In this article, we present the benefits of ferrite nanopowders for electromagnetic shielding and stealth. For this purpose, permittivity and permeability are introduced and described as key parameters for electromagnetic absorption. Their values are discussed in the case of Nickel-Zinc nanoferrites for the microwave range.
Electromagnetic shielding and stealth are made possible by ferrite composite materials
Electronics have invaded our lives! They are everywhere: in your car, your phone, your watch, your coffee maker… All those devices emit electromagnetic waves allowing them to communicate, to detect objects, but also to charge devices wirelessly using inductive charging. All these technologies are at the origin of an invisible electromagnetic fog. Consequently, it has become necessary to protect humans as well as electronic equipment against this pollution. Indeed, not only these waves can be hazardous for heath, but also they may interact with systems and components, thus preventing some devices from working. This is known as electromagnetic interference (EMI). Getting rid of EMI requires the use of specific materials capable of suppressing the propagation of the wave. This is called shielding.
Ferrites are a family of magnetic materials which have a chemistry of type XFe2O4, where X is generally a transition metal. These materials are good candidates for shielding thanks to their high electromagnetic wave absorption capacity. Moreover, this capacity of ferrites to absorb electromagnetic waves is useful for stealth. Stealth is required in various contexts, not only in the military field for aircrafts, but also in the civil domain to reduce the impact of wind turbines on weather radars, for example. In such cases, it is necessary to suppress the reflection as well as to absorb the wave inside the material.
In shielding and stealth applications in the Gigahertz range (microwaves), materials need to be increasingly light in weight and thin to meet the growing need for miniaturization and lightness. Existing shielding materials are no longer suitable, and it is therefore necessary to use composite materials. This is why HYMAG’IN develops technology, know-how and expertise at producing ferrite nanopowders as fillers in composite materials.
Permittivity and permeability determine absorption efficiency
In high frequency range, the dielectric and magnetic properties of ferrites are described by the permittivity ε and the permeability µ, respectively. These are two key parameters driving the efficiency of our products for shielding and stealth applications. Here are some details one should know about the underlying physics. Permittivity and permeability are complex quantities. Let’s introduce their real parts (ε', µ') and imaginary parts (ε'', µ'') according to the following: ε = ε' - iε'' and µ = µ' - iµ''. The real parts ε' and µ' are associated with the storage of electrical and magnetic energy, respectively. On the other hand, the dissipation of energy is controlled by the imaginary parts ε’’ and µ’’. This energy is dissipated through Joule effect and polarization mechanisms for the electrical part, and displacement of magnetic walls for the magnetic part.
For shielding and stealth applications, the ideal material would exhibit values ε' = µ' = 1 (value of permittivity and permeability in air), so that the wave penetrates the material, and values of ε'' and µ'' as high as possible in order to maximize the dissipation of energy in thin layers of materials. Unfortunately, such ideal material does not exist. This is why at HYMAG’IN, we develop the best compromise for your applications: ferrite powders with a maximum ε'' and µ'' while keeping an ε' and µ' as low as possible.
Nickel and Zinc proportions are key parameters to obtain suitable material properties
At HYMAG’IN, we produce our ferrite nanopowders through our unique, patented hydrothermal process. This allows us to obtain very high quality, ultrathin powders, at moderate costs. For shielding and stealth purposes, we developed a recipe with Nickel and Zinc as main ingredients. Nickel offers a high permittivity at high frequencies and a high electrical conductivity. Zinc has a low magnetic coercivity and an excellent chemical stability .
In the scientific literature, Ni0.5Zn0.5Fe204 ferrite nanopowders incorporated in a polymer exhibit values of ε' and ε'' that are almost constant over the frequency range 1 GHz - 10 GHz and are worth 4 and 0.3, respectively. The values of µ' and µ'' are more variable and decrease over the frequency range from 2.5 to 0.5 and from 2 to 0.2, respectively . The loading rate here is estimated around 30%.
Figure 1-Permittivity and permeability values for Ni0.5Zn0.5Fe204 ferrite (Yasukawa, 2021, Nature)
At 1 GHz, these values yield a reflection loss reaching 20 dB (first reflection). At 10 GHz, this value is still significant at around 10 dB.
Let’s introduce fr as the frequency where µ'' is maximum. The decrease of µ' observed in the very high frequencies is expected because the physics tells us the quantity µ’fr is constant. This is known as the Snoek’s limit .
Ferrites can be doped with many chemical elements (Co, Mn, Cr, ...) in order to modify the permittivity and permeability values. These dopings also allow to modify the frequency fr and to decrease the influence of the Snoek’s limit in the Gigahertz range.
In conclusion, the nice absorption properties of Ni0.5Zn0.5Fe204 ferrite nanopowders allow HYMAG’IN to offer them for electromagnetic shielding applications and to improve the stealth properties of various systems.
One should know that hexaferrites, another family of ferrites, also have remarkable absorption properties in the microwave range. Although more costly to produce and process, hexaferrite nanopowders keep these absorption properties at higher frequencies than Nickel-Zinc ferrites, therefore allowing to go beyond the Snoek’s limit. HYMAG’IN wills to develop such materials hand in hand with interested customers who need innovative solutions for electromagnetic shielding and stealth in radiofrequencies.
 Thakur, P., Taneja, S., Chahar, D., Ravelo, B., & Thakur, A. (2021). Recent advances on synthesis, characterization and high frequency applications of Ni-Zn ferrite nanoparticles. Journal of Magnetism and Magnetic Materials, 530, 167925. https://doi.org/10.1016/j.jmmm.2021.167925
 Jiang, N. N., Yang, Y., Zhang, Y. X., Zhou, J. P., Liu, P., & Deng, C. Y. (2016). Influence of zinc concentration on structure, complex permittivity and permeability of Ni–Zn ferrites at high frequency. Journal of Magnetism and Magnetic Materials, 401, 370‑377. https://doi.org/10.1016/j.jmmm.2015.10.003
 SHEN, X., WANG, Y. X., YANG, X., XIA, Y., ZHUANG, J. F., & TANG, P. D. (2009). Megahertz magneto-dielectric properties of nanosized NiZnCo ferrite from CTAB-assisted hydrothermal process. Transactions of Nonferrous Metals Society of China, 19(6), 1588‑1592. https://doi.org/10.1016/s1003-6326(09)60075-3