, pengjianing0922@163.com
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